A Mineralogical Perspective on Rare Earth Elements (REEs) Extraction from Drill Cuttings: A Review

Abstract

The growing demand for rare earth elements (REEs) in high-tech and green energy sectors has prompted renewed exploration of unconventional sources. Drill cuttings, which are commonly discarded during subsurface drilling, are increasingly recognized as a potentially valuable, underutilized secondary REE reservoir. This review adopts a mineral-first lens to assess REE occurrence, extractability, and recovery strategies from drill cuttings across various lithologies. Emphasis is placed on how REEs associate with specific mineral host phases ranging from ion-adsorbed clays and organically bound forms to structurally integrated phosphates, each dictating distinct leaching pathways. The impact of drilling fluids on REE surface chemistry and mineral integrity is critically examined, alongside an evaluation of analytical and extraction methods tailored to different host phases. A scenario-based qualitative techno-economic assessment and a novel decision-tree framework are introduced to align mineralogy with optimal recovery strategies. Limitations in prior studies, particularly in characterization workflows and mineralogical misalignment in leaching protocols, are highlighted. This review redefines drill cuttings from industrial waste to a strategic resource, advocating for mineralogically guided extraction approaches to enhance sustainability in the critical mineral supply chain.

1. Introduction

Rare earth elements (REEs) are essential to the functioning of modern technologies, playing a critical role in everything from renewable energy systems and electric vehicles to electronics, catalysts, and defense applications [1,2,3,4]. As global demand for clean energy and digital infrastructure continues to rise, so too does the strategic importance of REEs [5,6]. However, the global supply of these elements remains largely controlled by a few countries, making the market highly vulnerable to geopolitical tensions and trade restrictions [7,8]. This criticality has prompted governments and research communities worldwide to explore alternative, sustainable, and diversified sources of REEs to secure long-term supply [9,10].

In response to these challenges, attention has increasingly shifted toward unconventional sources of REEs, especially those that might be recovered as byproducts or from secondary materials [11,12,13]. Sedimentary formations such as coal seams and black shales have attracted interest due to their natural enrichment in light and heavy rare earths, often in forms more amenable to leaching compared to conventional hard rock ores [14,15]. These formations frequently host REEs in ion-adsorbed states or bound within organic matrices and clay minerals, offering an alternative pathway for extraction with potentially lower environmental and energy costs [16,17].

Among these unconventional sources, drill cuttings represent a largely overlooked but highly promising material [18,19]. Generated continuously during oil, gas, and geothermal drilling operations, drill cuttings are typically treated as waste [20,21,22]. However, these cuttings are in fact physical samples of the subsurface, often rich in geological information and occasionally in trace metals, including REEs [23,24]. Depending on the nature of the formation being drilled, cuttings can carry REEs in various mineralogical forms [25]. Since they are already brought to the surface as part of the drilling process, no additional mining or extraction is required to access them, making them a cost-effective and low-impact resource for further evaluation [26].

Basically, REEs represent a class of 17 elements: 15 lanthanides plus scandium (Sc) and yttrium (Y) which exhibit similar electronic configurations and chemical properties [27,28,29]. Despite their relative geochemical abundance, the rarity of economically recoverable concentrations, their fine dispersion in host matrices, and their difficult separation from one another make REEs both technically and economically challenging to exploit. With mounting global interest in energy transition technologies, securing a reliable and environmentally sustainable supply of REEs has become a strategic priority for many nations [30,31,32].

REEs are typically grouped based on their atomic number and geochemical properties. Traditionally, REEs are classified into light (LREEs) and heavy (HREEs) categories [33,34]. However, for improved granularity, especially relevant in mineralogical and extraction studies; REEs can also be reclassified into light, medium, and heavy rare earth elements as shown in Figure 1. Light REEs (LREEs) are Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm) while Medium REEs (MREEs) constitute Samarium (Sm), Europium (Eu), Gadolinium (Gd) and Heavy REEs (HREEs) are Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium (Yb), Lutetium (Lu), Yttrium (Y) and Scandium (Sc).

This tripartite classification better reflects subtle variations in ionic radii, crystal field effects, and geochemical partitioning, which influence how REEs behave during magmatic, sedimentary, and weathering processes. For example, LREEs are generally more compatible with phosphate and carbonate minerals, and are commonly enriched in monazite or bastnäsite [35,36]. MREEs show intermediate behaviour, sometimes partitioning into both light and heavy mineral hosts. HREEs, owing to their smaller ionic radii and higher charge density, tend to prefer silicates and oxide phases and are also more likely to be adsorbed onto clay surfaces under acidic, weathered conditions [37,38].

From an economic and technological standpoint, the HREEs especially Dy, Tb, and Y are in high demand due to their role in high-strength magnets, phosphors, and defense systems. However, these elements are often present in lower natural abundance, particularly in conventional REE deposits [39,40]. This scarcity has driven increased interest in unconventional HREE-rich sources such as ion-adsorption clays and organic-rich black shales. Understanding the mineralogical context and geochemical behaviour of each REE subgroup is therefore essential for designing effective recovery strategies and managing supply-demand constraints in critical mineral supply chains [41].

From a strategic materials perspective, REEs are considered indispensable to critical sectors such as defense, clean energy, automotive, and advanced electronics [4]. The permanent magnets made from neodymium (Nd), praseodymium (Pr), and dysprosium (Dy) are the backbone of electric motors and wind turbine generators. Yttrium (Y), europium (Eu), and terbium (Tb) are key constituents of phosphors in LEDs and screens, while gadolinium (Gd) is used in MRI contrast agents and neutron capture therapy [1,42]. More importantly, REEs are often non-substitutable in these applications, giving them an outsized importance despite their relatively low usage volumes. The majority of global REE supply and downstream processing is currently dominated by China, leading to significant geopolitical concerns. This has triggered a global response focused on supply chain diversification, stockpiling, and development of alternative sources both primary and secondary.

Several countries including the USA, Japan, Australia, Canada, and members of the EU have issued national strategies or funding initiatives for critical materials, with REEs consistently topping the list [43,44]. The geopolitical leverage of REE supply is no longer theoretical export restrictions, pricing fluctuations, and monopolistic behaviour have repeatedly influenced industrial policy and international trade dynamics. This context sets the stage for increased attention to non-traditional REE resources, including those associated with industrial byproducts like drill cuttings [45].

This review critically examines the potential of drill cuttings as a secondary source of REEs through a mineralogical lens. It explores how REEs occur in various mineral host phases and how drilling fluids may influence their detection and recoverability. By analysing the relationship between mineralogy and leachability, the review aligns REE-host types such as clays, phosphates, and organo-metallic complexes with suitable extraction methods. The work consolidates existing studies, highlights mineralogical controls on REE recovery, and proposes a scenario-based techno-economic assessment and novel decision-making framework to guide method selection. Ultimately, it reframes drill cuttings from a waste challenge to a resource opportunity within the critical mineral supply chain.

2. Structure and the Flow of the Review

This review is carefully structured to guide readers from foundational concepts to advanced critical insights surrounding REE recovery from drill cuttings (Figure 2). The Introduction (Section 1) lays the groundwork by explaining the strategic significance of REEs, challenges in conventional extraction, and the emerging interest in drill cuttings as a secondary source. It also introduces the core premise of the review, i.e., examining REE recovery through the lens of mineral host phases. Following this, Section 2 outlines the structure and logical flow of the paper to support reader navigation. Section 3 establishes the mineralogical context, detailing both conventional and unconventional REE sources and their associated host minerals, with special focus on how these modes of occurrence influence extractability. Section 4 synthesizes prior studies on REEs in drill cuttings, highlighting gaps in reporting, analytical inconsistencies, and variability in mineralogy and critical evaluations. It further transitions to Section 5, where analytical techniques, leaching strategies, and characterization methods are thoroughly discussed in context to REE extraction from drill cuttings. Section 6 features real time extraction of REE from drill site while these discussions culminate in Section 7, where a scenario-based qualitative assessment of techno-economic feasibility is presented, followed by a decision tree framework linking mineral host phases to suitable extraction and analytical methods in Section 8. The final section i.e., Section 9, emphasizes future challenges and research directions, reaffirming the need for mineral-first strategies in REE recovery from drill cuttings. Lastly, the study is concluded in Section 10.

3. Mineral Host Phases in REEs and Its Relevance to Drill Cuttings

3.1. Global Geological Sources of REEs: Deposit Types and Mineral Hosts

Conventional REE mining has historically relied on high-grade igneous and hydrothermal deposits, most notably carbonatites and alkaline granitic systems. Economically significant REE minerals include:

Bastnäsite [(Ce,La)(CO₃)F]—a fluorocarbonate rich in LREEs, prominent in deposits such as Mountain Pass (USA) and Bayan Obo (China) [46,47].

Monazite [(Ce,La,Nd,Th)PO₄]—a phosphate mineral common in carbonatites, beach placers, and metamorphic terrains; often associated with thorium, posing radiological handling concerns [35,48,49].

Xenotime [YPO₄]—a phosphate mineral rich in HREEs and yttrium; occurs in pegmatites and heavy mineral sands [50,51,52,53].

Eudialyte, allanite, and apatite; minor but locally significant hosts for REEs, often requiring complex processing [54].

Although these deposits contain relatively high REE concentrations (1–10 wt%), they are associated with complex gangue minerals, high processing costs, and environmental liabilities, particularly from radioactive waste e.g., thorium, uranium. The selective flotation, roasting, and acid leaching required for REE separation from hard rocks are capital- and reagent-intensive, making it difficult for many nations to develop their own upstream capabilities. Furthermore, most of the accessible and high-grade REE deposits are already developed or under tight environmental scrutiny, encouraging exploration into lower-grade, but more accessible and sustainable sources. Global distribution of REE deposits along with host minerals have been tabulated in Table 1.

Many other different types of minerals can also host rare REEs, incorporating them into their crystal lattices through elemental substitution. These REE-bearing minerals belong to a variety of mineral classes, including silicates (e.g., Allanite, Eudialyte, Mosandrite), phosphates (e.g., Apatite, Monazite, Xenotime), carbonates (e.g., Bastnaesite, Parisite, Synchysite), oxides (e.g., Fergusonite, Pyrochlore), and mixed-class or complex minerals (e.g., Loparite, Steenstrupine, Rinkite), each playing a significant role in REE enrichment and extraction. Allanite has the general formula (Y,La,Ca)₂(Al,Fe³⁺)₃(SiO₄)₃(OH), while Apatite is represented as (Ca,La)₅(PO₄)₃(F,Cl,OH). Bastnaesite is commonly expressed as (La,Y)(CO₃)F, and Eudialyte has a more complex composition: Na₄(Ca,La)₂(Fe²⁺,Mn²⁺,Y)ZrSi₈O₂₂(OH,Cl)₂. Fergusonite follows the formula (La,Y)NbO₄, and Gittinsite is defined as CaZrSi₂O₇. Limoriite is denoted as Y₂(SiO₄)(CO₃), while Kainosite has the formula Ca₂(Y,La)₂Si₄O₁₂(CO₃)·H₂O. Loparite is characterized by the formula (La,Na,Ca)(Ti,Nb)O₃, and Monazite is (La,Th)PO₄. Mosandrite’s composition is (Na,Ca)₃Ca₃La(Ti,Nb,Zr)(Si₂O₇)₂(O,OH,F)₄. Parisite is represented as Ca(La)₂(CO₃)₃F₂, and Pyrochlore has the formula (Ca,Na,La)₂Nb₂O₆(OH,F). Rinkite, also known as Rinkolite, is (Ca,La)₄Na(Na,Ca)₂Ti(Si₂O₇)₂(O,F)₂, while Steenstrupine is a highly complex mineral with the formula Na₁₄La₆Mn₂Fe₂(Zr,Th)(Si₆O₁₈)₂(PO₄)₇·3H₂O. Synchysite follows the formula Ca(La)(CO₃)₂F, Xenotime is YPO₄, and Zircon can incorporate rare earths as (Zr,La)SiO₄ [55,56].

Table 1. Global REEs deposits with host minerals (adopted after Kumari et al., 2015 [57]).

Deposit Location	Deposit Type (Appx. REE Content %) *	Main REEs	REE-Host Minerals
Bayan Obo, China	Carbonatite/hydrothermal (3%–12%)	La, Ce, Nd	bastnasite, parasite, monazite
Mountain Pass, USA	Carbonatite (8.9%)	LREE	bastnasite
Mount Weld, Australia	Laterite/Carbonatite (~30%)	LREE	apatite, monazite, synchysite, churchite
Illimaussaq, Denmark	Peralkaline igneous (1%–3%)	La, Ce, Nd, HREE	eudialyte, steenstrupine
Pilanesberg, South Africa	Peralkaline igneous (0.2%–0.5%)	Ce, La	eudialyte
Steenkampskraal, South Africa	Vein (~17%)	La, Ce, Nd	monazite, apatite
Hoidas Lake, Canada	Vein (1.5%–5.5%)	La, Ce, Pr, Nd	apatite, allanite
Thor Lake, Canada	Alkaline igneous (1%–2.5%)	La, Ce, Pr, Nd, HREE	bastnasite
Strange Lake & Misery Lake, Canada	Alkaline igneous/hydrothermal (1%–2%)	La, Ce, Nd, HREE	gadolinite, bastnasite
Nolans Bore, Australia	Vein (2%–4%)	La, Ce, Nd	apatite, allanite
Norra Kärr, Sweden	Peralkaline igneous (0.6%–1%)	La, Ce, Nd, HREE	eudialyte
Khibina & Lovenzero, Russia	Peralkaline igneous (0.7%–1.2%)	LREE + Y, minor HREE	eudialyte, apatite
Nkwombwa Hill, Zambia	Carbonatite (1.5%–2.5%)	LREE	monazite, bastnasite
Kagankunde, Malawi	Carbonatite (2%–3%)	LREE	monazite-Ce, bastnaesite-Ce
Tundulu, Malawi	Carbonatite (2%–3%)	LREE	synchesite, parasite, bastnasite
Songwe, Malawi	Carbonatite (1%–2%)	LREE, Nd	synchesite, apatite
Ion Adsorption Deposits, China	Ion adsorption (soil) (0.05%–0.2%)	La, Nd, HREE	clay minerals
Maoniuping, China	Carbonatite (3%–4%)	LREE	bastnasite
Dong Pao, Vietnam	Carbonatite (4%–6%)	LREE	bastnasite, parisite

* The REE content values are approximate and can vary within each deposit. They are based on available literature and are intended to provide a general understanding of the REE concentrations associated with each deposit type [45,58,59,60,61,62,63,64].

Table 1. Global REEs deposits with host minerals (adopted after Kumari et al., 2015 [57]).

Deposit Location	Deposit Type (Appx. REE Content %) *	Main REEs	REE-Host Minerals
Bayan Obo, China	Carbonatite/hydrothermal (3%–12%)	La, Ce, Nd	bastnasite, parasite, monazite
Mountain Pass, USA	Carbonatite (8.9%)	LREE	bastnasite
Mount Weld, Australia	Laterite/Carbonatite (~30%)	LREE	apatite, monazite, synchysite, churchite
Illimaussaq, Denmark	Peralkaline igneous (1%–3%)	La, Ce, Nd, HREE	eudialyte, steenstrupine
Pilanesberg, South Africa	Peralkaline igneous (0.2%–0.5%)	Ce, La	eudialyte
Steenkampskraal, South Africa	Vein (~17%)	La, Ce, Nd	monazite, apatite
Hoidas Lake, Canada	Vein (1.5%–5.5%)	La, Ce, Pr, Nd	apatite, allanite
Thor Lake, Canada	Alkaline igneous (1%–2.5%)	La, Ce, Pr, Nd, HREE	bastnasite
Strange Lake & Misery Lake, Canada	Alkaline igneous/hydrothermal (1%–2%)	La, Ce, Nd, HREE	gadolinite, bastnasite
Nolans Bore, Australia	Vein (2%–4%)	La, Ce, Nd	apatite, allanite
Norra Kärr, Sweden	Peralkaline igneous (0.6%–1%)	La, Ce, Nd, HREE	eudialyte
Khibina & Lovenzero, Russia	Peralkaline igneous (0.7%–1.2%)	LREE + Y, minor HREE	eudialyte, apatite
Nkwombwa Hill, Zambia	Carbonatite (1.5%–2.5%)	LREE	monazite, bastnasite
Kagankunde, Malawi	Carbonatite (2%–3%)	LREE	monazite-Ce, bastnaesite-Ce
Tundulu, Malawi	Carbonatite (2%–3%)	LREE	synchesite, parasite, bastnasite
Songwe, Malawi	Carbonatite (1%–2%)	LREE, Nd	synchesite, apatite
Ion Adsorption Deposits, China	Ion adsorption (soil) (0.05%–0.2%)	La, Nd, HREE	clay minerals
Maoniuping, China	Carbonatite (3%–4%)	LREE	bastnasite
Dong Pao, Vietnam	Carbonatite (4%–6%)	LREE	bastnasite, parisite

* The REE content values are approximate and can vary within each deposit. They are based on available literature and are intended to provide a general understanding of the REE concentrations associated with each deposit type [45,58,59,60,61,62,63,64].

3.2. Unconventional and Secondary Sources of REEs

The rising cost and environmental risks of traditional REE mining have catalyzed a shift in focus toward unconventional and secondary sources, which include materials derived from geological weathering, industrial waste, and sedimentary systems (Table 2) [65,66,67].

3.2.1. Ion-Adsorption Clays

Originating from deeply weathered granitic rocks, these are mostly composed of kaolinite, halloysite, or illite and host REEs through weak electrostatic adsorption. Found primarily in southern China and Myanmar, these deposits are especially enriched in HREEs and can be leached using simple ammonium salt solutions, making them environmentally less destructive. However, the mineralogical characterization is often poorly documented, and REE distribution can be highly heterogeneous at micro- and macro-scales [68,69,70,71].

3.2.2. Black Shales and Organic-Rich Mudstones

These rocks may contain REEs associated with organic matter, aluminosilicates, or phosphate phases. The depositional environment and redox conditions during diagenesis play a key role in REE enrichment. While REE grades in shales are generally lower (100–500 ppm), their vast distribution, shallow burial, and existing drilling infrastructure make them highly attractive for co-recovery [14,72,73,74,75,76].

3.2.3. Coal and Coal Byproducts

Certain coals particularly in Russia, the US, and China contain REE concentrations comparable to low-grade ores. REEs are often hosted in kaolinite, illite, and amorphous phases, or associated with organics. Fly ash, a byproduct of coal combustion, has shown good leaching behaviour using acidic or chelating agents, though mineralogy varies drastically with combustion temperature and feed coal composition [77,78,79,80].

3.2.4. Phosphogypsum, AMD, and Metallurgical Slags

These industrial wastes are underutilized but can contain economically interesting levels of REEs. Processing these materials can serve dual purposes resource recovery and environmental remediation [81,82].

What unifies these unconventional sources is that mineralogical form dictates extractability. For example, ion-adsorbed REEs can be recovered under mild conditions, while those within crystalline phases like monazite or apatite require intense treatment. Thus, a mineral-first approach is essential in evaluating the techno-economic feasibility of any REE recovery strategy.

Table 2. Comparison of unconventional REE sources by host minerals, grade, extraction methods, and estimated recovery potential [12,13,83].

Source	Typical Host Minerals/Associations	REE Grade	Enrichment Type	Key Advantages	Main Challenges	Approx. Recovery Potential (%)
Ion-Adsorption Clays	Kaolinite, halloysite, illite; REEs loosely adsorbed	200–1000 ppm	HREE-enriched	Low energy input, environmentally benign	Heterogeneous mineralogy; shallow resource base	60%–90% (High)
Black Shales/Organic-Rich Mudstones	Organic matter, aluminosilicates, apatite	100–500 ppm	LREE & sometimes HREE	Vast distribution, shallow depth, co-recovery with hydrocarbons	Low grade; REEs strongly bound; redox-sensitive	30%–60% (Moderate but scalable)
Coal & Coal Byproducts (e.g., Fly Ash)	Kaolinite, illite, amorphous phases, organics	100–800 ppm (can exceed 1000 ppm in ash)	LREE-dominant	Abundant waste material, good leachability in ash	Highly variable mineralogy and feedstock quality	40%–80% (up to 90% in fly ash)—Moderate to High
Phosphogypsum, AMD, Metallurgical Slags	Apatite, jarosite, amorphous/oxide-bound REEs	50–300 ppm (up to 1 wt% in slags)	Mostly LREE	Dual benefit of resource recovery and environmental remediation	Toxicity, regulation, process residue management	20%–60%, Moderate depending on feed quality

Table 2. Comparison of unconventional REE sources by host minerals, grade, extraction methods, and estimated recovery potential [12,13,83].

Source	Typical Host Minerals/Associations	REE Grade	Enrichment Type	Key Advantages	Main Challenges	Approx. Recovery Potential (%)
Ion-Adsorption Clays	Kaolinite, halloysite, illite; REEs loosely adsorbed	200–1000 ppm	HREE-enriched	Low energy input, environmentally benign	Heterogeneous mineralogy; shallow resource base	60%–90% (High)
Black Shales/Organic-Rich Mudstones	Organic matter, aluminosilicates, apatite	100–500 ppm	LREE & sometimes HREE	Vast distribution, shallow depth, co-recovery with hydrocarbons	Low grade; REEs strongly bound; redox-sensitive	30%–60% (Moderate but scalable)
Coal & Coal Byproducts (e.g., Fly Ash)	Kaolinite, illite, amorphous phases, organics	100–800 ppm (can exceed 1000 ppm in ash)	LREE-dominant	Abundant waste material, good leachability in ash	Highly variable mineralogy and feedstock quality	40%–80% (up to 90% in fly ash)—Moderate to High
Phosphogypsum, AMD, Metallurgical Slags	Apatite, jarosite, amorphous/oxide-bound REEs	50–300 ppm (up to 1 wt% in slags)	Mostly LREE	Dual benefit of resource recovery and environmental remediation	Toxicity, regulation, process residue management	20%–60%, Moderate depending on feed quality

3.3. Modes of REE Occurrence Based on Mineralogical Association

The recovery potential of REEs is fundamentally governed by their mode of occurrence within the host material. In drill cuttings, as in other geological and industrial sources, REEs are found in three broadly recognized forms: ion-adsorbed, structurally bound in minerals, and associated with amorphous or organic phases as depicted in

Table 3. Mode of occurrence of REEs.

Mode of Occurrence	Host Medium	Extractability	Example Deposits
Ion-adsorbed	Clays (kaolinite, halloysite)	Easy (mild leaching)	South China lateritic clays
Mineral-hosted (crystalline)	Monazite, bastnäsite, xenotime	Hard (acid/roasting)	Mountain Pass, Bayan Obo
Organic/amorphous-bound	Coal, black shale, Fe-Mn oxides	Moderate (oxidation)	Appalachian coals, fly ash

. Each mode differs significantly in terms of bonding strength, mineralogical host, extractability, and environmental implications. Understanding these distinctions is critical to designing appropriate and efficient recovery strategies.

3.3.1. Ion-Adsorbed REEs

Ion-adsorbed REEs are among the most accessible and environmentally preferred forms of occurrence. In this mode, REE ions are loosely held on the surface of clay minerals such as kaolinite, halloysite, and illite. These elements are bound primarily by weak ionic interactions or electrostatic forces, rather than being incorporated into mineral crystal lattices. This form of REE occurrence is especially prevalent in deeply weathered granitic crusts and tropical lateritic soils, with the most notable examples found in the ion-adsorption clay deposits of southern China and Myanmar [84,85,86].

The extractability of ion-adsorbed REEs is one of their key advantages. These elements can typically be leached under ambient conditions using mild reagents such as ammonium sulfate or organic acids. In recent studies, natural deep eutectic solvents (NADES) and biodegradable ligands like citric acid have also shown promise in recovering these elements with minimal environmental footprint. This low-impact leaching process, combined with relatively high HREE enrichment in ion-adsorbed deposits, makes this mode particularly attractive from both economic and sustainability standpoints [87,88].

3.3.2. Mineral-Hosted REEs

In contrast to ion-adsorbed forms, mineral-hosted REEs are structurally integrated into the crystal lattices of specific minerals. The most common host phases include monazite (a phosphate mineral), bastnäsite (a fluorocarbonate), xenotime (yttrium phosphate), and allanite (a complex silicate). These minerals accommodate REEs through chemical substitution for major cations such as Ca²⁺, Th⁴⁺, or other rare earths, making them relatively stable and resistant to natural weathering [45,89,90].

This mode of occurrence is characteristic of many high-grade primary deposits, including carbonatites, pegmatites, and heavy mineral placers [91,92]. While these deposits often yield significant concentrations of LREEs, the extraction of structurally bound REEs is technically more demanding. Liberation typically requires strong acid digestion, high-temperature roasting, or alkaline cracking, processes which can be energy-intensive and environmentally hazardous, particularly when radioactive elements like thorium are present [93]. Despite these challenges, most conventional REE mining operations are centered on this form due to the high resource concentration and established processing technologies [94,95].

3.3.3. REEs in Amorphous, Organic, or Colloidal Associations

A third, increasingly important mode of REE occurrence involves association with organic matter, poorly crystalline phases, or colloidal materials [96,97]. In this form, REEs may be bound to humic substances, trapped within coal or kerogen matrices, or adsorbed onto the surfaces of amorphous aluminosilicates and Fe/Mn oxides [98,99]. These complex associations are common in sedimentary environments such as coal seams and black shales, as well as in industrial byproducts including fly ash and acid mine drainage sludge [100].

The bonding mechanisms in these associations can be highly variable, ranging from surface adsorption and ion exchange to more complex chelation or incorporation into disordered structures [101]. As a result, the extractability of REEs in this form depends heavily on the host matrix and the specific leaching strategy employed. Techniques such as oxidative leaching, chelation, and bioleaching have shown promise, though challenges remain in scaling these processes and optimizing recovery efficiency. While generally lower in grade compared to primary ores, these materials are often available as industrial waste or byproducts, making them attractive for low-cost, sustainable resource recovery [102].

3.4. Mineralogical Controls on REE Recovery: Why It Matters in Drill Cuttings

In the context of drill cuttings, mineralogical understanding plays a pivotal role in assessing the potential for REE recovery. Drill cuttings are heterogeneous mixtures of fragmented rock produced as the drill bit penetrates subsurface formations, and they are transported to the surface by drilling fluids [20]. These materials serve as direct samples of the formation being drilled, capturing its lithology, geochemistry, and mineralogical makeup. However, they are often inconsistently preserved, poorly characterized, and subjected to chemical and mechanical alteration during the drilling process. Despite these challenges, drill cuttings may contain trace to moderate concentrations of REEs, hosted within a variety of mineralogical phases. These include ion-exchangeable REEs loosely adsorbed onto the surfaces of clay minerals such as kaolinite, smectite, or illite; crystalline REE-bearing minerals like monazite, xenotime, and apatite; organically-bound REEs within coal matrices or kerogen; and amorphous or poorly crystalline materials formed through diagenetic or weathering processes [103].

The mode of occurrence of REEs within these phases has direct implications for their extractability. Weakly adsorbed REEs can often be leached using mild reagents such as ammonium salts, citric acid, or natural deep eutectic solvents (NADES), whereas structurally bound REEs in minerals like monazite or xenotime require aggressive processing through strong acids or thermal activation [104,105,106]. Organically-associated REEs may necessitate oxidative or bioleaching approaches to liberate the elements from their host matrix [107]. Compounding this complexity, drilling fluids themselves may interfere with recovery by altering mineral surfaces, promoting occlusion of fine particles, or introducing contaminants that affect leaching efficiency [108]. Therefore, the lack of detailed mineralogical characterization can lead to inefficient extraction, underestimation of resource potential, or the application of environmentally and economically suboptimal recovery strategies.

Rather than being a supplementary consideration, mineralogy should be viewed as the central guiding factor in any REE recovery effort from drill cuttings. It informs not only the selection of appropriate leaching agents and processing conditions but also the broader design of exploration workflows and fluid formulation strategies. Recognizing the diversity of REE-hosting phases within cuttings and understanding their reactivity is essential for advancing sustainable and targeted resource recovery. The following section will explore in greater detail the mineral forms in which REEs occur within drill cuttings across different geological settings and how these influence downstream processing and extraction potential.

3.5. Mineralogical Forms of REEs Encountered in Drill Cuttings

The lithology of a formation plays a decisive role in determining the mineralogical form and mobility of REEs encountered during drilling. Since REEs are not uniformly distributed across geological environments, understanding their typical host phases in specific lithologies allows for targeted extraction strategies. The form in which REEs occur whether ion-adsorbed, structurally bound, or organically associated directly impacts their leachability, recovery efficiency, and environmental footprint. This section provides a critical assessment of the REE-bearing phases associated with key lithological units commonly encountered in drilling operations, such as laterites, carbonatites, shales, coal seams, and igneous rocks as tabulated in

Table 4. Summary of dominant host phases in drill cuttings (lithologies).

Lithology	Dominant REE-Host Phases	REE Type	Bonding Mechanism
Weathered Granites/Ion-Clay Zones	Ion-adsorbed REEs on kaolinite, halloysite, illite	HREE > LREE	Weak electrostatic adsorption
Carbonatites/Pegmatites/Plutonics	Monazite, bastnäsite, xenotime, allanite	LREE > HREE	Crystal lattice substitution (Ca2+, Th4+)
Black Shales/Organic Mudstones	Organically-bound REEs, phosphate microfossils, clays	Mixed LREE & HREE	Organo-metallic complexation, interlayer sorption
Coal Seams/Fly Ash	Amorphous aluminosilicates, Fe–Mn oxides, organic matter	Mixed (varies)	Variable: adsorption, chelation, occlusion
Marine Shales/Siliciclastic Sediments	Ion-adsorbed + interlayered REEs on smectite/illite clays	Variable (often HREE)	Weak ionic and surface bonding
Thermally Altered Zones/Alterites	Secondary phosphates, altered monazite, poorly crystalline oxides	LREE > HREE	Recrystallized or amorphous lattice

.

3.5.1. Weathered Granitic and Clay-Rich Sediments: Ion-Adsorbed REEs

Ion-adsorbed REEs are most commonly found in deeply weathered profiles developed over felsic igneous rocks, particularly granites and rhyolites. In these settings, prolonged tropical or subtropical weathering leads to the formation of clay-rich lateritic crusts dominated by kaolinite, halloysite, and illite [109,110]. REEs in these profiles are typically adsorbed onto the surfaces of clay minerals through weak electrostatic interactions. This form of REE association is prevalent in southern China, Myanmar, and parts of Southeast Asia, where ion-adsorption clay deposits serve as a primary global source of heavy REEs such as yttrium (Y), dysprosium (Dy), and terbium (Tb) [111,112].

Drilling through weathered felsic terrains, marine shales, or clay-dominated sedimentary basins often yields cuttings containing REEs in this loosely bound form. The weak binding energy makes them amenable to recovery via mild leaching agents such as ammonium sulfate, citric acid, or natural deep eutectic solvents (NADES). These environments are typically associated with low-grade but high-tonnage REE enrichment and are particularly significant for low-impact, scalable extraction technologies.

3.5.2. Carbonatites, Pegmatites, and Metamorphosed Terranes: Structurally Bound REEs

In hard rock lithologies such as carbonatites, alkaline igneous complexes, and high-grade metamorphic rocks, REEs occur predominantly in the form of discrete REE-bearing minerals. These include monazite [(Ce,La,Nd,Th)PO₄], xenotime [YPO₄], bastnäsite [(Ce,La)(CO₃)F], and allanite (a silicate phase). These minerals incorporate REEs into their crystal lattices through substitution mechanisms, forming stable, resistant host phases [92].

Carbonatite intrusions and associated hydrothermal zones, such as those in Mountain Pass (USA), Bayan Obo (China), and Mount Weld (Australia), are particularly rich in bastnäsite and monazite, with a dominance of light REEs like cerium (Ce), lanthanum (La), and neodymium (Nd). Similarly, xenotime is a major host of yttrium and HREEs in granitic pegmatites and heavy mineral placers [113,114].

When drilling intersects these lithologies, REEs in cuttings are likely to be structurally bound and require aggressive processing for extraction, including acid digestion, thermal cracking, or alkaline fusion. Identification of these minerals in cuttings often necessitates advanced mineralogical tools such as SEM-EDX (Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy), QEMSCAN (Quantitative Evaluation of Minerals by Scanning Electron Microscopy), or XRD (X-ray diffraction), as they are frequently fine-grained and dispersed.

3.5.3. Black Shales and Organic-Rich Mudstones: Mixed and Amorphous REE Associations

Black shales and organic-rich mudstones, commonly encountered in sedimentary basins during oil and gas exploration, represent a distinct lithological class where REEs occur in mixed or poorly crystalline forms. These environments are typically enriched in organic matter and phosphorus, creating geochemical conditions that favour the adsorption, complexation, or incorporation of REEs into organic matrices and amorphous aluminosilicates [115].

In black shales, REEs may be associated with phosphatic microfossils, adsorbed onto clay surfaces, or bound to organo-metallic complexes. The redox-sensitive behaviour of some REEs, especially cerium (Ce), also leads to selective enrichment or depletion under anoxic diagenetic conditions. These associations are moderately extractable using oxidative or chelating leaching agents, but their complex bonding mechanisms and variable mineralogy make recovery challenging [73].

Shale-derived cuttings may thus contain a mixture of ion-adsorbed and organically complexed REEs, requiring a combination of techniques for effective liberation. Pre-treatment steps such as oxidation or thermal conditioning may enhance extraction efficiencies in these materials.

3.5.4. Coal Seams and Fly Ash: Organically and Colloidally Bound REEs

Coal seams and associated combustion byproducts like fly ash represent another lithology with growing interest in REE exploration [116]. In coal, REEs are typically bound to organic matter (humic substances), or adsorbed onto clay minerals and amorphous phases such as Fe–Mn oxides. The mineralogical form is often amorphous, mixed-phase, and poorly crystalline, complicating direct identification and extraction [117,118]. Fly ash from REE-enriched coals has been shown to retain substantial concentrations of REEs, especially in the fine fractions. These REEs are associated with glassy aluminosilicates, iron oxides, and residual carbon. Although coal and fly ash are generally low-grade sources, they offer high availability and integration potential into existing energy infrastructure. Recovery strategies for such lithologies focus on oxidative leaching, ion-exchange resins, or bioleaching [80].

Drilling through coal measures or thermally altered coal seams often generates cuttings that, while less mineralogically well-defined, may present viable secondary sources of REEs if properly characterized and processed.

3.5.5. Stability of REE-Host Minerals During Drilling: Challenges and Data Gaps

The stability of REE-host minerals during drilling plays a significant role in determining how effectively these elements are preserved in cuttings and whether they remain recoverable through downstream processing. Many crystalline REE-bearing minerals such as monazite, bastnäsite, and xenotime exhibit moderate to high mechanical hardness, making them relatively stable in terms of particle survival. However, their abrasiveness poses risks to drilling tool wear and may influence the shape and fragmentation behaviour of cuttings. Conversely, ion-adsorbed REEs on clay minerals, while chemically reactive and easily leachable, are often susceptible to mechanical disaggregation, mud dispersion, or complete loss in ultrafine slurry fractions during drilling, especially in overbalanced or high-RPM conditions.

Another critical factor influencing REE liberation is grain size. Structurally bound REEs are typically hosted in fine-grained accessory minerals e.g., monazite <20 µm, which may be occluded within larger silicate matrices or locked within cemented rock fragments [119,120]. This fine grain size, coupled with the refractory nature of many host phases, necessitates fine grinding or thermal treatment to expose the REEs during processing. Additionally, weathering sensitivity varies considerably. Ion-adsorbed clays may undergo structural alteration or cation exchange upon interaction with drilling fluids, potentially modifying surface chemistry and reducing leach efficiency. Minerals like bastnäsite and allanite may partially dissolve in acidic or high-temperature conditions, leading to premature REE release into drilling mud, which risks loss and complicates recovery.

Despite the increasing interest in REEs from unconventional sources, there remains a significant gap in high-resolution mineralogical data from actual drill cuttings. Most mineralogical investigations rely on core samples or surface analogs, which do not reflect the physical and chemical alterations imposed by drilling. The aggressive environment of the borehole marked by elevated temperature, pressure, rotary motion, and fluid-rock interaction can cause mineral breakdown, phase transitions, or occlusion by filter cake components. Additionally, the fine and heterogeneous nature of cuttings often makes traditional characterization techniques such as XRD or SEM-EDX less effective unless combined with particle-size separation or targeted mineral concentration steps.

This lack of standardized mineralogical workflows for characterizing REEs in cuttings hampers accurate resource estimation, process design, and field-scale decision-making. Bridging this gap requires integrating analytical tools such as QEMSCAN, MLA, or synchrotron-based microanalysis with a sampling strategy that considers both particle size and drilling fluid chemistry. Without these improvements, opportunities for efficient REE recovery from drilling waste may remain underutilized.

4. Synthesis and Analysis of Prior Studies on REE Recovery from Drill Cuttings

The work done on REE identification and extraction from drill cuttings is still at nuance stage, but promising evidence has been collected from literature as shown in

Table 5. Summary of recent studies on REE in drill cuttings.

Research Group	Extracted Material Type	Basin/Region	Lithology Type	REE Host Phases Identified	Analytical Methods Used	Key Findings
Fowler & Zierenberg (2015) [121]	Drill cuttings and core samples	Reykjanes Peninsula, Iceland	Altered tholeiitic basalts (volcanic rocks)	Not mineral-speciated; implied REE association with clays, zeolites, and sulfates	XRF, ICP-MS	Vertical variation in REE content linked to hydrothermal alteration; LREE enrichment in altered zones
Bhattacharya et al. (2022) [122]	Core samples	Appalachian Basin (Marcellus), Haynesville Basin	Organic-rich black shale	Acid-soluble (carbonates, phosphates); minor organics and silicates	Sequential extraction, XRD, ICP-MS	Whole-rock REEs: 295–342 ppm; Haynesville higher than Marcellus; low extraction due to clay-bound REEs
Stuckman et al. (2021) [123]	Drill cuttings	Various U.S. Basins	Shale formations	Not specified	Advanced geochemical characterization techniques	Explored potential recovery of critical minerals, including REEs, from shale gas drill cuttings; emphasized sustainable recovery methods
Lopano et al. (2022) [124]	Drill cuttings	Various U.S. Basins	Shale formations	Not specified	Geochemical characterization	Developed novel treatments for optimizing drill cuttings for use as soil amendments; evaluated potential recovery of critical metals, including REEs, from waste materials across U.S. basins
Fontana et al. (2020) [18]	Drill cuttings	Offshore Brazil (Ultradeep waters)	Sedimentary rocks from offshore drilling	Not specified	Microwave-assisted acid digestion, ICP-MS, PCA, HCA	REE concentrations varied with depth; Ce, La, Nd, Sm, and Eu up to mg/kg levels; identified three sample groups based on REE composition; suggested drill cuttings as potential alternative REE source
Stuckman et al. (2019) [125]	Drill cuttings, core samples, and drilling mud	Marcellus Shale, USA	Organic-rich black shale	Not specified	Sequential chemical extraction, XRD, ICP-MS	Identified concentrations of REEs and other critical minerals; provided insights into their distribution and potential environmental impacts
Barczok et al. (2024) [126]	Drill cuttings and core samples	Various U.S. Basins	Shale formations	Not specified	Sequential extraction, ICP-MS	Identified concentrations of REEs and other critical minerals; developed a four-step sequential extraction process; demonstrated potential for converting drill cuttings into soil supplements
E-Tech Resources Inc. (2022) [127]	Drill cuttings and core samples	Eureka Project, Namibia	Carbonatite dykes	Monazite	XRF, ICP-MS	Intersected significant REE mineralization, including 8.2 m at 2.6% TREO from 83 m depth; mineralization open at depth and along strike

and later analysed in subsequent sections.

4.1. Geographical Trends in REE in Drilling Cutting Related Studies

The current landscape of REE research in drill cuttings reveals a distinct geographical concentration, with the United States leading the field, accounting for over 60% of documented studies (Figure 3). This dominance reflects active national interest in critical mineral recovery from unconventional sources, driven by the strategic focus of institutions like the National Energy Technology Laboratory (NETL). The U.S.-based studies typically emphasize shale formations such as the Marcellus and Haynesville, leveraging drill cuttings from hydrocarbon basins to evaluate REE potential. In contrast, Iceland and Brazil each contribute singular but noteworthy studies; one investigating hydrothermally altered volcanic rocks in the Reykjanes Peninsula, and the other exploring REEs in offshore sedimentary rocks in ultra-deepwater basins. Namibia, represented by a diamond drilling study from the Eureka carbonatite project, highlights the emerging role of African carbonatite-hosted systems in REE exploration. Overall, the geographic distribution reflects both regional geology and institutional priorities, with the U.S. showing the most comprehensive integration of REE recovery into existing energy exploration frameworks.

4.2. Extraction and Characterization Techniques

Figure 4 summarizes the frequency of analytical techniques used across studies focusing on REE recovery from drill cuttings. Among characterization techniques, ICP-MS (Inductively Coupled Plasma–Mass Spectrometry) stands out as the most widely applied. This prominence is unsurprising given ICP-MS’s exceptional sensitivity and capability for precise trace-level quantification of REEs, which are often present in low concentrations within complex matrices like drill cuttings.

In contrast, mid-range usage and usually accompanied with ICP-MS techniques are observed for XRF (X-ray Fluorescence) and XRD (X-ray Diffraction), both used in two studies. XRF offers rapid bulk elemental analysis, while XRD helps identify mineral phases both valuable but sometimes limited in resolution or specificity for REEs. Notably, multivariate statistical methods such as PCA (Principal Component Analysis) and HCA (Hierarchical Cluster Analysis) appear in only one instance, despite their potential to uncover hidden geochemical trends and correlations. Likewise, advanced geochemical characterization (AGC) techniques are minimally represented, possibly due to inconsistent terminology or limited adoption in REE-specific workflows.

The inset table illustrates the frequency of extraction techniques used. Sequential extraction, including chemical variants, is the most frequently employed method (three studies), highlighting its usefulness in partitioning REEs based on binding strength and mineral association. Microwave-assisted acid digestion, though effective in enhancing leaching kinetics and efficiency, appears only once likely due to equipment accessibility issues or a lack of standardization in methodology when applied to drill cuttings.

This analysis suggests a methodological skew toward high-sensitivity elemental analysis over mineralogically targeted strategies. While ICP-MS remains indispensable, the underutilization of complementary tools such as PCA-HCA, microwave-assisted leaching, and mineral-phase-guided extraction workflows indicate a gap in integrated, host-phase-informed approaches. Bridging this gap aligns with the decision-tree framework proposed in this review and supports a more informed, efficient pathway for REE recovery from unconventional sources.

4.3. Type of Drilling Cutting Studied

The lithological analysis of REE-related studies in drill cuttings reveals a strong dominance of shale and organic-rich shale formations, which account for over 60% of the total studies (Figure 5). This trend reflects the growing recognition of shales as viable repositories of rare earth elements, particularly due to their fine-grained nature, high surface area, and complex geochemistry that favours REE adsorption and incorporation. Other lithologies such as sedimentary offshore rocks, altered volcanic basalts, and carbonatite dykes are less frequently studied but represent important niches with distinct mineralogical characteristics. The inclusion of carbonatite-hosted systems, for instance, highlights their economic relevance due to the presence of monazite as a primary REE host. These lithological patterns underline the need for tailored extraction and characterization techniques, which will be critically examined in the following sections.

4.4. Occurrence of REEs in Drilling Cuttings

The occurrence of individual REEs across different lithologies shows clear patterns shaped by both mineralogical host phases and geochemical environments. In shale and organic-rich formations, REEs such as La, Ce, Nd, Sm, Dy, and Y are commonly reported, often adsorbed onto clay surfaces or associated with organic matter and phosphates as depicted in

Table 6. Presence of REEs in drill cuttings.

* Lithology	La	Ce	Nd	Pr	Sm	Eu	Dy	Y	Tb	Gd
Shale/Organic-rich shale
Sedimentary rocks (offshore)
Carbonatite dykes
Altered volcanic rocks

* The presence of REEs listed in this table is based on representative findings from literature and project-specific studies. Their occurrence may vary depending on regional geological settings, mineralization styles, and diagenetic conditions. This table reflects typical trends rather than universal distribution across all global occurrences of each lithology.

. These formations tend to favour heavy REE (HREE) retention under reducing conditions, though light REEs (LREEs) are generally more abundant. Offshore sedimentary rocks, while less studied, exhibit moderate concentrations of LREEs, particularly Ce, La, and Nd, likely due to diagenetic enrichment and input from detrital phases. In contrast, carbonatite-hosted systems such as those intersected in the Eureka Project demonstrate the most comprehensive REE spectrum, including both LREEs and HREEs (e.g., Pr, Dy, Tb, Gd, and Y), primarily bound within monazite and other phosphate minerals. Altered volcanic lithologies like hydrothermally influenced basalts show modest REE enrichment, dominated by Ce, La, and Nd, often linked to secondary minerals such as zeolites and clays. These lithology-specific trends not only dictate the choice of extraction strategies but also impact the economic viability of drill cuttings as REE resources.

4.5. Host Minerals vs. Lithology

The distribution of REE-hosting minerals varies significantly across lithologies, reflecting differences in depositional environments, diagenetic processes, and alteration history. In shale and organic-rich formations, REEs are predominantly hosted in ion-adsorbed clays, organic matter, and poorly crystalline Fe–Mn oxides as supported by

Table 7. Drill cuttings vs. host mineral.

Lithology	Ion-Adsorbed Clays	Monazite	Xenotime	Organic Matter	Fe-Mn Oxides	Apatite	Zeolites Amorphous
Shale/Organic-rich shale		/minor				/minor
Sedimentary rocks (offshore)	/moderate	/trace		maybe
Carbonatite dykes
Altered volcanic rocks	/weak	maybe			/weak

. These phases favour surface sorption and complexation, making the REEs more mobile and potentially recoverable via mild leaching approaches. Zeolitic and amorphous aluminosilicates also appear in thermally altered shales and contribute to low-grade but accessible REE pools. In contrast, offshore sedimentary rocks though underexplored reveal REE associations with Fe–Mn oxides and detrital clays, suggesting secondary enrichment rather than primary mineralization. The most mineralogically distinct lithology is that of carbonatite dykes, where REEs occur almost exclusively within monazite, xenotime, and apatite, all of which are structurally bound and require aggressive chemical or thermal treatments for liberation. Altered volcanic rocks, such as hydrothermally influenced basalts, often retain REEs in secondary clays and zeolitic phases, though the concentrations are typically lower and more diffuse. These host-mineral associations are not only pivotal in determining REE accessibility but also dictate the choice of extraction reagents, processing severity, and environmental impact.

4.6. Reliance on Core Samples vs. Drill Cuttings: Implications for REE Recovery

Historically, many studies investigating the mineralogical occurrence and extractability of REEs have relied on core samples due to their intact stratigraphy, minimal contamination, and preserved texture. For instance, Bhattacharya et al. in 2021 and Lopano et al. in 2020 (Table 5) utilized core materials to assess REE enrichment in shale formations, achieving detailed mineralogical characterization via SEM, XRD, and sequential leaching protocols. While such studies have undeniably advanced our understanding of REE host phases, their applicability to drill cuttings remains limited.

Core samples offer idealized conditions that are not representative of the heterogeneous, fragmented, and drilling-fluid-affected nature of cuttings. In contrast, drill cuttings are often subjected to mechanical disintegration, which alters grain size and exposes new mineral surfaces; drilling fluid contamination, which can lead to cation exchange, adsorption-desorption shifts, and pH buffering; sampling loss or depth-mixing, reducing spatial resolution and affecting stratigraphic accuracy. Recent investigations (e.g., Barczok et al. in 2023; Stuckman et al. in 2021) have attempted to bridge this gap by directly using drill cuttings to evaluate REE leachability. However, even in these cases, limited attention is given to how mineralogical signatures evolve during drilling or how the interaction with drilling fluids modifies REE availability. For example, Stuckman et al. reported REE recovery efficiencies of only 9%–22%, which may reflect both the loss of structurally bound REEs and the misalignment between leaching strategy and actual mineral hosts in drill cuttings. Moreover, the mineralogical fidelity of drill cuttings is rarely validated prior to leaching trials. Without phase-specific confirmation (e.g., clay vs. phosphate vs. oxide-bound), bulk REE concentrations can be misleading in terms of economic potential. This underscores the importance of integrating mineralogical fingerprinting techniques (SEM-EDS, EPMA, synchrotron XRF) before designing extraction pathways.

In practice, drill cuttings are more accessible, especially during active exploration campaigns, and represent a more scalable and sustainable feedstock for REE recovery particularly when real-time data is needed. However, current literature underutilizes this resource, and few studies have adopted an integrated approach that considers both the mineralogical integrity and operational constraints of using drill cuttings.

4.7. Limitations in Analytical Techniques Used in Previous Studies

Despite the increasing interest in REE recovery from subsurface materials, the analytical strategies employed in many prior studies suffer from significant limitations that undermine the interpretability and applicability of their findings particularly when applied to drill cuttings. Most investigations have relied almost exclusively on bulk geochemical assays, such as ICP-OES or ICP-MS, to determine total REE concentrations. While these methods provide precise quantitative data, they are inherently blind to the mineralogical context of the REEs. They do not distinguish whether REEs are structurally integrated within mineral lattices, loosely adsorbed onto particle surfaces, or organically complex distinctions that are critical for predicting leachability and designing appropriate extraction methods.

This methodological gap has led to a concerning trend in the literature: overinterpretation of bulk REE concentrations as indicative of extractable potential. In reality, high total REE content does not necessarily equate to high recovery efficiency. Without concurrent mineralogical or surface-sensitive analyses, there is no way to determine whether the REEs measured are even accessible to leaching reagents. As a result, studies frequently report low recovery yields without identifying the fundamental causes—whether due to reagent mismatch, drilling-induced alteration, or mineral encapsulation.

Compounding this issue is the tendency of some studies to apply uniform leaching strategies without accounting for the heterogeneous nature of REE host phases. For example, REEs hosted in resistant phosphate minerals such as monazite or xenotime require entirely different treatment than those loosely bound to clays or iron-manganese oxides. Yet, few studies employ diagnostic extractions or selective dissolution protocols to differentiate these forms. Even when such attempts are made, they often lack mineralogical verification. Techniques such as SEM-EDS, EPMA, or synchrotron XRF which could provide spatially resolved data on REE associations—are either underutilized or treated as secondary to leachate chemistry. This analytical imbalance perpetuates assumptions about REE behavior and inhibits progress toward a mechanistic understanding of their mobility in drill cuttings.

Moreover, an overlooked but crucial factor is the alteration of mineral surfaces during the drilling process itself. Drilling fluids can introduce chemical species that modify surface charges, induce secondary mineral formation, or even displace loosely bound REEs; all of which directly affect subsequent leaching behavior. However, these interactions are rarely studied using appropriate tools. Surface-sensitive techniques such as XPS or FTIR, which could detect chemical speciation and surface bonding environments, are virtually absent in the reviewed literature. The consequence is a systematic underestimation of drilling-induced artifacts and a skewed picture of REE availability.

4.8. Oversights in Existing Leaching Approaches for REEs from Drill Cuttings

Leaching remains the most commonly investigated pathway for REE recovery from drill cuttings and related geological substrates [128]. However, a critical examination of the existing literature reveals a pattern of methodological oversights, particularly in the mismatch between leaching strategies and the actual mineralogical binding states of REEs. Most studies adopt generic acid leaching protocols predominantly using hydrochloric acid (HCl), nitric acid (HNO₃), or sulfuric acid (H₂SO₄) without first establishing whether the REEs of interest are surface-adsorbed, organically bound, or structurally integrated into resistant mineral lattices such as monazite or xenotime [85].

This indiscriminate application of leaching reagents often leads to underwhelming recovery efficiencies, especially when attempting to extract REEs from highly crystalline or phosphate-rich phases. For instance, studies that report low extraction yields seldom interrogate whether the leachant used was chemically or kinetically suitable for the mineral host phase. This reflects a broader issue: REE extraction trials are frequently decoupled from prior mineralogical characterization. The result is that extraction inefficiencies are attributed to low REE content or fluid limitations rather than to a poor understanding of phase-specific dissolution behavior.

Furthermore, sequential extraction methods while more refined in concept are often applied without rigorous validation [129]. Many such protocols, adapted from soil science or environmental geochemistry, assume idealized partitioning of REEs into “exchangeable”, “carbonate”, or “residual” phases. Yet, in drill cuttings, where mechanical grinding, fluid contamination, and thermal gradients affect mineral integrity, these assumptions rarely hold. Very few studies calibrate their sequential leaching steps against mineralogical data to verify whether each fraction truly corresponds to a distinct host phase. As a result, the apparent selectivity of these methods may be more notional than real.

Another underexplored dimension is the role of drilling fluids in modifying the surface chemistry and extractability of REEs. In real drilling environments, water-based muds (WBMs), oil-based muds (OBMs), and synthetic-based muds (SBMs) introduce additives that interact with mineral surfaces through adsorption, complexation, or secondary mineral precipitation [130,131]. Yet, most leaching studies either neglect to mention the type of drilling fluid used or fail to assess its impact on REE mobility. This represents a significant blind spot, given that fluid residues can mask active adsorption sites or displace loosely bound REEs prior to laboratory extraction.

Equally concerning is the lack of innovation in leaching reagent design. Despite growing environmental concerns, few studies have explored alternatives to strong mineral acids. The use of biodegradable organic acids (e.g., citric, gluconic), chelating agents (e.g., EDTA, NTA), or novel green solvents such as deep eutectic solvents (DES) remains extremely limited [132,133]. This suggests a field that remains anchored in traditional metallurgical paradigms, with little emphasis on eco-efficiency, selectivity, or downstream separation considerations. Where newer approaches are trialed, they are often poorly optimized or not benchmarked against conventional methods, making it difficult to evaluate their true performance or scalability.

Finally, most extraction studies remain confined to bench-scale conditions with limited parametric variation. Critical factors such as temperature, leaching time, solid-to-liquid ratio, reagent concentration, and kinetic modeling are either fixed or insufficiently reported [134,135]. Without systematic optimization or kinetic insight, it is challenging to determine whether low REE recoveries stem from equilibrium limitations, diffusion barriers, or surface passivation. Moreover, the absence of scale-up studies or pilot trials further distances laboratory findings from industrial applicability, especially in the context of high-throughput drill cuttings generated during real drilling campaigns.

4.9. Drilling Fluids and Their Influence on REE-Bearing Minerals

Drilling fluids play a crucial yet underexplored role in the context of REE recovery from drill cuttings [136]. While the primary function of drilling muds has traditionally been wellbore stabilization, lubrication, and cuttings transport, their interactions with REE-bearing minerals may significantly influence the retention or loss of REEs. Case studies from REE-rich formations in southern China (ion-adsorption clays), the Appalachian Basin (USA) (black shales), and Madagascar (carbonatite-hosted monazite) reveal that little consideration is given to the geochemical sensitivity of REE host phases during drilling operations. This oversight poses a risk to both accurate resource estimation and downstream recovery potential.

4.9.1. Fluid–Mineral Interactions in REE-Rich Formations

The chemical composition of drilling fluids especially pH and ionic strength can induce mineralogical transformations that alter REE mobility [137,138]. Weakly bound REEs, such as those adsorbed on clays or incorporated into amorphous iron oxides, are particularly vulnerable to changes in pH. For instance, alkaline muds can desorb LREEs like La and Ce from clay surfaces, while acidic filtrates can dissolve phosphate-hosted REEs [139,140]. Similarly, high ionic strength fluids can trigger cation exchange, displacing REEs in ion-adsorbed clays and enhancing their leachability into the fluid phase (Table 8).

4.9.2. Mechanical Dispersion and Particle Loss

Beyond chemical effects, mechanical dispersion during drilling may also influence REE retention [141]. Ultrafine particles, which often carry a high concentration of surface-adsorbed REEs, are particularly susceptible to being flushed out with drilling fluids. These particles may bypass solids control systems such as shale shakers and hydrocyclones due to their colloidal or sub-micron size, resulting in the inadvertent loss of REE-enriched fractions. Such losses are particularly critical in lithologies where REEs are predominantly associated with loosely bound or ion-adsorbed phases, as observed in weathered granitic terrains and clay-rich formations. For example, studies from ion-adsorption clay deposits in China have reported the presence of REEs in mud filtrate returns, suggesting the mobilization of REE-bearing fines during circulation. However, these observations remain largely anecdotal, as most drilling operations lack systematic protocols for monitoring REEs in the liquid phase or filtrate waste streams. This represents a critical gap in current REE recovery strategies from drill cuttings, where both chemical leachability and physical losses must be jointly considered. Future studies should incorporate particle size distribution analyses and closed-loop fluid monitoring to more accurately account for REE losses during drilling and to develop mitigation strategies that enhance overall recovery potential [142].

4.9.3. REE Leaching into Fluids vs. Preservation in Cuttings

There exists a delicate and formation-dependent balance between the retention and loss of REEs during drilling particularly between their potential leaching into the drilling fluid and their preservation within the solid drill cuttings. In hard, crystalline formations such as carbonatites, where REEs are primarily hosted in structurally bound phases like monazite or bastnäsite, the risk of significant chemical dissolution during drilling is comparatively low. However, even in these settings, the interaction with drilling fluids may alter the surface chemistry of REE-bearing minerals, potentially forming passivation layers or modifying surface charge, which can subsequently hinder or enhance leaching efficiency during downstream recovery processes.

In contrast, sedimentary and fine-grained shale formations present a more vulnerable matrix. REEs in these lithologies are often associated with clay minerals, organic matter, or loosely adsorbed to particle surfaces, making them susceptible not only to chemical dissolution under the influence of drilling fluids (especially if fluids are acidic, chelating, or thermally active) but also to mechanical entrainment. The latter is particularly relevant for ion-exchangeable REEs or those associated with ultrafine particles, which may be lost to the circulating fluid system through dispersion or bypassing of surface solids control units.

This dual vulnerability, i.e., chemical and physical; underscores the importance of selecting drilling fluid systems and solids handling protocols that minimize REE mobilization and loss, especially when targeting REE-rich zones for recovery. Moreover, pre-screening lithology and mineral host phases can aid in designing fluid compositions that balance drilling performance with REE preservation. Thus, lithology-specific interactions between REEs and drilling operations must be carefully considered to avoid underestimating the true REE content of cuttings or compromising their economic viability as a secondary resource [14]. Thus, cutting preservation is not merely a function of recovery efficiency but also of fluid chemistry and particle retention.

Table 8. Effect of various aspects of drilling mud on REEs’ extraction.

Research Group	Aspect	Impact on REEs Extraction
Bamforth et al. (2024) [143]	pH Effect	Altered pH can destabilize REE-hosting minerals like monazite and apatite, leading to REE mobilization or deposition.
Madruga et al. (2018) [144]	Ionic Strength	Increased ionic strength induces cation exchange, mobilizing surface-adsorbed REEs from clay minerals.
Morariu et al. (2022) [145]	Clay Dispersion	Enhanced clay dispersion increases fine particle suspension and it can be deduced that REE-rich fines through solids control systems.
Fontana et al. (2020) [18]	Physical Particle Loss	Mechanical dispersion can entrain REE-hosting particles, reducing solid-phase REE yield in cuttings.
Rasool et al., 2024 [146]	Green Fluids (NADES/ILs)	Based on properties of NADES, REE can extracted in-situ with proper modification.
Fontana et al. (2020) [18]	REE Leaching into Fluid	Loosely bound REEs are susceptible to leaching into drilling fluids, though this phenomenon is rarely quantified in field studies.
Lopano et al., 2022 [124]	REE Preservation in Cuttings	Preservation of REEs in cuttings is favored in minerals like monazite; however, drilling fluid chemistry significantly influences this retention.

Table 8. Effect of various aspects of drilling mud on REEs’ extraction.

Research Group	Aspect	Impact on REEs Extraction
Bamforth et al. (2024) [143]	pH Effect	Altered pH can destabilize REE-hosting minerals like monazite and apatite, leading to REE mobilization or deposition.
Madruga et al. (2018) [144]	Ionic Strength	Increased ionic strength induces cation exchange, mobilizing surface-adsorbed REEs from clay minerals.
Morariu et al. (2022) [145]	Clay Dispersion	Enhanced clay dispersion increases fine particle suspension and it can be deduced that REE-rich fines through solids control systems.
Fontana et al. (2020) [18]	Physical Particle Loss	Mechanical dispersion can entrain REE-hosting particles, reducing solid-phase REE yield in cuttings.
Rasool et al., 2024 [146]	Green Fluids (NADES/ILs)	Based on properties of NADES, REE can extracted in-situ with proper modification.
Fontana et al. (2020) [18]	REE Leaching into Fluid	Loosely bound REEs are susceptible to leaching into drilling fluids, though this phenomenon is rarely quantified in field studies.
Lopano et al., 2022 [124]	REE Preservation in Cuttings	Preservation of REEs in cuttings is favored in minerals like monazite; however, drilling fluid chemistry significantly influences this retention.

5. Leaching and Characterization Techniques of REEs: Mineralogy Based Overview

Effective REE recovery from drill cuttings hinges on aligning extraction methods with the specific mineralogical host phases of the REEs. However, many past studies overlook this critical link, applying generic protocols without considering whether REEs are adsorbed, structurally bound, or organically complexed. Characterization efforts are similarly limited, often relying on bulk assays that fail to distinguish extractable from non-extractable forms. A mineralogy-informed approach is therefore essential, integrating both targeted extraction and high-resolution analytical tools to accurately evaluate REE potential in drill cutting matrices. Significant leaching techniques and their association with mineral host phase has been discussed in the subsequent section.

5.1. Acid Leaching

Among conventional approaches, mineral acid leaching using hydrochloric acid (HCl) or nitric acid (HNO₃) remains the most widely applied method for mobilizing REEs, especially from phosphate-rich phases such as monazite, xenotime, or apatite. These strong acids are effective in breaking down crystalline lattices, but they also pose significant environmental risks, require corrosion-resistant infrastructure, and generate large volumes of acidic waste [147]. In response to these challenges, milder organic acids such as citric acid have been explored as greener alternatives, particularly for recovering REEs that are ion-adsorbed on clay minerals. However, these organic systems are less effective when REEs are locked within more stable mineral matrices [34,128,141,148].

5.2. Chemical Sequential Extraction

Chemical sequential extraction has emerged as an insightful technique not only for partial recovery but also for understanding the binding environment of REEs within drill cuttings. By progressively applying reagents that target specific mineral fractions such as exchangeable ions, carbonates, oxides, and silicates this method provides both analytical and preparative value [149]. While often used for speciation studies, sequential leaching can also be scaled for recovery, particularly when REEs are loosely held in reactive phases. Nevertheless, its operational complexity and time requirements limit its scalability in industrial applications [150,151,152].

5.3. Microwave-Assisted Leaching

Microwave-assisted acid leaching represents another promising route, especially when coupled with mineral acids or chelating agents. The application of microwave energy enhances reaction kinetics, improves mineral breakage at the microstructural level, and allows for shorter leaching durations at lower reagent concentrations [153,154,155]. This technique has shown increased efficiency for recovering REEs from refractory and crystalline phases, such as in offshore sedimentary drill cuttings. Microwave energy interacts preferentially with minerals that exhibit high dielectric loss, such as iron oxides, clays, and carbonaceous matter, leading to selective and localized heating. This makes MAL particularly effective for REEs that are ionically adsorbed on clay surfaces or associated with amorphous Fe-Mn oxides, as the microwaves enhance desorption and increase surface reactivity. In contrast, REEs structurally incorporated within thermally stable, crystalline phosphate minerals like monazite or xenotime show limited responsiveness to microwave treatment alone due to their poor microwave absorption and low dielectric properties. In such cases, MAL must be coupled with chemical additives or oxidants to induce lattice breakdown. Despite its potential, very few studies have systematically evaluated MAL performance across different REE host phases in drill cuttings. This lack of mineralogical targeting often results in inconsistent or modest recovery gains [156].

5.4. Bioleaching

Bioleaching presents a more sustainable yet mechanistically constrained alternative. Utilizing acidophilic bacteria and certain fungi, REEs can be solubilized from both oxide-bound and organically complexed phases through the production of organic acids, siderophores, and chelating metabolites [157,158]. The mechanism relies on microbial action either through direct enzymatic attack or indirect acid production to solubilize REEs. This makes bioleaching particularly suitable for REEs associated with ion-exchangeable phases, adsorbed onto clays, or loosely bound to amorphous iron or manganese oxides, where microbial acids or metabolites can effectively mobilize REEs without requiring aggressive chemical conditions. However, its application becomes severely limited when REEs are locked within crystalline lattices of phosphate or silicate minerals, such as monazite or allanite, which are largely resistant to microbial dissolution. Moreover, drill cuttings present an additional challenge due to their heterogeneous composition, fluid contamination, and possible biocidal additives from drilling muds, which may inhibit microbial growth or activity. Despite these limitations, bioleaching remains underexplored for drill cuttings, and the few available studies rarely tailor microbial consortia or operational parameters to the mineralogical form of REE. While this method aligns with environmental and energy goals, it suffers from slow kinetics, sensitivity to environmental conditions, and limited control over selectivity. Moreover, in drill cuttings where pH buffering and toxic metals may be present, microbial activity can be inhibited, making bioleaching a less predictable strategy [159,160,161].

5.5. Hybrid and Green Approaches

Several emerging hybrid and green methods are under exploration, combining physical pretreatments such as grinding or ultrasonication with mild leaching agents to overcome limitations of accessibility and kinetics. Microwave-assisted systems, chelator-based aqueous leaching, and polymer-stabilized solvents fall under this category. While they demonstrate promising recovery in laboratory conditions, questions of scale, fluid recyclability, and solid-liquid separation continue to limit their industrial readiness [41,162,163].

Ultimately, the choice of extraction technique must be dictated by mineralogical context. Ion-adsorbed REEs in clays are amenable to mild or organic leaching, whereas REEs embedded in phosphate or silicate matrices demand more aggressive or thermally assisted methods. Moreover, operational considerations such as sludge formation, filterability, and energy inputs must be integrated into method selection frameworks. A critical gap persists in the absence of workflows that couple mineralogical assessment with tailored extraction schemes highlighting the need for integrative, adaptable approaches in future REE recovery from drill cuttings [164].

After leaching, the accurate identification and quantification of REEs after leaching is fundamental to evaluating the efficiency, selectivity, and viability of extraction methods applied to drill cuttings. Post-leaching characterization not only informs on total REE recovery but also provides insights into mineralogical selectivity, residual phase persistence, and potential reagent–mineral interactions. Given the mineralogical heterogeneity and trace-level concentrations typical of drill cuttings, a robust characterization framework must integrate both quantitative and spatially resolved techniques. Various characterization techniques for REE quantification and identification have been listed in subsequent sub-sections.

5.6. ICP-MS: The Benchmark for Quantification

ICP-MS is the most widely adopted technique for leachate analysis due to its ultra-trace detection capabilities, multi-element coverage, and rapid throughput [165,166,167]. It provides precise REE concentration data across the full lanthanide series, allowing for the evaluation of total recovery and leaching selectivity [168,169]. ICP-MS remains the analytical workhorse for determining trace concentrations of rare earth elements (REEs) in geological materials, including drill cuttings, due to its exceptional sensitivity, broad elemental range, and low detection limits (sub-ppb levels). In the context of drill cuttings, ICP-MS is typically used to quantify total REE concentrations in acid-digested samples, providing a first-pass estimate of REE abundance. However, while this technique is powerful for elemental quantification, it offers no information about the mineralogical form or phase association of the REEs. Consequently, ICP-MS data alone cannot distinguish whether REEs are ionically adsorbed on clay surfaces, bound to organics, or locked within resistant mineral lattices information that is critical for predicting extraction behavior. Additionally, the sample preparation usually involves complete digestion with strong acids destroys any information about host phase relationships. In drill cuttings, where heterogeneity and drilling fluid contamination are common, such total digestion approaches may further obscure meaningful distinctions between extractable and non-extractable REE fractions. Moreover, matrix effects from residual drilling fluids, clays, or salts can introduce interferences and require careful calibration and internal standardization to maintain accuracy. While ICP-MS is indispensable for baseline REE quantification, it must be coupled with phase-resolved techniques such as SEM-EDS, LA-ICP-MS, or sequential leaching to gain a comprehensive understanding of REE occurrence and recovery potential in drill cuttings [170,171,172].

5.7. ICP-OES and AAS: Less Sensitive Alternatives

While ICP-OES (Inductively Coupled Plasma—Optical Emission Spectroscopy) presents a cost-effective option for analyzing higher-concentration samples, its typical detection limits in the low ppm range may limit its suitability for dilute leachates. Both ICP-OES and Atomic Absorption Spectroscopy (AAS) have long-standing roles in elemental analysis within geochemical studies; however, their application in REE quantification particularly within the complex matrices of drill cuttings can pose challenges. ICP-OES is valued for its multi-element detection capabilities and operational efficiency, yet its detection limits for REEs are generally higher than those of ICP-MS, which can restrict its utility in tracing low-abundance REEs or detecting subtle enrichment trends. Additionally, spectral interferences and overlapping emission lines may arise in complex matrices such as drill cuttings, where coexisting elements and residual drilling additives can affect measurement accuracy.

AAS, while robust for targeted elemental analysis, is typically limited to single-element detection and offers relatively higher detection limits, making it less practical for comprehensive REE profiling. Furthermore, both techniques involve total digestion, which removes mineralogical context and does not provide insight into the binding state or host phase of REEs. Nevertheless, in scenarios involving high-grade or simplified matrices, ICP-OES and AAS may still serve as economical tools for preliminary screening or supporting analysis, particularly when integrated with more sensitive or phase-specific techniques such as ICP-MS or LA-ICP-MS [167,173,174]. AAS is rarely used in modern REE workflows due to its single-element limitation and moderate detection limits, although it can be useful for select REEs in simplified matrices.

5.8. XRD and SEM-EDS: Residual Solid Phase Analysis

The XRD and SEM-EDS are among the most widely employed techniques for residual solid-phase analysis in geochemical studies, offering essential insights into the mineralogical context of REE-bearing substrates. In the case of drill cuttings, these techniques are particularly valuable for evaluating the post-leaching residue, helping to determine which REE-hosting phases persist after extraction attempts and whether they contribute to incomplete recovery. XRD is effective at identifying crystalline phases such as monazite, xenotime, zircon, and bastnäsite that are commonly resistant to acid leaching While XRD has also been used to analyze paracrystalline minerals, such as imogolite, an aluminosilicate [175] and amorphous phases, such as amorphous silica [176], Fe-Mn oxides [177], and organo-metallic complexes [178], its lower limit of detection is often less than satisfactory.

SEM-EDS, on the other hand, offers spatial resolution and elemental identification at the micron scale, enabling direct observation of REE-bearing mineral grains and their association with other matrix components. Yet, it is largely qualitative and may struggle to detect REEs due to low atomic concentration and overlapping spectral lines, especially in samples with heavy contamination or fine particle size. More critically, both XRD and SEM-EDS are often underutilized in drill cuttings studies, where bulk leaching is prioritized, and solid-phase characterization is treated as an afterthought. This results in a fragmented understanding of REE phase persistence and undermines efforts to correlate extraction efficiency with mineralogical behavior. To improve the predictability and design of REE recovery processes, solid-phase analyses like XRD and SEM-EDS should be integrated as standard post-leaching diagnostic tools [179,180,181,182].

5.9. PCA and HCA: Multivariate Interpretation Tools

Given the complex geochemical behavior of REEs, multivariate statistical techniques like PCA and HCA have emerged as valuable tools for interpreting leachate composition data [183]. PCA and HCA offer powerful statistical frameworks for uncovering hidden patterns in complex REE datasets derived from drill cuttings. Given the geochemical and mineralogical heterogeneity of drill cuttings exacerbated by drilling fluid contamination, stratigraphic mixing, and variable REE host phases univariate approaches often fail to capture meaningful relationships between elements, phases, and extraction behavior. PCA enables dimensionality reduction by transforming correlated REE variables into uncorrelated principal components, thereby revealing underlying geochemical signatures, associations between REEs and matrix elements, or trends linked to specific lithologies or fluid interactions. HCA complements this by grouping samples or variables based on similarity, which can help classify REE-enriched zones, identify cuttings affected by specific drilling fluids, or cluster REEs by likely host phase affinity (e.g., clay-associated LREEs vs. phosphate-bound HREEs). However, despite their potential, multivariate techniques remain underutilized in the REE-from-cuttings literature. Most studies rely solely on elemental concentrations without leveraging statistical tools to interpret inter-element relationships, assess variability across depth or lithology, or trace the influence of process parameters on extraction efficiency. Moreover, PCA and HCA are often applied superficially without prior geochemical rationale or post-hoc validation via mineralogical data, leading to misleading classifications or overfitting. When integrated thoughtfully with phase-resolved data, these tools can provide a deeper interpretive layer, enabling more targeted leaching strategies and improved decision-making frameworks for REE recovery [184,185].

5.10. Sequential Extraction and Speciation Studies

Sequential extraction protocols, often adapted from environmental geochemistry, allow researchers to assess REE partitioning across operationally defined phases such as exchangeable, carbonate-bound, oxide-associated, or residual fractions [186]. Sequential extraction procedures have been employed in REE studies to operationally define the chemical speciation of elements across various binding phases such as exchangeable, carbonate-bound, oxide-associated, organic-bound, and residual fractions. These methods offer a valuable, though indirect, approach to assess the relative mobility and leachability of REEs in complex substrates like drill cuttings. However, their application in this context is often methodologically unrefined and poorly validated, particularly given the unique challenges that drill cuttings present. Many sequential protocols are adapted from soil science or environmental geochemistry and assume phase boundaries that may not exist in high-temperature, mechanically altered, and fluid-contaminated drill cutting matrices. Moreover, these studies typically lack mineralogical confirmation (e.g., via SEM-EDS or XRD) of the phases being targeted at each extraction step, making it difficult to ensure that the operationally defined fractions correspond to actual REE host phases such as monazite, clay minerals, or Fe-Mn oxides.

In some cases, the extraction reagents themselves often designed for soft or oxidizable phases may be too weak or chemically mismatched for the robust mineral matrices typical of drill cuttings, leading to underrepresentation of structurally bound REEs. Conversely, overly aggressive reagents can cause leaching across multiple phases, resulting in fraction overlap and loss of interpretive resolution. Additionally, the cumulative errors in each step, potential readsorption, and the effect of residual drilling fluid compounds are rarely addressed. As a result, while sequential extraction can provide broad trends on REE mobility, it is not a definitive tool for mechanistic understanding unless tightly coupled with mineralogical and surface chemistry analyses. To truly inform selective recovery strategies, speciation studies must evolve beyond generalized protocols and move toward phase-validated, cuttings-specific workflows that recognize the complex interplay between mineralogy, surface reactivity, and process-induced alteration [150,187].

5.11. Solid–Liquid Partitioning and Mass Balance Considerations

Achieving an accurate mass balance between liquid and solid fractions is essential to validating recovery efficiency. ICP-based quantification of leachates must be complemented with solid-phase mineralogical characterization to confirm whether non-recovered REEs are retained in recalcitrant phases or were lost due to experimental error. Incomplete separation, colloidal carryover, or post-leaching precipitation can all introduce uncertainties. Integrated analysis using multiple techniques allows for the most robust assessment of overall process performance [188,189].

Understanding solid–liquid partitioning is central to evaluating REE extraction efficiency, yet this aspect remains poorly addressed in most studies involving drill cuttings. Many investigations report leaching yields solely as percentages of total REE content without quantifying or characterizing the residual solid fraction, leading to an incomplete or even misleading assessment of process effectiveness. This lack of attention to mass balance closure not only obscures where REEs remain post-extraction—adsorbed, precipitated, or occluded but also prevents proper identification of process limitations such as surface passivation, secondary precipitation, or incomplete dissolution of resistant phases. Drill cuttings, in particular, present a complex matrix where such partitioning behavior can be highly variable due to fine particle size, phase intergrowth, and fluid contamination. Additionally, studies rarely differentiate between genuinely solubilized REEs and those present in colloidal or suspended particulate form in the leachate, which can lead to overestimated extraction efficiencies if not properly filtered or stabilized before ICP analysis.

Furthermore, without comprehensive solid–liquid mass balance calculations, it becomes impossible to optimize reagent dosages, solid-to-liquid ratios, or residence times for scale-up. Many studies fail to track REEs lost through side reactions, adsorption onto reactor surfaces, or entrained in precipitated gangue phases, resulting in mass deficits that compromise the credibility of their findings. This oversight is particularly critical when aiming to transition from laboratory testing to process design for field implementation. A rigorous, mineralogically-informed mass balance supported by both leachate analysis and residual solid characterization (e.g., via XRD, SEM-EDS, or EPMA) is essential to accurately interpret partitioning behavior, identify extraction bottlenecks, and refine the thermodynamic or kinetic models underlying REE recovery from drill cuttings [190].

6. Potential for Real-Time or Near-Drill Site REE Assessment

6.1. Shifting Toward On-Site Critical Element Screening

With growing interest in drill cuttings as a secondary source of REEs, the potential for real-time or near-wellbore REE assessment is gaining relevance. Traditionally, the evaluation of REEs has been conducted in centralized laboratories using high-resolution geochemical methods. However, the ability to assess REE content and distribution during drilling either through mud-logging systems or rapid on-site analysis could transform current workflows by enabling dynamic decision-making, stratigraphic targeting, and more efficient resource characterization.

6.2. Mud-Logging as a Screening Tool for REEs

Mud-logging, a standard component of hydrocarbon exploration, is increasingly being explored for broader geochemical monitoring, including critical metals [191,192]. While historically used to track hydrocarbons and gas anomalies, its infrastructure and workflows could be adapted to assess elemental trends, including REEs. The primary limitation is that conventional mud-logging does not involve the level of analytical precision required to detect REEs at typical crustal abundances. Nonetheless, developments in geochemical sensors and portable detection tools may allow integration of REE screening protocols into mud-logging setups. For example, pre-concentration modules or ion-exchange filtration units may enable the accumulation of REEs from mud filtrates, facilitating downstream detection.

6.3. Portable Spectroscopy Tools: XRF, LIBS, and ICP-OES

Several portable or semi-portable analytical techniques are being evaluated for near-rig REE analysis. Handheld XRF has seen increasing use in elemental mapping of drill cuttings and cores due to its non-destructive nature and rapid output. However, XRF is limited in its sensitivity to lighter REEs and suffers from matrix effects in clay- and water-rich samples [30,179,193,194]. Laser-Induced Breakdown Spectroscopy (LIBS) offers better spatial resolution and multi-element detection, including some capacity for rare earths. LIBS systems are also compatible with automation, enabling integration with core scanners or conveyor-based cuttings systems. That said, LIBS performance depends heavily on calibration, sample homogeneity, and matrix complexity conditions not always met in field environments [195,196].

Meanwhile, portable ICP-OES units though less common offer a bridge between high sensitivity and field applicability. These systems require sample digestion, which adds time and operational complexity, but they can provide reliable REE quantification at sub-ppm levels. Their use near drill sites remains limited due to logistical challenges, but mobile lab configurations are emerging that could house such systems for continuous or batch-mode analysis during drilling campaigns.

6.4. Integration into Exploration and Resource Workflows

For REE-bearing drill cuttings to be systematically evaluated and valorized, analytical feedback must be embedded into the broader exploration and decision-making workflows. Real-time REE data could be used to flag intervals for detailed geochemical follow-up, optimize drilling fluid formulations to preserve REE content, or even influence bit selection and drilling rate based on lithology-dependent REE potential. To realize this, data integration platforms must link analytical outputs from mud-logging, cuttings scanners, and lab assays into geochemical databases and subsurface models.

Pilot efforts by several geological surveys and national laboratories have begun demonstrating the feasibility of incorporating rapid REE characterization into field operations, though these remain at early stages. The critical path forward involves reducing analytical costs, improving instrument ruggedness, and developing REE-specific calibration protocols tailored to the lithologies encountered in energy and mineral exploration.

While the technological foundation for real-time or near-drill site REE analysis is emerging, significant gaps remain. These include the absence of standardized REE detection protocols for field use, challenges in differentiating between total and extractable REE content, and limited datasets validating the correlation between on-site signals and lab-confirmed concentrations. Nevertheless, as demand for critical minerals grows, the integration of rapid REE assessment into the drilling and exploration lifecycle is likely to become a core component of future mineral intelligence frameworks.

7. Qualitative Assessment of Economic Viability and Deployment Models

The viability of recovering rare earth elements (REEs) from drill cuttings extends beyond technical feasibility and into the realm of economic realism [1]. While prior studies have predominantly focused on leaching yields, mineral associations, and analytical advancements, few have rigorously explored the techno-economic context in which such recovery efforts would operate. This section provides a qualitative yet critical techno-economic commentary, highlighting cost drivers, value opportunities, and realistic implementation pathways based on the state of existing research [197].

7.1. Resource Potential vs. Ore-Grade Feedstock

Drill cuttings typically contain REE concentrations ranging from 100 to 400 ppm, as reported in several studies. These concentrations fall below the conventional cutoff grades (>1000 ppm) for commercially mined REE ores such as bastnäsite or monazite [198]. However, drill cuttings represent a distinct category of feedstock characterized not by high grade, but by high availability, zero mining cost, and accessibility during active drilling operations [18]. Unlike conventional ores, drill cuttings do not require additional capital investment in exploration, excavation, or transportation costs that significantly influence the economics of primary mining operations. Their availability as a byproduct of routine drilling activity in resource exploration, geothermal development, or oil and gas wells enables the possibility of co-recovery, which could drastically reduce operational costs and environmental impacts associated with separate extraction efforts.

Moreover, when recovery strategies are guided by mineralogical screening, even lower-grade materials like cuttings can become economically viable, especially in high-throughput or decentralized processing models. As the critical minerals industry shifts toward sustainability and circular resource utilization, the strategic value of drill cuttings lies not in their ore-grade equivalency, but in their potential to supplement the global REE supply chain with minimal disruption and infrastructure adaptation. When viewed through this lens, their economic value lies in their abundance, their role as a waste stream, and the potential to integrate recovery into existing exploration and development workflows.

7.2. Cost Drivers in Drill Cuttings-Based Recovery

Unlike traditional REE mining, recovery from drill cuttings avoids capital-intensive operations such as blasting, crushing, haulage, and tailings management. The primary cost drivers shift toward sample preparation, leaching reagent consumption, solid-liquid separation, and chemical purification. Additional challenges include the cost of decontaminating samples exposed to drilling mud additives and addressing lithological variability across depths. In pilot or field settings, chemical cost and operational labor are likely to dominate the operating expenditure (OPEX), particularly when extraction yields are modest or inconsistent [199].

The most significant cost components include sample preparation, particularly drying and homogenization of highly variable material; leaching reagent consumption, especially when targeting low-grade or complex matrices; and solid–liquid separation and purification stages, which require careful optimization to ensure selectivity and minimize losses. Furthermore, samples retrieved from drilling operations often contain residual drilling mud additives, surfactants, and lubricants that may interfere with extraction chemistry, necessitating pre-cleaning or decontamination steps that add to both cost and complexity.

A particularly challenging aspect of cuttings-based recovery is lithological variability across depth intervals. This variability influences mineralogy, REE speciation, and leaching behavior, thereby requiring adaptive workflows or real-time mineralogical screening another potential cost factor in scaled operations. In pilot or field-scale implementations, chemical reagents and labor-intensive workflows are expected to dominate OPEX, particularly in scenarios where extraction yields are modest, inconsistent, or dependent on aggressive processing.

7.3. Recovery Limitations and Variability Challenges

The heterogeneity of drill cuttings poses a significant barrier to economic predictability. Variations in lithology, mineralogy, grain size, and drilling fluid interaction make it difficult to standardize extraction workflows [200]. Moreover, the sub-economic REE grade of most cuttings means that even modest fluctuations in recovery efficiency or reagent cost can drastically affect the unit economics. Without a phase-targeted approach, aggressive leaching may recover non-selective metal fractions, increasing downstream separation costs and reducing product purity.

Furthermore, the sub-economic REE concentrations typical of drill cuttings (usually <500 ppm) amplify the impact of even minor fluctuations in process parameters. Variability in extraction efficiency, reagent consumption, or processing time can disproportionately affect the unit cost of recovery, making the process sensitive to operational inefficiencies [201,202]. In the absence of a phase-targeted strategy, non-selective leaching approaches may solubilize a broad spectrum of elements, including unwanted base metals, thereby increasing the complexity and cost of downstream purification and ultimately compromising product quality.

These challenges reinforce the need for adaptive, mineralogy-informed workflows that can dynamically adjust to changes in material composition. Incorporating real-time mineral characterization, stratified processing strategies, and selective reagent schemes are crucial for improving recovery consistency and ensuring that REE extraction from drill cuttings is both technically feasible and economically defensible.

7.4. Value Opportunities and Strategic Advantages

Despite these challenges, drill cuttings offer several economic advantages. They are readily available during hydrocarbon or mineral exploration, often without incurring additional collection costs. Integrating REE recovery into on-site or near-site processing workflows, particularly for lithologies rich in ion-adsorbed or loosely bound REEs, could further reduce costs by minimizing transportation, storage, and material handling requirements. Such integration also presents opportunities for real-time recovery or selective depth-based processing, improving efficiency while reducing environmental footprint. Moreover, this approach aligns with broader national critical mineral strategies, especially in regions lacking conventional REE ore bodies [203,204]. By valorizing an existing byproduct, drill cuttings-based recovery supports the goals of supply chain diversification, resource circularity, and domestic resilience, making it especially attractive for countries striving for technological independence in rare earth supply. When coupled with government incentives, green procurement policies, or low-carbon technology mandates, these efforts could also benefit from non-market value drivers, including subsidies, tax relief, or regulatory fast-tracking.

Collectively, these factors position drill cuttings not just as a marginal supplement to traditional sources, but as a strategic asset capable of filling niche supply gaps while promoting sustainable resource development [205,206]. Additionally, such recovery efforts could complement national critical mineral strategies, particularly in regions with limited access to conventional REE ore bodies. If linked to government incentives or green technology targets, these initiatives could attract non-market value support through policy and subsidy mechanisms.

7.5. Scenario-Based Qualitative Assessment

To further elucidate the economic spectrum of potential recovery models, two conceptual scenarios are proposed. These are designed not to present exact financial projections but to highlight contrasting implementation routes one rooted in centralized, conventional processing logic, and the other leveraging real-time, in-field adaptability. Each scenario reflects differing assumptions around REE concentration, host phase accessibility, operational integration, and economic scalability.

In the first scenario, a pilot-scale plant is envisioned where stored drill cuttings from various wells are aggregated and processed at a centralized facility. This model assumes the use of strong mineral acids (e.g., HCl, HNO₃) for total digestion, followed by solvent extraction to isolate REEs. The average REE concentration in such cuttings is expected to be around 200 ppm. Although this method can achieve moderate recovery yields (~30%), it incurs considerable costs for logistics, infrastructure, chemical consumption, and waste management. The viability of this model is limited unless the facility is co-located with existing hydrometallurgical infrastructure or offset by policy incentives such as critical mineral subsidies or tax breaks.

The second scenario considers a field-integrated portable extraction system tailored for ion-adsorbed or loosely bound REEs, particularly in clay-rich or weathered lithologies. This setup involves mild organic acid leaching (e.g., citric or acetic acid), membrane-based separation, and minimal energy input, all integrated with real-time mineralogical screening. The reduced need for sample transport and chemical intensity makes this approach more environmentally friendly and logistically agile. Though likely to yield lower absolute recovery due to selective targeting, the economic trade-off is favorable in cases where REEs are present in accessible forms. This model is especially promising for integration with existing exploration programs or mobile lab infrastructure and aligns well with emerging sustainable and decentralized processing trends.

These scenarios emphasize that economic viability is not solely a function of REE concentration but of process alignment with mineralogical form, logistics, and recovery goals. A rigid, high-throughput system may suit centralized processing of diverse cuttings, while lithology-specific, agile systems may thrive in field-based deployments where host phase targeting can be executed in real time (

Table 9. Scenario based qualitative assessment of REE extraction from drill cuttings.

Scenario	Key Assumptions	Viability Summary
1. Centralized Pilot Plant with Acid Leaching	– Stored drill cuttings aggregated at a fixed facility Average REE content ~200 ppm – Uses strong mineral acids (HCl, HNO₃) and solvent extraction – Moderate (~30%) recovery efficiency	– Technically feasible but economically marginal – High CAPEX/OPEX for logistics, reagents, and waste handling – Only viable if co-located with existing infrastructure or subsidized
2. Field-Integrated Portable Unit	– On-site processing during active drilling – Targets ion-adsorbed REEs in clay-rich or weathered lithologies – Uses mild organic acids (e.g., citric acid) – Incorporates membrane separation and real-time mineralogical screening	– Lower absolute recovery but higher operational efficiency – Reduced chemical and transport costs – Suitable for agile, lithology-targeted deployment; aligns with green processing trends

).

The techno-economic outlook for REE recovery from drill cuttings is highly context-dependent. While the overall grade may limit standalone commercial interest, the combination of free feedstock, reduced infrastructure requirements, and integration into exploration workflows presents a viable niche opportunity. Key factors influencing economic performance include mineralogical selectivity, reagent cost, and the ability to target high-yield intervals during drilling [19,207]. To fully assess viability, future research should incorporate life cycle costing, reagent recycling, and small-scale pilot demonstrations to validate scalability and environmental performance. Ultimately, a mineralogically-informed, field-adapted, and selectively optimized approach may offer the best chance to transform drill cuttings from a waste liability into a strategic resource for REE supply diversification.

8. Decision Tree Based on Mineral Host Phase in Drill Cuttings

The decision tree proposed in this review provides a mineralogy-guided framework for selecting appropriate extraction and characterization methods for REEs in drill cuttings based on host mineral phase, addressing the complexity and heterogeneity of this underutilized resource as shown in Figure 6. Unlike conventional ores, drill cuttings are often geochemically diverse, variably altered, and partially degraded by drilling fluids necessitating a more diagnostic, mineral-phase-specific approach to recovery. The decision tree starts with a fundamental mineralogical question: What is the dominant REE host phase in the cutting material?

If drill cuttings are derived from shale formations, weathered granite crusts, or lateritic profiles, the REEs are often loosely bound on clay mineral surfaces through weak electrostatic or ionic interactions. These ion-adsorbed REEs are relatively easy to recover and are characteristic of many Chinese ion-adsorption clay deposits [84]. In such cases, mild leaching agents such as citric acid, ammonium salts, or weak organics are sufficient to mobilize the REEs [128,208,209]. Characterization is best conducted using ICP-MS for solution analysis and SEM-EDS to visualize distribution on clay surfaces. This scenario offers the highest leaching efficiency with the lowest environmental cost, making it ideal for pilot-scale valorization.

Drill cuttings from carbonatite intrusions, phosphate-rich shales, or offshore sedimentary sequences may contain REEs bound within structurally stable phosphate minerals [210]. These minerals require strong mineral acids e.g., HCl or HNO₃ and sometimes microwave-assisted leaching to break down the lattice and liberate REEs. The inclusion of thermal or mechanical pre-treatment e.g., grinding or roasting may also enhance exposure. Post-leaching XRD analysis is critical to confirm phase dissolution, while SEM helps assess surface corrosion and grain boundaries. This path yields high recovery but with greater chemical input and residue management needs.

In organic-rich cuttings such as those from the Marcellus or Haynesville shales, or drill intervals intersecting coal seams REEs may be chelated with organic functional groups or embedded within amorphous organo-metallic matrices. These require bioleaching strategies e.g., using Aspergillus niger or Acidithiobacillus or mild oxidative acids to liberate the REEs without destroying organic matter completely. While recovery is generally lower and slower than with acid leaching, bioleaching offers sustainability advantages and may mobilize otherwise inaccessible REE fractions. TOC analysis and sequential extraction are used to monitor the organic phase before and after treatment.

Cuttings influenced by oxidized environments or hydrothermal alteration may contain REEs associated with iron or manganese oxides or other amorphous phases. These are semi-mobile under acidic or reducing conditions. Reductive leaching agents, such as ascorbic acid or oxalic acid, can help release REEs from these hosts. The leaching pathway here is sensitive to pH buffering and solution redox potential, and characterization requires selective leaching tests, along with SEM-EDS to confirm mineral replacement textures and elemental maps.

When drill cuttings originate from granite, volcanic rocks, or metamorphic intervals, REEs are often tightly locked in aluminosilicate structures making them the most refractory and challenging to process [211]. In such cases, thermal cracking (calcination), alkali fusion with NaOH, or microwave-enhanced strong acid digestion is necessary to break down the silicate matrix. This pathway is energy-intensive but essential for unlocking silicate-bound REEs. Characterization via XRD confirms crystalline breakdown, while petrography and ICP-MS of digests quantify the REE release.

If none of the above host phases dominate, or characterization results are inconclusive, the tree suggests returning to detailed mineralogical investigation including XRD, whole-rock geochemistry, and laser ablation ICP-MS, if available. This loop acknowledges the reality of mixed or cryptic REE occurrence in many cuttings, particularly those that have undergone diagenetic alteration or fluid overprinting.

This decision tree serves as more than just a selection guide; it reflects an evidence-based, workflow-oriented approach to resource recovery from drill cuttings. It bridges the knowledge gaps between mineralogy, geochemistry, and extractive metallurgy, and underscores that efficient REE recovery begins with accurate mineral identification. By rooting decisions in host mineralogy rather than generalized leaching, the framework aligns recovery techniques with real-world geochemical behaviour, helping avoid reagent waste, low yields, and overprocessing.

9. Challenges and Future Directions

9.1. Challenges

Despite the growing interest in utilizing drill cuttings as a secondary source of REEs, several scientific, technical, and logistical challenges persist, limiting their integration into the critical mineral recovery chain. One of the foremost challenges is the inconsistent mineralogical composition of drill cuttings. Unlike core samples, cuttings are often fragmented, mixed, and poorly preserved due to circulation through drilling fluids. This heterogeneity makes it difficult to accurately assess REE host phases, distribution, and abundance. Moreover, many studies report bulk REE concentrations without identifying the precise mineralogical association, limiting process optimization and extraction predictability.

Also, drilling muds and additives can significantly alter the surface chemistry of REE-hosting minerals, either by promoting adsorption, forming secondary coatings, or chemically reacting with exposed grains. These alterations not only affect the efficiency of leaching processes but also complicate post-leaching characterization. Additionally, contamination from drilling equipment or additive-rich fluids can skew analytical results, especially in trace-level REE quantification. Currently, there is no standardized protocol for sampling, preparing, or analysing REEs in drill cuttings. Different studies use varying digestion techniques, leaching agents, and analytical instruments, making cross-study comparisons difficult. This lack of harmonization limits the development of universally applicable workflows and hinders the translation of lab findings into field-scale practices. Furthermore, when compared to conventional ores, REE concentrations in drill cuttings are generally low and spatially dispersed. This poses a challenge in both economic feasibility and recovery efficiency. Without enrichment mechanisms or targeted stratigraphic zones, bulk processing of cuttings may not be viable unless co-recovered with other valuable elements e.g., lithium, vanadium, and cobalt.

Notably, most available data is derived from laboratory-scale investigations. Few pilot-scale or industrial-scale recovery studies exist for REEs from drill cuttings, making it difficult to assess real-world performance, environmental impact, or economic viability. This also means that critical factors such as reagent recycling, waste disposal, and process integration into existing drilling operations remain underexplored.

9.2. Future Directions for Research and Development

Future research must focus on the design of selective leaching systems that align with specific REE host phases. This includes integrating mineralogical characterization data (XRD, SEM-EDS, sequential extraction) into pre-leach screening tools that inform process design. Novel combinations of mild acids, bioleaching organisms, and green solvents should be tested systematically across known mineral types. Emerging technologies such as portable XRF, LIBS, and ICP-OES, as well as on-site pre-concentration modules, can be further developed and validated for near-rig REE screening. These tools would allow exploration teams to identify REE-rich zones during drilling campaigns, enabling real-time sample triage and more strategic sample collection.

Moreover, drilling operations could be adapted to accommodate REE-specific mud-logging protocols, cuttings preservation standards, and fluid compatibility considerations. Interdisciplinary collaboration between drilling engineers, geochemists, and mineral processors is needed to develop drilling fluids that minimize REE loss or alteration, rather than focusing solely on shale inhibition or lubrication. Furthermore, to evaluate commercial potential, future studies should focus on pilot-scale leaching experiments, followed by techno-economic modelling and life cycle analysis. These should consider not just REE recovery but also the possibility of co-recovery of other critical metals, and the reuse of processed cuttings in construction or agriculture, enhancing the overall value proposition.

The creation of open-access databases that catalogue REE concentrations, host minerals, lithologies, and leaching performance across different basins would support comparative studies and model development. Similarly, the adoption of reference materials and standardized leaching protocols will increase reproducibility and foster global collaboration.

10. Conclusions

This review underscores the untapped potential of drill cuttings as a secondary source for REEs, positioning them not merely as waste byproducts but as viable contributors to the critical mineral supply chain. This review critically highlights that while drill cuttings have been explored sporadically for REE recovery, most previous efforts lacked mineralogical resolution and methodological alignment. A major finding of this work is that REE recovery cannot be approached as a generic process; instead, it must be guided by the nature of the host mineral phase, which fundamentally dictates extractability, leaching efficiency, and analytical suitability. Our synthesis revealed that many prior studies applied “black-box” leaching protocols without considering whether REEs were ion-adsorbed, organically bound, or locked within resistant mineral lattices leading to inconsistent or sub-optimal outcomes.

By dissecting these gaps, this review introduces a novel decision-tree framework that aligns mineral-host associations with tailored extraction and analytical strategies a key contribution aimed at improving recovery precision. Furthermore, the qualitative techno-economic assessment presented in this paper demonstrates that while drill cuttings may not match ore-grade materials in concentration, they offer processable volumes, zero mining cost, and integration potential within existing drilling workflows.

Critically, this work transforms the narrative of drill cuttings from a disposal challenge to a strategic and sustainable resource, provided that recovery methods are mineralogically informed and economically grounded. Future studies must embrace this mineral-first mindset, optimize extraction protocols based on host characteristics, and incorporate standardized analytical pipelines to advance the field meaningfully.