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Article

REE Mineralogy and Geochemistry of the Lower Karstic Bauxite Strata (b1), in the Parnassos-Ghiona Unit, Greece

by
Nikolaos Sofis
,
Efthymios Panagiotis Ntouros
and
Stavros Kalaitzidis
*
Department of Geology, University of Patras, Rio, 26504 Patras, Greece
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(8), 804; https://doi.org/10.3390/min15080804 (registering DOI)
Submission received: 20 June 2025 / Revised: 18 July 2025 / Accepted: 25 July 2025 / Published: 30 July 2025
(This article belongs to the Special Issue Trace Elements in Bauxite Deposits: Critical Georesource and Significant Indicators of Paleoenvironmental Conditions)
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Abstract

The Parnassos-Ghiona region constitutes the most significant bauxite-bearing province in Greece, with a well-documented history of research highlighting its geotectonic complexity and its importance for bauxite exploitation. Among the three principal bauxite horizons, the lower stratum (b1) remains the least thoroughly investigated, in contrast to the upper (b3) and intermediate (b2) strata. This disparity is primarily attributed to the limited surface exposure of the b1 horizon within the broader Parnassos-Ghiona Unit. The present study examines the characteristics of the b1 strata through an integrated mineralogical and geochemical approach. For the first time, the confirmed presence of rare earth element (REE) minerals within the b1 horizon is documented. Geochemical proxies, including REE distribution patterns and elemental ratios, indicate a genetic relationship with igneous parent rocks of intermediate to basic affinity.

1. Introduction

Karst-type bauxite deposits form through the accumulation of residual mud, originating from intense erosion and transportation of lateritic (bauxitic) profiles, within karstic depressions and cavities [1,2,3]. This clastic bauxitic material is primarily composed of Al-oxyhydroxides with lesser amounts of Fe-oxyhydroxides, Ti-oxides, and clay minerals [2]. Rare earth elements (REE) become concentrated in these deposits and occur either as authigenic REE-bearing minerals or REE-rich phosphates, or can be absorbed by clay minerals, Fe-oxyhydroxides, Al-oxyhydroxides and Ti-oxides [4] and references therein.
Rare earth elements (REE) include the 15 lanthanides, with atomic numbers ranging from 57 (lanthanum) to 71 (lutetium), and typically encompass scandium (atomic number 21) and yttrium (atomic number 39) due to their comparable chemical and physical characteristics. Although REE occur relatively frequently in the Earth’s crust, economically exploitable concentrations are uncommon. Their distinctive physicochemical properties make them indispensable across a wide array of applications in defense, energy, industrial processes, and advanced technologies. For instance, lanthanum-based catalysts are vital in petroleum refining, while cerium-based catalysts are extensively utilized in automotive catalytic converters [5].
While bauxite is primarily known as the main source of aluminum, bauxite ore deposits also represent a potential source of critical metals such as scandium (Sc), gallium (Ga), and possibly rare earth elements [6,7,8,9,10,11,12,13], being recognized also within the 2024 European Union critical raw materials report [12,13].
The distribution of REE (Y + Sc) has been studied for decades in numerous deposits globally and a variety of REE minerals has been reported [14]. In the Italian Apulian karst bauxites, fluoro-carbonate minerals of the bastnäsite group have been identified [9,15]. In Greece monazite prevails [16], while other phosphate REE minerals (churchite, monazite, florencite, rhabdophane, xenotime) have been reported [17,18]. In China, a plethora of REY minerals (parisite, synchysite, bastnaesite, cerianite, rhabdophane, and churchite) have been the subject of study for many karstic bauxite deposits [7,19,20], while in Brazil xenotime and thorite were identified in REY-bearing bauxite profiles. Furthermore, in Montenegro Maksimović and Pantó [16] and Radusinović et al. [21] studied the mode of occurrence of monazite and xenotime in the Zagrad bauxite deposit of Montenegro.
Regarding the studied Parnassos area (Figure 1), to date, most research has focused on the b3 bauxite strata, which is currently mined, while studies on b2 strata remain limited. According to Valeton et al. [22], REE-bearing phases in Greek bauxites occur in low concentrations and may originate from authigenic, detrital or material from the parent rocks. Generally, LREEs are more abundant than HREEs [23]. LREEs are mostly associated with phosphate minerals, such as detrital monazite, rhabdophane and florencite, whereas HREEs are linked with Y-bearing phosphates like detrital churchite and xenotime [17,23]. Other REE-bearing minerals identified in Greek bauxites include cerianite, detrital bastnäsite-group minerals and parisite, often found infilling pores, veins and fractures [23,24,25,26].
In certain localities, particularly at the upper levels of the b3 horizon, Kalaitzidis et al. [32] documented the presence of thin coal seams (up to 50 cm). These seams often contain oxidized pyrite, suggesting the infiltration of acidic fluids, which likely led to bleaching of the overlying bauxite zones [23,32]. Such acidic alteration could have influenced REE mobility, as under low-pH conditions, REE are easily leached from aluminosilicates and tend to migrate downward, precipitating as secondary, authigenic mineral phases in deeper layers [6,23].
Although the b2 strata has been also partially exploited, it has received less attention in the recent literature. However, Maksimovic and Pantó [25] reported occurrences of REE minerals such as bastnäsite-(La) and monazite-(Nd) in the Marmaras deposit. These phases were found near the base of the bauxite profile, associated with the Ni-rich serpentine mineral brindleyite, which has also been observed in lower bauxite zones [25]. Regarding REE geochemistry, Valeton et al. [22], Laskou [33], and Laskou & Economou-Eliopoulos [34,35] observed LREE enrichment over HREE in Greek bauxites. Europium anomalies—especially the Eu/Eu* ratio—serve as indicators of the parent rock type: below 1 anomalies suggest felsic sources, while above or equal to 1 point to mafic or ultramafic origins. Moreover, above 1 Ce anomalies reported in the upper parts of b2 and b3 strata [17,23] can be attributed to oxidizing conditions, which favor Ce4+ precipitation as cerianite (CeO2). Scandium, on the other hand, is mainly hosted in iron oxides such as hematite and goethite, as well as in titanium-rich phases [36,37,38]. Finally, the total REE content (ΣREE) varies significantly among bauxite horizons: b3 ranges between 123 and 444 ppm, while data on the b2 horizon remain scarce [23,26].
This study investigates the mineralogical and geochemical characteristics of the b1 bauxite deposits, with particular emphasis on the distribution, concentration, and modes of occurrence of rare earth elements (REE). Special attention is given to identifying the key mineral associations and understanding the geochemical behavior of REE within the bauxitic strata. The study aims to clarify whether REE are incorporated into major mineral phases such as boehmite, diaspore, and iron oxides, or occur as discrete REE-bearing minerals. These findings enhance the understanding of REE enrichment processes within the lower bauxite (b1) horizon of the Parnassos-Ghiona Unit (Figure 1). The broader geological and textural context of the bauxite bodies, along with a detailed petrographic and mineralogical analysis will be addressed in a subsequent publication.

2. Geological Setting

The Parnassos-Ghiona Unit, situated in the central part of mainland Greece (Figure 1), represents a carbonate sedimentary platform hosting three principal karst-type bauxite horizons: the upper (b3), intermediate (b2), and lower (b1) strata. Historically, mining activity has primarily targeted the intermediate b2 horizon; however, current exploitation is limited to the uppermost b3 stratum. The present study focuses on the stratigraphically deepest and chronologically oldest b1 horizon (Figure 2 and Figure 3).
Geographically, the Parnassos-Ghiona Unit spans the mountainous regions of Oiti, Ghiona, Parnassos, and Helicon. It is predominantly characterized by neritic carbonate sedimentation, which persisted from the Upper Triassic to the Early Paleocene. This long-lasting depositional regime was interrupted by at least three distinct stratigraphic discontinuities associated with the formation of bauxite deposits [39]. The extended carbonate sedimentation and platform architecture classify the unit as a typical Mesozoic carbonate platform [40]. Stratigraphically and structurally, the Parnassos-Ghiona Unit correlates with the “Westmontenegrisch-Kroatische Hochkarstzone” [41,42], as well as with the “Piattaforma Campano-Lucana” [43] and the “Laziale-Lucana” of southern Italy [44]. Sedimentation in the Unit terminated with the deposition of the Paleocene–Eocene flysch.
The allochthonous karst-type bauxite deposits of the Parnassos-Ghiona Unit form part of the broader Mediterranean bauxite province and have been the subject of numerous studies (e.g., [2,22,26,45]). These deposits are hosted within Mesozoic carbonate sequences (Upper Triassic to Upper Cretaceous) and formed as allochthonous accumulations through a complex interplay of processes, in which redox (Eh) and pH variations, both during deposition as well as during diagenesis, critically influenced their mineralogical and geochemical evolution. The genesis of these bauxites was governed by multiple factors, including the regional lithology and weathering history of the parent rocks, the dynamics of erosion and transport of bauxite lateritic material, and the paleogeographic conditions prevailing at the time of deposition (e.g., [26]).

3. Materials and Methods

Fieldwork and sampling were conducted across the northern, northeastern, eastern, and southeastern sectors of the Parnassos-Ghiona Unit, encompassing the sites of Palaiochori village (PL2 site, 9 samples), near Drymaia at the foothills of Mount Kallidromo; Elateia (EL2 site: 12 samples; EL3 site: 6 samples; EL3A site: 6 samples; EL4 site: 12 samples); and the Monastery of Timios Prodromos (PR1 site: 11 samples; PR2 site: 10 samples) in the greater Desfina area (Figure 1, see Supplementary Data Table S1).
The objective of each sampling campaign was to obtain representative material from each stratigraphic profile by applying channel sampling. Where feasible, samples were collected from the overlying and underlying limestones, transitional lithologies, and bauxites.
X-ray powder diffraction (XRD) analyses were performed to qualitatively determine the mineralogical composition of the samples. For this purpose, the samples were ground to a grain size of <50 µm using a laboratory mill with agate components. The analyses were carried out using a BRUKER D8 ADVANCE diffractometer (Bruker-AXS, Fitchburg, WI, USA), using Ni filtered Cu-Ka radiation, operating at 40kV/40mA, and employing a Bruker Lynx Eye fast detector, at the Minerals and Rocks Research Laboratory (MRRL) of the Department of Geology, University of Patras. Analytical conditions included a 2θ range of 2–70°, with a scan step of 0.015°/0.1 s. Mineral phase identification was performed using DIFFRACplus EVA software (Bruker-AXS, USA) based on the ICDD Powder Diffraction File (2006 version), and semi-quantitative mineral content was determined using. Diffrac plus TOPAS v. 3.0 (Manual) software.
X-ray fluorescence (XRF) analyses were conducted at the Laboratory of Electron Microscopy and Microanalysis, University of Patras, using a RIGAKU ZSX PRIMUS II spectrometer equipped with a Rhodium anode (Rigaku Corporation, Tokyo, Japan). Additional XRF analyses were conducted at the National Technical University of Athens. Approximately 0.8 g of powdered sample was mixed with 0.2 g of wax binder and pressed into pellets for analysis.
Scanning Electron Microscopy (SEM) analyses were carried out at the Hellenic Survey of Geology and Mineral Exploration (HSGME) on metallographically polished block sections of bauxite ore collected from all sampling sites. These analyses were used to determine the chemical composition of the minerals of the samples and were performed using a JEOL JSM-IT500 LV SEM (JEOL Ltd., Tokyo, Japan) operating at 20 mm working distance and 20 kV accelerating voltage. The SEM was equipped with an Oxford Instruments EDS system (UNTIM MAX X-stream processor, Emerson, St. Louis, MO, USA) and AZTEC software version 6.1. Calibration was performed using certified standards, including albite (Na), wollastonite (Ca), pure Fe (Fe), SiO2 (Si, O), rutile (TiO2), Al2O3 (Al), pure Mn (Mn), pure Zr (Zr), ThO2 (Th), pure Cr (Cr), MgO (Mg), pyrite (S), pure V (V), KCl (Cl), MAD-10 feldspar (K), and GaP (P).
Geochemical analyses of major, minor, and trace elements were also conducted. Rare earth elements (REE), along with trace elements, were analyzed at SGS Canada Inc. using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) following Na2O2/NaOH fusion. Loss on Ignition (LOI) was determined at the Laboratory of Economic Geology, Department of Geology, University of Patras, by heating the samples to 950 °C for 2 h.
Rare earth element concentrations were normalized to chondrite [46]. The Ce and Eu anomalies were calculated as Ce/Ce* = CeN/√(LaN*PrN) and Eu/Eu* = EuN/√(SmN*GdN), respectively [9,26], where the subscript “N” refers to normalized values for chondrite. The (La/Yb)ch ratio was calculated following the formula (La/Yb)ch = (La/Lach)/(Yb/Ybch).
XRF analysis was performed on all collected samples (66 in total), and based on this screening, ICP-MS analysis was conducted on a selection of bauxite (in total 27 samples: PR1-3, PR1-6, PR2-1, PR2-3, PR2-5, PR2-6, PL2-2, PL2-3, PL2-4, PL2-5, EL2-2a, EL2-3a, EL2-5, EL2-6a, EL2-6b, EL2-6c, EL3-2, EL3-4, EL3A-5, EL3A-6, EL4-1, EL4-2, EL4-3, EL4-5, EL4-7, EL4-8, EL4-9), limestone (EL2-2, EL3-5), eight transitional lithologies (PR1-2, PR2-7, PL2-7, EL2-3, EL2-4, EL3-3, EL3A-2, EL4-6), and two various other lithologies encountered within the bauxite profiles (PR1-11, EL3A-4).

4. Results

4.1. Macroscopical Lithological Features

The footwall Middle and Lower Jurassic limestones typically exhibit a dark grey coloration, interspersed with lighter, irregular mottling. In contrast, the hanging wall Upper Jurassic limestones are generally darker and occur as thick, well-bedded sequences. Transitional lithologies vary in color from light red to light brown. The bauxite samples themselves are characterized by a red to deep red hue and frequently display a distinctive oolitic texture.

4.2. Mineralogical Components

Mineralogical analysis revealed that the dominant mineral phases of the bauxites include hematite, goethite, diaspore, boehmite, and anatase (Figure 4). Calcite predominates in the limestones and is also a major phase within the transitional lithologies, as reasonably expected. Boehmite was identified exclusively in the Prodromos deposits, where it coexists with diaspore.
Kaolinite was detected in several bauxite samples from the Prodromos area but was absent in samples from other localities. Chlorite and chamosite were identified as accessory minerals in several samples from the Elateia and Paleochori areas. Pyrite was observed as a trace phase only at sites PL2 and PR2, while gibbsite was present also in a few bauxitic samples. It should be noted that mineral concentrations below approximately 1%–2%, depending on crystallinity, could not be reliably detected using the applied methods.
Optical microscopy confirmed the XRD results, particularly the presence of calcite and oolitic iron oxides and hydroxides embedded in the clay matrix of both bauxite and transitional lithologies. According to XRD data, the iron-bearing phases correspond to hematite and goethite. Calcite frequently occurs as veinlets crosscutting the bauxitic matrix.
The predominant texture observed in the bauxitic samples is matrix-supported and oolitic (Figure 5). Most oolites possess an iron-rich core surrounded by rims that may be either iron- or aluminum-rich, though aluminum-rich boundaries are more common. These aluminum-rich oolites often contain dispersed hematite fragments within the clay matrix or exhibit concentric zones enriched in hematite, sometimes forming multiple bands within a single oolite. The matrix in most samples is iron-rich and contains sporadic inclusions of hematite, anatase, boehmite, and diaspore.
To achieve more precise mineral identification, scanning electron microscopy (SEM) was employed. SEM-EDS analyses confirmed the presence of Al-rich phases (primarily diaspore and boehmite), Fe-rich phases (mainly hematite), and Ti-bearing phases (anatase) (Figure 6a,c, see also Supplementary Data, Figure S1). Various accessory minerals were also detected, including REE-bearing phases such as monazite, xenotime, and cerianite (Figure 6b). Trace components included nimite and Ni-bearing chromite (nichromite) (Figure 6d), as well as thorianite, pyrite and galena. Cerianite was typically found as agglomerates associated with Fe-rich phases, particularly hematite (Figure 6a), whereas monazite and xenotime were distributed more broadly within the bauxitic matrix (Figure 6a,c). Most REE minerals were characterized by high cerium content, with additional light rare earth elements (LREE), notably lanthanum and neodymium, frequently present. However, the obtained SEM data are considered semiquantitative and thus indicative of the mineralogical phases, while for more accurate determinations microprobe data are required.

4.3. Geochemical Features

4.3.1. Bauxites

Most of the major oxides display a wide variation, with the concentration of Al2O3 in the bauxite samples ranging from 27.4 to 63.90 wt.%, of Fe2O3 from 0.47 to 28.8 wt.%, SiO2 from 1.25 to 23.9 wt.%, CaO from 0.14 to 17.56 wt.%, and TiO2 from 0.02 to 3.92 wt.%. The contributions of K2O, MgO, MnO, Na2O, and P2O5 are generally insignificant, typically remaining below or around 1 wt.% (see Supplementary Data Table S1). Notable exceptions include sample EL2-2a, where CaO reaches 3.22 wt.%, and EL4-12, where it is 1.17 wt.%. As expected, the average concentrations of Al2O3 and Fe2O3 are relatively high, at 42.80 wt.% and 17.33 wt.%, respectively. The average values for SiO2 and TiO2 are 7.43 wt.% and 2.67 wt.%, respectively (Table 1, Figure 7; see Supplementary Data, Table S2).
Trace elements in the bauxite samples can be grouped based on their concentration ranges. Chromium, Mn, Ni, V, Zn, and Zr each exceed 100 mg/kg. Elements such as Ba, Cu, Li, Sr, As, Bi, Cd, Co, Cs, Ga, Hf, In, Mo, Nb, Pb, Sb, Sn, Ta, Th, Tl, U, W fall within the 10–100 mg/kg range. Elements such as Bi, Cd, Cs, Hf, Rb, Sb, Sn, Ta, Tl, and U are generally considered insignificant due to concentrations below 10 mg/kg (see Supplementary Data, Table S1).
Among the trace elements, several exhibit relatively high concentrations. Chromium averages 355 mg/kg (maximum 446 mg/kg), Li 96 mg/kg (maximum 678 mg/kg), Ni 217 mg/kg (maximum 800 mg/kg), V 324 mg/kg (maximum 430 mg/kg), and Zr 468 mg/kg (maximum 605 mg/kg). Gallium—a notable by-product of bauxite mining and currently a target in metallurgical applications [26,47]—has an average concentration of 65 mg/kg, with a maximum of 95 mg/kg. Lithium (average Li 96 mg/kg) displays significant variability with values in most samples from 10 to 100 mg/kg, whereas for 8 samples it exceeds 100 and actually reaches 678 mg/kg, a value that falls within the upper limits recently reported by Economou-Iliopoulos and Kanellopoulos [48] and Kalaitzidis 2010 [32] for the b3 strata.
The total rare earth element plus Y + Sc [ΣREE (REE + Y+Sc)] content in the bauxite profiles ranges from 182.65 to 2781.62 mg/kg. Light REE (LREE, i.e., La, Ce, Pr, Nd, Sm, Eu, Gd) concentrations range from 86.79 to 2485.17 mg/kg, while heavy REE (HREE, i.e., Tb, Dy, Er, Ho, Tm, Yb, Lu, +Y + Sc) values fall between 65.99 and 296.45 mg/kg. The average ΣREE concentration is 527 mg/kg. Elements with consistently high concentrations include La (average 115 mg/kg), Ce (204 mg/kg), Nd (75 mg/kg), and Sc (45 mg/kg). These elevated values are most frequently observed in the lower sections of the bauxite profiles.
Across the dataset, the average ΣREE is 647 mg/kg, with a mean value of LREE of 496 mg/kg and HREE of 151 mg/kg. The data confirm that LREE are significantly enriched relative to HREE, with La, Ce, Nd, and Sc being the most abundant, exhibiting mean concentrations of 131, 230, 80, and 45 mg/kg, respectively (see Supplementary Data, Table S4).

4.3.2. Transitional Lithologies

Samples representing the transitional lithologies exhibit elevated CaO concentrations, ranging from 8.16 to 56.55 wt.%, as expected for carbonate-rich material. Fe2O3 values vary between 3.28 and 39.32 wt.%, Al2O3 from 2.42 to 33.2 wt.%, SiO2 from 1.84 to 23.97 wt.%, and TiO2 from 0.12 to 2.03 wt.%. The contents of K2O, MgO, MnO, Na2O, and P2O5 remain consistently low, not exceeding 1.7 wt.% (see Supplementary Data, Table S2).
Regarding trace elements, only Cr, Cu, Mn, Ni, V, Zn, As, Pb, and Zr display relatively elevated concentrations. The total rare earth element content (ΣREE) in transitional lithologies ranges from 102.54 to 842.16 mg/kg (see Supplementary Data, Table S3).
Sample PR1-2, representative of a transitional lithology, is characterized by elevated concentrations of Al2O3 (27.39 wt.%), SiO2 (23.97 wt.%), and Fe2O3 (11.13 wt.%), alongside lower concentrations of CaO (8.16 wt.%), TiO2 (1.88 wt.%), and K2O (1.21 wt.%). Other oxides, including MgO, MnO, Na2O, P2O5, and TiO2, are present in minor amounts, each contributing less than 0.64 wt.% (see Supplementary Data, Table S2). For trace elements, elevated values are recorded for Cr, Cu, Mn, Ni, V, Zn, and Zr, with ΣREE again ranging from 102.54 to 842.16 mg/kg (see Supplementary Data, Table S5).

4.3.3. Other Lithologies

Sample PR1-11, representing a clay aggregate layer at the uppermost part of the Prodromos bauxite profile, displays a notably high concentration of Al2O3 (57.98 wt.%), with comparatively low levels of SiO2 (4.75 wt.%) and Fe2O3 (2.06 wt.%) (see Supplementary Data, Table S4). The concentrations of CaO, K2O, MgO, MnO, Na2O, P2O5, and TiO2 are minimal, all remaining below 0.64 wt.%. Among the trace elements, only Cu, Li, Mn, Ni, Zn and Co exhibit relatively elevated values. The total rare earth element content (ΣREE) in this sample is 800.27 mg/kg (see Supplementary Data, Table S6).

5. Discussion

5.1. Bauxite Lithological Attributes

The results of the mineralogical analysis indicate that paragenesis varies significantly depending on the sampling location. In most cases, the bauxite samples are composed of hematite, goethite, anatase, and either diaspore or boehmite. Kaolinite is present only at the PR1 and PR2 sampling sites, where it occurs as the dominant clay mineral. The transitional lithologies are primarily composed of calcite, often accompanied by either hematite or goethite. A classification of the PR2 bauxite samples is illustrated in Figure 8.
Diaspore and boehmite are the principal aluminum-bearing phases in both the b1 bauxites and the associated transitional lithologies. As noted by Kloprogge et al. [50], diaspore—being a polymorph of boehmite—typically forms under relatively high pressures and temperatures, as a result of diagenesis or low-grade metamorphism. According to Mondillo et al. [26], the diaspore in the Parnassos–Ghiona Unit originated through post-depositional diagenetic metasomatic processes. Supporting this interpretation, Kalaitzidis et al. [32] reported vitrinite reflectance values below 0.7% in thin coal layers overlying the b3 bauxites, indicative of an early catagenetic stage in the Unit.
The irregular distribution of kaolinite (mean value 27 wt% at PR1 and 28 wt% at PR2) at the Prodromos sites along with the SiO2 wt% content (mean value 10.7 and 6.2 wt%) suggest that the PR1 and PR2 bauxites were influenced by moderate to strong secondary lateritization processes, meaning in situ weathering [2,22,26,33,34]. Additionally, and regarding the participation of pyrite, Valeton et al. [22] proposed that in the Parnassos–Ghiona Unit, pyrite formed through secondary mobilization of iron under reducing conditions associated with stagnant groundwater during the early diagenesis of the bauxite deposits—a conclusion also supported by Mondillo et al. [26]. As Kalaitzidis et al. [32] proposed, the presence of pyrite in coals overlying the Upper Cretaceous bauxites, b3 bauxite strata, indicates reducing conditions in a paralic to lacustrine depositional environment; moreover, Economou-Eliopoulos and Kalatha [51] also proposed the contribution of microorganisms in iron mobility and resultant formation of bacterio-morphic pyrite.

5.2. Factor Analysis and REE Mode of Occurrence

In order to obtain a better understanding of the geochemical characteristics of the studied b1 samples, R mode factor analysis [52] was implemented by using the IBM SPSS V29.0.2 Statistics factor analysis package. A 6-factor model was selected, representing 81.3% of the total variance of the eigenvalues, in which three factors display bipolar mode. The correlation of the factor loadings provided the following groupings and affiliations (Figure 9; see Supplementary Data, Tables S8–S10).
The first factor displays a bipolar mode by grouping in the positive pole the elements: Cr, V, Bi, Ga, Hf, Nb, Sn, Ta, Th, Zr, Er, Ho, Tm, Yb, Lu, Sc and the Al2O3, and TiO2; negative factor loadings are provided for CaO, Na2O, LOI, Mn, Cd and Co. The positive pole represents the bauxite paragenesis, by revealing an affinity of some HREE (i.e., Er, Ho, Tm, Yb, Lu) plus Sc and Ga with Al-bearing phases; particularly Ga is frequently substituting Al3+ in the crystal lattices of Al-phases [53,54]. Other elements revealing an affiliation with Al and Ti bearing phases include Cr, V, Bi, Hf, Nb, Sn, Ta and Th, indicating common origin and co-variability. The negative pole represents the carbonate sedimentation and increased marine influx.
The second factor groups La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Ho and Y; this factor indicates the expected strong affiliation for these elements since they have similar ionic radii, especially in their most common oxidation state (+3), resulting in concentration in the same mineral phases such as monazite, bastnasite, and xenotime; moreover, the factor is also a measure of REE enrichment, affecting mostly samples PL2-5, EL3A-2 and EL4-1.
The third factor groups Cu, Zn, As, Cd, Mo, Pb and Sb, elements having a usual affiliation to sulfides, which has been identified in the studied bauxite samples with SEM analysis (e.g., galena). Factor scores indicate that the above grouping and/or relative enrichment occurs in the bauxites PR2-3, EL2-2a, EL3-4, EL4-5, as well as the transitional lithologies EL3A-2, EL4-6.
The fourth factor displays high positive loadings for Fe2O3, V and Th; an indication that V and Th are incorporated in Fe-bearing minerals, particularly in samples EL3A-2, EL3A-4 and EL3A-5. The negative pole groups CaO and LOI, as a measure of carbonate influx.
The fifth factor exhibits positive loadings for K2O, SiO2, Li, Cs and Rb suggesting a probable geochemical association with clay minerals, particularly illite and kaolinite, which are known to incorporate these elements. K2O and Rb imply a contribution from K-bearing clays, while SiO2 and Li are consistent with kaolinite and/or residual quartz [55,56,57]. The latter are expressed particularly for samples PR1-2 and PR1-3 from the PR1 sampling site, which are located at the uppermost part of the profile, being typically zones of intense weathering and clay formation, with kaolinite prevailing in these samples (52 and 39%, respectively).
Finally, the sixth factor displays a bipolar mode and groups in the positive pole: Al2O3, Be, Ni, Co, Th, Ce and in the negative: CaO and Sr particularly in samples PR1-11 and PR2-1, located at the bottom parts of the respective bauxite profiles. Ni and Co can isomorphically substitute for Fe3+ in goethite, often found at the lower parts of bauxitic profiles; Th and Ce are more likely to be concentrated in thorianite (identified in b1 by SEM studies), which contains substantial Ce in its structure due to isomorphic substitution of Ce4+ for Th4+ [58,59,60]. It is possible that the grouping of the elements Be, Ni, Co (±Th, Ce) with Al2O3 implies a possible adsorption of them onto aluminum hydroxides (boehmite and diaspore). CaO and Sr are grouped together and reflect marine influx during the deposition of the studied profile, as Sr is enriched in seawater and gets incorporated into marine precipitated carbonate rocks [61] ).

5.3. Bauxite Geochemical Attributes and Provenance

According to Mason and Moore [62], the average concentrations of selected elements in the continental crust are as follows: Sc (22 mg/kg), V (135 mg/kg), Cr (100 mg/kg), Co (25 mg/kg), Ni (75 mg/kg), Cu (55 mg/kg), Zn (70 mg/kg), Rb (90 mg/kg), Sr (375 mg/kg), Y (33 mg/kg), Zr (165 mg/kg), Ba (425 mg/kg), W (1.5 mg/kg), Pb (13 mg/kg), La (30 mg/kg), and Ce (60 mg/kg). All analyzed bauxite samples exhibit notable enrichment in Sc, V, Cr, Ni, Cu, Zn, Y, Zr, W, Pb, La, and Ce, while appearing depleted in Co, Rb, Sr, and Ba. Transitional lithologies show further depletion in Sc relative to bauxite samples. The clay aggregate layers are comparatively depleted in V, Cr, Zr, and La but exhibit enrichment in Co. Similarly, the iron-rich lithologies display elevated Co concentrations.
Geochemical analyses reveal significant variation in total rare earth element (ΣREE) content based on lithological characteristics (Figure 7). On average, bauxite samples are more enriched in REE (average ΣREE: 647 mg/kg) than transitional lithologies (average ΣREE: 404.97 mg/kg). Specifically, ΣREE values in bauxite samples range from 182 to 2781 mg/kg, while transitional lithologies show lower values, from 102 to 842 mg/kg (see also Supplementary Data, Tables S1 and S4 for bauxites and Tables S3 and S5 for transitional lithologies). Among other lithotypes, sample PR1-11 (clay aggregates) exhibits high REE content (800 mg/kg), whereas samples EL3A-3 and EL3A-4 (iron-rich lithologies) show relatively low REE values (315 mg/kg) (see Supplementary Data, Tables S3 and S6).
ΣREE concentrations vary widely across different bauxite profiles, ranging from 224 to 1169 mg/kg depending on the sampling site. The La/Y ratio spans from 0.47 to 5.85, while the Ce/Ce* ratio ranges from 0.41 to 7.43 (see Supplementary Data, Tables S4–S6). Chondrite-normalized REE patterns (Figure 10) are similar across sites, displaying consistent negative anomalies for Eu and Ho, and a positive anomaly for Er across all sites.
At relatively high-pH conditions (typical of Greek karstic bauxitization) Ho3+ forms particularly stable aqueous carbonate (e.g., HoCO3⁺, Ho(CO3)2−) and organic complexes. Those complexes greatly inhibit Ho adsorption onto any mineral surface so Ho stays in solution and is carried away during leaching, resulting in a negative Ho anomaly. Er3+, by contrast, forms weaker carbonate/organic complexes and is more available to adsorb to the residual solid phases [63,64].
Ce shows a positive anomaly in the PR1, PR2, and EL2 profiles, but appears as negative or absent in PL2, EL3, and EL4 (see Supplementary Data, Tables S4–S6). The similarity implies common provenance of the bauxitic material in the studied strata, but the minor variability of the Ce anomalies indicates differentiation in the syndepositional conditions in each location.
Minerals 15 00804 g010
Figure 10. REE/chondrite normalized patterns of the studied bauxite samples (chondrite reference values from Taylor and McLennan [65]).
Figure 10. REE/chondrite normalized patterns of the studied bauxite samples (chondrite reference values from Taylor and McLennan [65]).
Minerals 15 00804 g010
Based on the literature [25,66,67], positive La/Y ratios indicate alkaline conditions during the early depositional stage, whereas negative values suggest acidic environments. Alkaline conditions were inferred at PR1, PR2, PL2, EL2, EL3A, and EL4, while acidic conditions were dominant at EL3. Similarly, the Ce/Ce* index is interpreted as a redox indicator: values exceeding 1 suggest oxic conditions, while values below 1 indicate reducing conditions. Oxic conditions prevailed at PR1, PR2, EL2, EL3A, and EL4, whereas PL2 and EL3 likely developed under reducing conditions.
Notably, this index varies significantly among samples (0.4 to 7.4). In profiles PR1, PR2, EL3A, and EL4, elevated Ce/Ce* values are observed in the basal parts of the profiles, while in EL2, EL3, and PL2, the highest values occur in the uppermost bauxite layers (see Supplementary Data, Tables S4–S6). The observed variability can possibly be attributed to fluctuations in groundwater level, which dissolves cerianite and REE fluorocarbonates at the uppermost parts of the profile, resulting in Ce3+ leaching and later precipitation at the lower parts [14,68,69,70].
The LaN/YbN ratio among the studied bauxite profiles ranges from 3.15 to 59.87 (see Supplementary Data, Tables S4–S6). At the PR1 site, the LaN/YbN ratio decreases toward the bottom of the profile, indicating that light rare earth elements (LREEs) were preferentially fractionated in the upper layers. Conversely, at PR2, PL2, and EL3A, this ratio increases toward the base of the profiles, suggesting LREE enrichment in the lower strata. Profiles from EL2, EL3, and EL4 do not display a consistent trend. Notably, at the PL2 site, the LaN/YbN ratio reaches a maximum value of 59.87, reflecting strong LREE fractionation relative to heavy rare earth elements (HREEs) (see Supplementary Data, Tables S4–S6).
The Eu/Eu* ratio is below 1 in all bauxite samples, with a mean of 0.62, indicating negative europium anomalies. This ratio serves as a useful proxy for assessing the nature of the parental rocks of bauxite deposits. The Eu/Eu* values in this study range from 0.56 to 0.69. When compared with published data on the parental sources of Greek bauxites from the Parnassos–Ghiona in Greece, Sardinia and Zagros Belt regions [9,26,41,42], these values along with the Zr/Cr ratio suggest a derivation from igneous rocks of intermediate to basic affinity (see Supplementary Data, Tables S4–S6).
According to Valeton et al. [22], Cr concentrations typically range from 500 to 2000 ppm, while Ni values range from 200 to 1000 ppm in b3 strata. Laskou and Economou-Eliopoulos [34] report average values of approximately 1200 ppm for Cr and 650 ppm for Ni in the Parnassos–Ghiona deposits. These averages are corroborated by Mondillo et al. [26], who noted similar enrichment in the b3 horizon. In this study, the average values of Cr and Ni are 356 mg/kg and 217 mg/kg, respectively, considerably lower than those of b3, indicating basic and/or ultrabasic parent rocks for the b3. On the Cr–Ni diagram (Figure 11), most b1 bauxite samples plot within the karst bauxite field, between the domains of mafic and ultramafic precursors. A few samples fall in transitional zones between the karst bauxite and low-iron lateritic bauxite fields, with one sample plotting near the shale-slate and carbonate areas.
To further evaluate the geochemical characteristics of the parental material, elemental proxies such as the TiO2/Al2O3 ratio, Eu/Eu* ratio and Zr/Cr ratio were utilized. In the first approach (Figure 12a), TiO2/Al2O3 was plotted against Eu/Eu* content. Bauxites derived from basic rocks (e.g., b3 strata) typically exhibit Eu/Eu* ratios around ~0.7 to 0.8 and relatively high TiO2/Al2O3 ratios (around ~0.1) [9,26,72]. TiO2/Al2O3 and Eu/Eu* ratios in this study present lower values (mean values 0.06 and 0.63, respectively), indicating intermediate to basic affinity. In the second approach (Figure 12b), TiO2/Al2O3 was plotted against Zr/Cr. According to Mondillo et al. [73], bauxites of the b3 strata show low Zr/Cr ratio (~0.5), indicating basic parental material. Bauxite samples from b1 strata (this study) present Zr/Cr values greater than 1, which indicate contribution of intermediate to basic parental material, whereas values below 1 suggest a mafic origin.
The main REE-bearing mineral phases detected were monazite, cerianite and xenotime. EDS analysis revealed that monazite and xenotime occur as individual crystals in the diasporic matrix. As phosphate minerals they are highly weathering-resistant and are typically detrital, and in the b1 strata are characterized by high sphericity, most likely indicating transportation from the original site of the parent rocks of b1 [26,77]. The presence of other detrital minerals (e.g., nichromite, thorianite, zircon) common to ophiolitic and mafic rocks suggests that mafic rocks of basic to igneous affinity were significant parental rock sources. Cerianite was detected at the uppermost parts of the bauxite profiles. The presence of cerianite can be used as an elemental and effective proxy of pH and Eh conditions, as it is stable in acidic and oxidizing conditions. As reported, it precipitates in the upper parts of weathering profiles and bauxite deposits. Typically, in sediment pores filled with groundwater, acidic (pH range 5 to 6) and oxidizing (Eh values ranging between 0.38 V and 0.61 V) conditions prevail, and Ce3+ precipitates as CeO2 [14,20,78,79,80]. In the studied b1 case, indeed cerianite is mainly detected in the upper parts of the profiles, corroborating the establishment of more oxidizing and acidic conditions. However, it should be noted that a more comprehensive evaluation of the geodynamic setting and paleoenvironmental evolution is required for the accurate interpretation of the b1 forming conditions.
Although currently it is not economically viable to exploit bauxites for REE, the data of this study indicate a promising potential for recovering ΣREE (647 mg/kg), La (mean value 115 mg/kg), Ce (mean value 204 mg/kg), Nd (mean value 75 mg/kg), and Sc (mean value 45 mg/kg) from these bauxites. The average content of scandium in Earth’s crust is about 22 mg/kg; therefore, materials with a scandium content between 20 and 50 mg/kg can be considered as resources [81]. In this study, the Sc average concentration is 40 mg/kg, classifying the b1 strata as a potential source. However, it should be taken into account that during the metallurgical processing of bauxite using the Bayer method, almost the whole quantity of rare earth elements (REE) ends up in the bauxite residue, and their concentration doubles in comparison to that of the original bauxite [82,83]. A potential exploitation of the bauxites of the b1 strata would involve both the recovery of aluminum from bauxites and the extraction of rare earth elements from the bauxite residue using hydrometallurgical methods. The combination of both methods could contribute to the economic viability of REE.

6. Conclusions

Despite their broad geographic distribution, the b1 bauxite samples from different localities exhibit consistent mineralogical characteristics, with only minor variations primarily related to the aluminum-bearing mineral phases. The dominant texture observed is oolitic and matrix-supported, with oolites enriched either in iron or aluminum, while the matrix is generally more iron-rich.
Geochemically, most bauxite samples are characterized by elevated Al2O3 and Fe2O3 contents, averaging 42.80 wt.% and 17.33 wt.%, respectively, along with moderate concentrations of SiO2 (7.43 wt.%) and TiO2 (2.67 wt.%). The total rare earth element (REE) content in the bauxite profiles ranges from 182.65 to 2781.62 mg/kg, with an average of 527 mg/kg. REEs with notably high concentrations for lanthanum (average: 115 mg/kg), cerium (204 mg/kg), neodymium (75 mg/kg), and scandium (45 mg/kg), with the highest concentrations typically found in the lower portions of the bauxite strata. Identified REE-bearing minerals include monazite, xenotime, and cerianite, with monazite being the main phase. The elevated Sc values (mean value 45 mg/kg) within the bauxite lithologies in b1 samples upgrade the strata to a potential target for future exploitation.
Based on statistical analysis, REE are affiliated more to Al- and Ti-bearing phases and less to Fe phases, while SEM data confirmed the participation of REE minerals within the clay/bauxitic mass.
Geochemical indicators suggest that the parent rocks of these bauxites are igneous in origin, with intermediate to basic affinity. This interpretation is supported by the Eu/Eu* ratio (ranging from 0.56 to 0.69), TiO2/Al2O3 values, and Zr/Cr ratios. Additionally, the mean chromium concentration of approximately 360 mg/kg and Zr/Cr values slightly exceeding 1 further corroborate the involvement of intermediate-composition igneous rocks in the genesis of the b1 bauxites.
The distribution of kaolinite in the studied profiles, as a weathering index, indicates the influence of secondary lateritization after the deposition of the clastic material. However, the overall forming conditions of the b1 bauxite strata are still under research, requiring a more comprehensive dataset including regional geological processes.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15080804/s1, Table S1: Sampling locations; Table S2: Geochemical data of the studied b1 strata (Bauxite samples); Table S3: Geochemical data of the studied b1 strata (transitional lithologies); Table S4: Geochemical data of the studied b1 strata (Fe-rich and clayey facies; Table S5: Sum and REE ratios for b1 bauxite samples; Table S6. Sum and REE ratios for b1 transitional lithologies; Table S7: Sum and REE ratios for b1 Fe-rich and clayey facies; Table S8: R-type Factor analyses data; Table S9: Factor loadings of R-type Factor analyses; Table S10: Factor scores of R-type Factor analyses; Figure S1: (a) Monazite spectrum. (b) Cerianite spectrum. (c) Xenotime spectrum. (d) Nickel-bearing chromite (Nichromite) spectrum.

Author Contributions

Conceptualization, N.S. and S.K.; Formal analysis, N.S.; Funding acquisition, S.K.; Investigation, N.S. and E.P.N.; Methodology, N.S. and E.P.N.; Resources, S.K.; Software, N.S. and E.P.N.; Supervision, S.K.; Validation, S.K.; Writing—original draft, N.S.; Writing—review and editing, S.K. All authors have read and agreed to the published version of the manuscript.

Funding

Nikolaos Sofis was funded by the Andreas Mentzelopoulos Foundation (No. 53557/2019).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material). Further inquiries can be directed to the corresponding author.

Acknowledgments

N. Sofis would like to acknowledge the financial support by the “Andreas Mentzelopoulos Foundation”. We would like to thank Michalis Sakkalis, Laboratory of Electron Microscopy and Microanalysis at the Hellenic Survey of Geology and Mineral Exploration (HSGME), for the valuable contribution on SEM-EDS analysis. We also would like to acknowledge Paraskevi Lampropoulou, Department of Geology, University of Patras; Vaia Xanthopoulou, Laboratory of Electron Microscopy and Microanalysis, Faculty of Natural Sciences, University of Patras; and A. Xenidis and K Vaxevanidou at the School of Mining and Metallurgical Engineers, National Technical University of Athens for their support on XRD and XRF analysis. We would also like to thank the three anonymous reviewers for their constructive comments, which improved our manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Geological map of Parnassos-Ghiona Unit presenting the sampling sites (modified after [27,28,29,30,31]).
Figure 1. Geological map of Parnassos-Ghiona Unit presenting the sampling sites (modified after [27,28,29,30,31]).
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Figure 2. (a): Synthetic stratigraphic column of Parnassos-Ghiona Unit (after [32]) and (b): lithological column of PR2 sampling site.
Figure 2. (a): Synthetic stratigraphic column of Parnassos-Ghiona Unit (after [32]) and (b): lithological column of PR2 sampling site.
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Figure 3. (a) Field view of the PR sampling site; (b) view of the different lithologies.
Figure 3. (a) Field view of the PR sampling site; (b) view of the different lithologies.
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Figure 4. Diffractometer of PR2-7 (transitional lithology) sample in which the mineral phases of boehmite, diaspore, anatase, hematite and calcite are recognized.
Figure 4. Diffractometer of PR2-7 (transitional lithology) sample in which the mineral phases of boehmite, diaspore, anatase, hematite and calcite are recognized.
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Minerals 15 00804 g005
Figure 5. SEM backscattered image illustrating the matrix-supported oolitic texture of PR3-3 bauxite sample.
Figure 5. SEM backscattered image illustrating the matrix-supported oolitic texture of PR3-3 bauxite sample.
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Figure 6. SEM images illustrating the microtextural features of the bauxite samples. (a) Monazite (Mnz) crystals associated with hematite (Hem) and anatase (Anat); (b) Hematite (Hem) cavity (darker areas) partially filled with cerianite (Ce) (light-toned aggregates); (c) Crystalline xenotime (Xtm) alongside anatase crystal; (d) Nimite (Nim) and nickel-bearing chromite (Nichromite, Ni-Chr) crystals within the bauxitic groundmass (see also Supplementary Data, Figure S1).
Figure 6. SEM images illustrating the microtextural features of the bauxite samples. (a) Monazite (Mnz) crystals associated with hematite (Hem) and anatase (Anat); (b) Hematite (Hem) cavity (darker areas) partially filled with cerianite (Ce) (light-toned aggregates); (c) Crystalline xenotime (Xtm) alongside anatase crystal; (d) Nimite (Nim) and nickel-bearing chromite (Nichromite, Ni-Chr) crystals within the bauxitic groundmass (see also Supplementary Data, Figure S1).
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Figure 7. Boxplots of selected elements according to the lithologies.
Figure 7. Boxplots of selected elements according to the lithologies.
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Figure 8. Lithologic column indicating the positions of PR2 samples and ternary diagram Al2O3-Fe2O3-SiO2, showing the type of bauxites of PR2 samples (after [49]).
Figure 8. Lithologic column indicating the positions of PR2 samples and ternary diagram Al2O3-Fe2O3-SiO2, showing the type of bauxites of PR2 samples (after [49]).
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Figure 9. (a) scatter plot of Factor 1 vs. Factor 2 loadings, and (b) Factor 3 vs. Factor 4 loadings.
Figure 9. (a) scatter plot of Factor 1 vs. Factor 2 loadings, and (b) Factor 3 vs. Factor 4 loadings.
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Figure 11. Cr–Ni concentration contents of b1 bauxite strata in relation to the parent rock fields (fields after [71]).
Figure 11. Cr–Ni concentration contents of b1 bauxite strata in relation to the parent rock fields (fields after [71]).
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Figure 12. Gevslots of (a) TiO2/Al2O3 vs. Eu/Eu* and (b) TiO2/Al2O3 vs. Zr/Cr of b1 bauxite samples (green circle) and the Mediterranean area. Data from: Spinazzola–Apulia–Italy (grey rhombus) [9]; Parnassos–Ghiona–Greece (blue rhombus) [26]; Otranto–Apulia—Italy (light grey rhombus) [72];Provenve–France (red square) and Langedoc–France (orange square), [73], Catalan Coastal Range—Spain (orange circle) [74]; Sardinia—Italy (yellow circle) [75]; Campania—Italy (green triangle) [76], (plots modified after [73]).
Figure 12. Gevslots of (a) TiO2/Al2O3 vs. Eu/Eu* and (b) TiO2/Al2O3 vs. Zr/Cr of b1 bauxite samples (green circle) and the Mediterranean area. Data from: Spinazzola–Apulia–Italy (grey rhombus) [9]; Parnassos–Ghiona–Greece (blue rhombus) [26]; Otranto–Apulia—Italy (light grey rhombus) [72];Provenve–France (red square) and Langedoc–France (orange square), [73], Catalan Coastal Range—Spain (orange circle) [74]; Sardinia—Italy (yellow circle) [75]; Campania—Italy (green triangle) [76], (plots modified after [73]).
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Table 1. Summary of geochemical data (see also Supplementary Data, Tables S1–S6).
Table 1. Summary of geochemical data (see also Supplementary Data, Tables S1–S6).
BauxiteTrans. LithologiesOther Lithologies
wt %MinMaxAvg.MinMaxAvg.MinMaxAvg.
SiO21.9520.556.060.5423.985.992.159.845.58
TiO20.203.822.540.112.181.180.030.150.10
Al2O329.2563.9045.212.9533.2017.082.8357.9923.49
Fe2O31.1928.7717.943.2939.3211.622.0673.0948.16
MnO<0.010.170.030.010.400.100.010.420.16
MgO0.031.120.340.170.510.330.170.200.19
CaO0.1416.691.898.1756.5532.420.281.210.71
Na2O0.030.120.060.040.400.150.050.050.05
K2O<0.011.100.150.031.220.360.010.210.14
P2O50.010.940.110.030.270.090.010.220.12
mg/kg
Ba1013530127124878787
Be597555788
Cr124463562937618217330174
Cu1014462242518273130102
Li106789710412756010382
Mn17112725986282990439326391516
Ni2980021776861299172907540
Sr167453136846444444
V604303242147321644286165
Zn4411173671104196889260631446
Ag111111111
As6194441427189131414
Bi<153<142<131
Cd<151<142<132
Co132636111795112512262
Cs<131<151<111
Ga695653632935328
Ge172122111
Hf116134118131313
In<11<1<11<1111
Mo2721042010131313
Nb2564524018444444
Pb195891021315671606563
Rb<11031207253
Sb12151218121
Sn114113137101010
Ta343132333
Th3594423920134931
Tl111111222
U521911156139
W5180282472174124
Zr326054692437619227428228
La259431312417272277652
Ce4265223023280136170583377
Pr41542433617131313
Nd11557801312359425146
Sm288152221181411
Eu1163<152232
Gd375133281281310
Tb182<132122
Dy636132179111111
Er41491105576
Ho263<142222
Tm121<111111
Yb3119184486
Lu121<111<111
Y2216168259751346147
Sc17604554622373938
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Sofis, N.; Ntouros, E.P.; Kalaitzidis, S. REE Mineralogy and Geochemistry of the Lower Karstic Bauxite Strata (b1), in the Parnassos-Ghiona Unit, Greece. Minerals 2025, 15, 804. https://doi.org/10.3390/min15080804

AMA Style

Sofis N, Ntouros EP, Kalaitzidis S. REE Mineralogy and Geochemistry of the Lower Karstic Bauxite Strata (b1), in the Parnassos-Ghiona Unit, Greece. Minerals. 2025; 15(8):804. https://doi.org/10.3390/min15080804

Chicago/Turabian Style

Sofis, Nikolaos, Efthymios Panagiotis Ntouros, and Stavros Kalaitzidis. 2025. "REE Mineralogy and Geochemistry of the Lower Karstic Bauxite Strata (b1), in the Parnassos-Ghiona Unit, Greece" Minerals 15, no. 8: 804. https://doi.org/10.3390/min15080804

APA Style

Sofis, N., Ntouros, E. P., & Kalaitzidis, S. (2025). REE Mineralogy and Geochemistry of the Lower Karstic Bauxite Strata (b1), in the Parnassos-Ghiona Unit, Greece. Minerals, 15(8), 804. https://doi.org/10.3390/min15080804

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