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Article

Mechanochemical Treatments of Commercial Vermiculites

by
Celia Marcos
1,*,
Javier F. Reynes
2 and
Pedro Álvarez-Lloret
1
1
Department of Geology, Faculty of Geology, University of Oviedo, 33005 Oviedo, Spain
2
Department of Organic and Inorganic Chemistry, Faculty of Chemistry, University of Oviedo, 33006 Oviedo, Spain
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(4), 383; https://doi.org/10.3390/min15040383
Submission received: 5 March 2025 / Revised: 26 March 2025 / Accepted: 30 March 2025 / Published: 5 April 2025
(This article belongs to the Special Issue Use of Clay Minerals in Adsorption and Photocatalysis Technologies)
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Abstract

:
This study investigates the mechanochemical transformation of commercial vermiculites from Uganda and China, processed for 30 minutes (30 min), 8 hours (8 h), and 24 hours (24 h). Structural and textural modifications were analyzed using X-ray diffraction (XRD), thermogravimetric analysis (TGA), BET surface area measurements, and scanning electron microscopy (SEM). Characterization via X-ray diffraction (XRD), thermogravimetric analysis (TGA), BET surface area measurements, and scanning electron microscopy (SEM) revealed substantial structural and textural modifications. Crystallinity decreased significantly, from 66.37% to 3.47% in the Ugandan sample, whereas the three mixed-phase Chinese samples exhibited greater structural resilience, with final crystallinity ranging from 3.82% to 6.30%. Mechanochemical treatment induced mineral phase transformations, including hydrobiotite formation in the Ugandan sample and Fe3Si, quartz, moganite, and NaMgH3 in the Chinese samples. Particle size reduced significantly, reaching submicrometric dimensions after 24 h, with C1 showing the smallest mean size (0.39 µm). BET analysis showed an initial increase in specific surface area, peaking at 31.83 m2/g for C1 after 8 h, followed by a decrease due to pore collapse. The optimal treatment time varied by sample, with 30 min maximizing adsorption in C2 and C3, while 8 h was most effective for C1. These findings highlight mechanochemical treatment as a viable method for tuning vermiculite properties for applications in adsorption, catalysis, and composite materials.

Graphical Abstract

1. Introduction

Vermiculite is a phyllosilicate mineral that belongs to the silicate group. It shares visual similarities with mica and can appear in shades of green, yellow, or brown. Typically, vermiculite has a layered structure, a Mohs hardness of about 2, and a density ranging between 2.4 and 2.7 g/cm3. Structurally, it is classified as part of the 2:1 group [1], characterized by two T–O–T layers linked through an interlayer. The T–O–T structure consists of an octahedral sheet of Mg2+ sandwiched between two tetrahedral sheets of Si4+. The interlayer, in turn, contains an octahedral sheet of Mg2+ bonded to oxygen or hydroxyl (OH−) groups, along with intercalated water. Isomorphic substitutions, particularly the replacement of Si4+ with Al3+ in the tetrahedral layers, are commonly observed. Vermiculite has the capacity to undergo cycles of hydration and dehydration due to the presence of water and OH− groups, with these changes being influenced by variables such as temperature, pressure, particle size, humidity, and chemical composition [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. Apart from pure vermiculite, there are also “commercial vermiculites”, which include various interstratifications of mica and vermiculite, vermiculite with differing hydration states, as well as mica-vermiculite mixtures. These materials exhibit a layered structure comprising both ordered and disordered phases distributed in a mosaic-like arrangement. A key characteristic of commercial vermiculites is their ability to expand and exfoliate when subjected to rapid heating, a process driven by the release of water molecules trapped between silicate layers. Research by Midgley and Midgley [15] and Couderc and Douillet [16] has indicated that the highest degree of exfoliation occurs in regularly interstratified mica-vermiculite structures. Additionally, vermiculites with higher potassium levels in the interlayer tend to retain less water, have lower crystallinity, and show a greater degree of interstratification. Vermiculite is valued for its abundance, affordability, ease of exfoliation, and high porosity and surface area, making it a widely utilized material [15,16,17].
Thanks to its distinctive physicochemical properties—particularly its thermal resistance and exfoliation capability—vermiculite is widely used across various industrial and technological fields. In the construction sector, it is highly valued for its lightweight nature and insulating properties, making it an ideal component in the production of concrete, plaster, coatings, fire-resistant materials, and thermal flooring [18]. Additionally, its particle size and natural porosity make it well-suited for technological applications such as catalysts in chemical processes and filtration media for fluids [19,20,21]. Recent studies have also underscored vermiculite’s potential in biomedical engineering, particularly in cancer theranostics [22]. These diverse applications demonstrate the material’s versatility, as its properties can be tailored through physical, chemical, or mechanical modifications to suit the requirements of various industries.
The mechanochemical process is characterized by low energy consumption, low processing temperatures, and low costs, and it is a powerful, sustainable, time-efficient, environmentally friendly, and more economical alternative for the preparation of functional materials. Transformation mechanisms include mechanical energy and activation that produces phase transformations in polymorphic solids, structural disorder, formation of solid solutions, ionic exchanges, complex formation, oxidation-reduction reactions, acid-base reactions, amorphization of polymers, etc. [23,24]. According to Baláž [25], milling-induced disorder and defects act as nucleation sites, providing activation energy for chemical reactions, including mineral decomposition, redox reactions, ion exchange, and hydration/dehydration. These effects have technological applications in agriculture, metallurgy, waste treatment, materials engineering, construction, pharmaceuticals, and the coal industry. The solid-state grinding or high pressure state or the application of high pressure to certain compounds could produce the same effects on their structure [25,26]. Additionally, mechanochemical grinding physically disintegrates minerals, reduces particle size, and disrupts crystalline structures [27,28]. Hiroshi Takahashi [29,30] observed that dry mechanochemical treatment of kaolinite and natural clay led to structural disorder and amorphization, varying with milling time and initial crystallinity.
The aim of the current study was to investigate the mechanochemical transformation of commercial vermiculites from Uganda and China at different processing times (30 min, 8 h, and 24 h). For this purpose, the structural and textural changes during mechanochem-ical treatment were analyzed using X-ray diffraction (XRD), thermogravimetric analysis (TGA), BET surface area measurements, and scanning electron microscopy (SEM). The research evaluates the alteration crystallinity and particle size properties and phase transformations due to the induced mechanical pressure. These findings highlight optimal treatment conditions for enhancing vermiculite technological applications, for example related to adsorption, catalysis, and composite materials.

2. Materials and Methods

2.1. Materials

The materials investigated in this study were commercial vermiculites; one vermiculite comes from Uganda and was supplied by the company Vermiculita y Derivados S.L. (Gijón, Spain); it has been labeled as U. The other vermiculites come from China and they were supplied by China National Non-Metallic Minerals Industrial Import and Export Corporation (CNMIEC) (China); they have been labeled as C1, C2, and C3, respectively. The particle size of all vermiculites is <5 mm in diameter and the thickness varies from 0.5 to 1 mm. With respect to their appearance, the Ugandan vermiculite and the Chinese C3 are golden in color and the Chinese vermiculite C1 and C2 are silver in color (Figure 1).
The chemical analyses of the Ugandan sample and the Chinese samples were previously published [31,32,33]. The percentage of K2O for the untreated samples was: 0.36 for U, 3.92 for CHG, 4.80 for CHS, and 5.08 for CHO; the high values for the Chinese samples indicate that these are composed of various interstratified layers of mica/vermiculite, vermiculite with different states of hydration, mixtures of mica and vermiculite, etc., that is, the samples are “commercial vermiculites”. The Ugandan vermiculite is a pure commercial sample.

2.2. Mechanochemical Treatments

Mechanochemical treatments were made using the raw vermiculites with the Retsch MM500 Vario (Sheffield, UK) equipment, with up six grinds at a time. Milling frequency of 30 Hz, 440 C stainless steel Retsch screw closure jar of 10 mL with thin PTFE vessel ((BR-100, Berghof Products + Instruments GmbH, Eningen, Germany)), 1 × 10 mm diameter 440 C stainless steel milling ball. For the present study, three experimental mechanochemical treatment times have been performed: 30 min, 8 h, and 24 h. The samples were labeled as shown in Table 1.

2.3. Samples Characterization

Untreated and treated vermiculites were subjected to characterization of the identification of the mineral composition by X-ray diffraction (XRD), thermal behavior using thermal gravimetric equipment (Mettler Toledo, Columbus, OH, USA), and analyses of textural parameters with BET.
XRD patterns of the samples, previously ground with an agate mortar, were taken with a PANalyticalX’pert Pro (Malvern Panalytical, Malvern, UK) diffractometer. Setting conditions were 40 mA and 45 kV (Cu-Kα radiation; λ = 1.5418 Å), 2θ range of 5–70 degrees (in which the most important phases are reflected), 2θ step scans of 0.007°, and a counting time of 1 s per step. The standard reference material used was 660a NIST LaB6 with Full Width at Half Maximum (FWHM) of 0.06° for 2θ = 21.36°. Changes in the intensity and position of the basal reflections were used to indicate changes in the structural order and hydration states. For the identification of the mineral phases and the calculation of its crystallinity, the software X’Pert HighScore Plus 2.2d (2.24) 2008 was used. Crystallinity was calculated based on the intensity ratio of diffraction peaks to total measured intensity using Formula (1):
C r y s t a l l i n i t y   ( % ) = 100 ∑ I n e t . / ( ∑ I t o t . − ∑ I c o n s t . b g r . )
After determining the background intensity and separating crystalline peaks from the amorphous hump, the software automatically calculates the crystallinity percentage.
The thermal gravimetric analyses were made between 25 and 1100 °C on the samples. The equipment was a Mettler Toledo Stare System Thermobalance (Mettler Toledo, Columbus, OH, USA) with alumina crucible, a heating rate of 10 °C/min, and flowing oxygen at 50 mL/min. The total mass loss was determined gravimetrically by heating the samples in air at 1000 °C.
Textural parameters of the powdered samples were determined with the ASAP 2020 equipment (Iberfluid Instruments, Barcelona, Spain) under the following conditions: nitrogen adsorption at −195.8 K, with σm (N2) of 0.162 nm2; unrestricted evacuation of 30.0 mm Hg; vacuum pressure of 10 μm Hg; evacuation time of 1 h; and temperature of sample evacuation prior to N2 adsorption measurements of 22 °C. The data were recorded with equilibration times (p/p0 ranging 0.001 and 1.000) between 50 and 25 s and a minimum equilibrium delay of 600 s at p/p0 ≥ 0.995. Specific surface area and pore size data have been determined by using a mathematical description of the adsorption isotherms with the software of the equipment.
A JEOL-6610LV (JEOL, Tokyo, Japan) scanning electron microscope at 20 kV and 10 mA, with a vacuum of 2.0 × 10−4 Pa, was used to view the superficial morphology of the samples. The length of the maximum length of the particles mechanochemically treated was measured with ImageJ 1.53e software (LOCI, University of Wisconsin, Madison, WI, USA) from the SEM images, and the histograms of the particle length was carried out with Origin 8.5 SR1 software (OriginLab, Northampton, MA, USA).

3. Results

3.1. XRD Analyses

The XRD diffractograms for the untreated and treated samples U, C1, C2, and C3 are presented in Figure 2a–d, covering the 5–70° 2θ range. The mineral phase compositions are detailed in Table 2 and the crystallinity of the investigated samples is presented in Table 3.
The mechanochemical treatment induced, in the sample U, a structural transition in vermiculite, but no additional mineral phases formed after 8 h of processing. The mechanochemical treatment on the C1 sample initially promoted the transient formation of carbonates and borosilicates (after 30 min), but over time, after 24 h, vermiculite and hydrobiotite disappeared and Fe3Si appeared. The mechanochemical treatment of the C2 sample altered vermiculite and micas, leading to the appearance of quartz after 8 h and metal-silicate phases after 24 h. The mechanochemical treatment of the C3 sample led to the progressive decomposition of micas into siliceous and metal-silicate phases.
The mineral phases changed over time during mechanochemical treatment. Vermiculite, initially present in all untreated samples, disappeared completely in C1 and C2 after 24 h but remained in U with structural modifications. Hydrobiotite generally persisted until 8 h but vanished in C1 and C3 after 24 h. Phlogopite and biotite underwent transformations into different structural variants and appeared at different stages, indicating instability under mechanochemical processing. Quartz progressively formed in C1, C2, and C3. Fe3Si formed after 8 h and remained after 24 h in C1, C2, and C3. Therefore, the treatment time directly affected phase transformation and crystallinity reduction. Longer treatments led to greater crystallinity reduction and new phase formation. All samples showed a significant reduction in crystallinity with mechanochemical treatment. The samples responded differently to treatment. For example, sample U showed a more drastic reduction in crystallinity compared to C3.

3.2. Particle Size of Mechanochemical Treated Samples

Figure 3a–c shows the histograms of the longest particle length of the analyzed particles of the mechanochemically treated samples for 30 min, 8 h, and 24 h, respectively. Table 4 presents the maximum, minimum, mean, and the standard deviation length of the maximum length of the particles mechanochemically treated.
The detailed analysis of the histograms and tabulated data reveals significant patterns in the behavior of particles subjected to mechanochemical treatment. For the 30-min treatment, the samples exhibited a heterogeneous size distribution, with maximum values ranging from 85.31 µm (sample U) to 146.61 µm (sample C1). The distributions of particle size are asymmetric and show an extended tail towards larger sizes, which was reflected in the high standard deviations, particularly notable in C1 (35.64 µm) and C2 (25.52 µm). After 8 h of treatment, relatively large particles persisted, with maxima similar to those observed at 30 min, except for sample C3-mq8h, which showed a significant reduction in both its maximum value (44.67 µm) and standard deviation (7.77 µm). The most dramatic change is observed after 24 h of treatment, where all samples experienced a substantial reduction in particle size, reaching submicrometric dimensions. At this point, the distributions become narrower and more symmetric, as demonstrated by the low standard deviations range (0.11–0.41 µm).

3.3. Thermogravimetric Analysis

The TG, DTG, and SDTA curves of the starting and treated samples U, C1, C2, and C3 are presented in Figure 4, Figure 5, Figure 6 and Figure 7, respectively, for each mechanochemical experimental time. The integral thermogravimetric analysis of the samples reveals distinctive patterns and significant transformations associated with mechanochemical treatment. In the TG curves, the untreated U sample exhibited a gradual mass loss characteristic of pure vermiculite, with well-defined stages of surface dehydration, interlayer water loss, and dehydroxylation. Mechanochemical treatment induced a progressive reduction in total mass loss, which was more pronounced after 24 h.
The DTG profiles provide detailed information about the mass loss rate. Sample U showed well-defined peaks that become less intense and broader with prolonged treatment. In the C1, C2, and C3 samples, DTG peaks are significantly modified with treatment, observing the development of new peaks and displacement of existing ones, especially notable in C3 after 24 h.
The SDTA curves revealed thermal events associated with structural transformations. Sample U showed well-defined endothermic events corresponding to dehydration and dehydroxylation, which gradually modified with treatment. Samples C1, C2, and C3 exhibited additional thermal events, particularly notable in the high-temperature region, related to recrystallization and formation of new mineral phases.
The TG curves of all investigated samples (Figure 4a, Figure 5a, Figure 6a and Figure 7a, respectively) show two steps. The first step, from 25 to about 250 °C, is due to the loss of adsorbed water on the surface and/or localized in the interlayer space of vermiculites [34,35]. The second step, at temperatures above 800 °C, show an additional mass loss due to recrystallization into new phases or to a transformation process associated with the dehydroxylation of OH− anions in the octahedral layer [7,34]. In general, all treated samples lost less mass than untreated ones. Water loss varied significantly among treated samples, with C2 losing the least and C1 the most.
Up to five different steps can be observed in the DTG curves of the untreated and treated samples of each type of vermiculite analyzed (Figure 4b, Figure 5b, Figure 6b and Figure 7b, respectively). The first step, ranging from approximately 50 to 150 °C, is due to the loss of surface adsorbed water. In a second step, at temperatures above 200 °C, a loss of the interlayer water and water bound to the interlayer cations is observed. The third step, at temperatures between 550 °C and 650 °C, is due to the loss of hydroxyls. The fourth step is due to CO2 decomposition, and the fifth step, at temperatures above 850 °C, is due to phase recrystallization.
The SDTA curve of each analyzed sample (Figure 4c, Figure 5c, Figure 6c and Figure 7c, respectively) indicates endothermic processes; from 850 °C the baseline of the SDTA curve becomes exothermic, probably due to a phase change in the solid state.

3.4. Textural Analysis (BET)

The nitrogen adsorption-desorption isotherms of the studied vermiculite samples (Figure 8) correspond to type IV and V of the IUPAC classification, with characteristics of mesoporous solids [36]. These isotherms revealed significant changes in their textural properties after mechanochemical treatment. The structural modifications observed through these isotherms demonstrated significant changes in the adsorption capacity and porous structure of the material. The U sample treated for 30 min (U-mq30m) showed a remarkable increase in adsorption capacity, reaching approximately 2.21 mmol/g, and the hysteresis loop widens. Samples treated for longer periods (U-mq8h and U-mq24h) show a similar trend but with different magnitudes. Sample U-mq8h reached approximately 1.78 mmol/g, while U-mq24h showed a slightly lower adsorption capacity. The mechanochemical treatment resulted in a notable increase in adsorption capacity, reaching maximum values of 4.0 mmol/g for C1-mq8h and 2.26 mmol/g for C2-mq30m, representing a substantial increase compared to untreated samples. The peaks in the dV/dlog(w) Pore Volume (cm3/g) vs Pore Width (nm) indicate the most predominant pore sizes in the material (Figure 9a–c). The specific surface area (SBET), adsorption capacity (Qm), average cumulative pore volume (Vp) and average pore width (nm), BET constant (C), and correlation coefficient (R2) values obtained from the adsorption-desorption experiments are shown in Table 5. The specific surface area (SBET) was measured using the BET mathematical model. The model used for the pore size calculation was the Barrett–Joyner–Halenda (BJH) model [37], which is applied only to type IV isotherms, considering the Faass correction [38], which adjusts for the change in thickness of the multilayer during the intervals in which the cores are not emptied. The initial treatment (30 min) showed significant increase in SBET for all samples (U, C1, C2, C3); the treatment for 8 h revealed variable behavior, with the maximum increase in C1-mq8h (31.83 m2/g), but with decreases in U, C2, and C3; with treatment for 24 h, a general decrease in SBET across all samples was displayed. Generally, average pore volume increased with initial treatment and then decreased with prolonged treatments. The maximum average Vp value was provided by the C1-mq8h sample (0.10 cm3). All samples exhibited mesopores, with average pore width ranging between 17 nm to 40 nm. Mechanochemical treatment tends to reduce the average pore width.
The untreated U sample shows a bimodal distribution with peaks at ~4 nm and ~20 nm. With 30 min mechanochemical treatment (U-mq30m), reduction of the 20 nm peak is observed. At 8 h (U-mq8h), significant decreases in both peaks were observed. At 24 h (U-mq24h), the porous structure almost disappears, indicating pore collapse. Untreated C1 shows more uniform distribution with main peak around 4–5 nm. C1-mq30m maintains a similar distribution but with lower pore volume. C1-mq8h shows a more pronounced reduction in pore volume. C1-mq24h presents the lowest porosity, while maintaining structure partially. Initial C2 shows broad distribution with a maximum at ~4 nm. C2-mq30m shows a slight reduction in pore volume while C2-mq8h maintains distribution shape but with lower intensity. C2-mq24h shows the greatest porosity reduction. Untreated C3 has distribution similar to C2. C3-mq30m shows moderate porosity reduction. C3-mq8h presents more significant decrease while C3-mq24h maintains some porous structure but significantly reduced.
All samples show progressive pore structure degradation with mechanochemical treatment: U samples exhibit the most dramatic changes with near-complete pore collapse. In contrast, samples C1, C2, and C3 show greater resistance to structural changes. Treatment time directly impacts pore volume reduction.

4. Discussion

The initial composition of the analyzed vermiculites influenced the phases formed during mechanochemical treatment. In sample U, vermiculite transformed into hydrobiotite after 8 h due to atomic rearrangement and defect introduction. In sample C1, the formation of Fe3Si and quartz after 8 h might suggest reactions facilitated by mechanical energy. Fe3Si (iron silicide) forms during extended mechanochemical treatment of phyllosilicates through several key mechanisms: (1) Source materials: iron comes from the octahedral sheets of phyllosilicates (especially biotite and vermiculite), while silicon comes from the tetrahedral silicate sheets; (2) Reduction process: the high-energy impacts during grinding create localized high-pressure and high-temperature conditions that can reduce Fe³⁺ to Fe²⁺ and eventually to metallic iron (Fe0); (3) Structural breakdown: the milling process first delaminates the layered structure, then breaks Fe-O and Si-O bonds, and finally causes complete amorphization that allows new bonds to form; (4) Formation pathway: after sufficient grinding (8 h), the nanoscale mixing of Fe and Si under high-energy conditions enables direct Fe-Si bonding and crystallization of Fe3Si; (5) Environmental factors: an inert milling atmosphere favors reduction reactions, while grinding media may contribute additional iron. Local heating facilitates diffusion and new phase formation. This process is remarkable because it achieves direct Fe-Si bonding at nominal room temperature, something that typically requires high-temperature metallurgical processes. In sample C2, the appearance of sodium magnesium hydride (NaMgH3) after 24 h indicated a chemical reaction (Na+ + Mg2+ + 1.5H2 → NaMgH3) induced by mechanical energy between sodium and magnesium. In sample C3, the formation of moganite and Fe3Si after 8 and 24 h might suggest significant atomic rearrangement and chemical reactions. The reduction in crystallinity with mechanochemical treatment resulted from induced disorder in the crystalline structure of minerals.
Mechanochemical treatment induced the progressive transformation of phyllosilicates (vermiculite, hydrobiotite, phlogopite, biotite) into siliceous (quartz, moganite) and metal-silicate phases (Fe3Si). In siliceous phases, there is high thermal (up to >1400 °C) and chemical stability, making them resistant to most acids and bases. Fe3Si demonstrates moderate-high thermal stability (800–1000 °C) in oxygen-free environments, but is susceptible to oxidation in air and moisture. Modified vermiculite demonstrates reduced thermal stability (400–500 °C) and increased chemical reactivity compared to original vermiculite. Transient phases (carbonates and borosilicates) demonstrate low stability, and are naturally prone to transformations. These phases can undergo additional transformations through: (1) Natural aging, gradual oxidation of Fe3Si, and partial rehydration of modified vermiculites; (2) Thermal treatments, formation of iron oxides, cristobalite, or mullite at different temperatures; (3) Chemical treatments, development of porous materials, alkaline silicates, or zeolites; (4) Additional mechanochemical treatments, further amorphization, or formation of new nanostructures and alloys. These properties and potential transformations make these materials promising for applications in catalysis, adsorption, or as precursors for advanced materials. These mechanisms are influenced by treatment time and intensity. In the short term (30 min), structural modifications occurred, and transient carbonate and borosilicate phases appear in C1. In the long term (24 h), micas almost completely disappeared in C1 and C2, while C3 stabilized into quartz and Fe3Si. Sample U was the most resistant to changes, maintaining vermiculite in a structurally modified form without the formation of significant new phases.
The high standard deviations of the mean length of the maximum length of the particles mechanochemically treated in C1 and C2 could suggest greater agglomeration and particle growth at these treatment times. The significant reduction in both maximum value and standard deviation in sample C3-mq8h could suggest a higher susceptibility to prolonged mechanical treatment. After 24 h, particle size distributions became narrower and more symmetric, as demonstrated by low standard deviations (0.11–0.41 μm). Notably, after 24 h, particle size followed the order C1 = 0.39 µm < C2 = 0.54 µm < C3 = 0.75 µm, indicating that chemical composition influences resistance to size reduction during prolonged treatment. Despite the significant reduction in particle size during mechanochemical treatment of vermiculites, the observed decrease in surface area can be explained by five key mechanisms: (1) Particle aggregation: while individual particles become smaller, they tend to form aggregates during prolonged treatment, especially after 24 h; (2) Collapse of interlayer spaces: the vermiculite’s expansive layered structure, which initially provides high surface area, progressively collapses during treatment, particularly as it transforms to hydrobiotite; (3) Phase transformations: the formation of new crystalline phases (especially Fe3Si) coincides with surface area reduction, as these phases have different morphologies than the original layered silicates; (4) Pore coalescence: extended treatment causes micropores to merge into larger voids, reducing the total available surface area; (5) Crystallinity relationship: an optimal degree of structural disorder maximizes surface area, but complete amorphization (below 7% crystallinity) leads to significant surface area reduction. This explains why the samples with the smallest mean particle size after treatment (C1 = 0.39 μm) did not necessarily show the highest surface area, confirming that structural evolution during mechanochemical treatment follows a complex pattern rather than a linear relationship.
The correlation between particle behavior during mechanochemical treatment and the mineralogical composition of the samples revealed fundamental aspects of transformation mechanisms and potential applications. Sample U, initially composed of pure vermiculite, exhibited a controlled evolution of particle size, transforming into hydrobiotite while maintaining a modified vermiculite structure after 24 h of treatment. The presence or absence of hydrobiotite in the mechanochemically treated samples, in our experimental context, could be due to the fact that the mechanochemical treatment created favorable conditions for the redistribution of potassium from completely unweathered mica domains to vermiculite domains. In general, the transformation of vermiculite to hydrobiotite occurs through a partial potassium enrichment process [39]. Specifically, the reaction can be described as vermiculite + K⁺ → hydrobiotite. This transformation involves: (1) Cation Exchange with potassium ions replacing hydrated interlayer cations (primarily Mg2+) in some of the interlayer spaces of vermiculite; (2) Partial dehydration with the replacement of hydrated cations by K⁺ leading to collapse of some interlayer spaces, reducing the overall water content (from 4H2O to nH2O, where n < 4); (3) Regular interstratification with the resulting hydrobiotite consisting of regularly alternating vermiculite-like layers (expanded, hydrated) and biotite-like layers (collapsed, potassium-filled). In our experimental context, we propose that the mechanochemical treatment created conditions favorable for potassium redistribution from completely unweathered mica domains to vermiculite domains, facilitating the transformation. This stability was reflected in uniform size distribution and controlled reduction to submicrometric dimensions (mean value of 0.32 µm). Samples C1, C2, and C3, with more complex initial compositions, including phases such as phlogopite and hydrobiotite, displayed heterogeneous behavior during treatment. The appearance of new crystalline phases, such as quartz, Fe3Si, and moganite, coincided with the progressive increase in mean particle sizes (from 0.39 to 0.75 µm). The formation of Fe3Si in all samples after 24 h might suggest a significant mechanochemical reaction contributing to increased resistance to size reduction.
The progressive reduction in total mass loss induced by the mechanochemical treatment would coincide with partial transformation to hydrobiotite. The more complex thermal behavior of samples C1, C2, and C3 would reflect their initial mixed mineralogical composition. Particularly, C1 and C2 presented higher mass losses in the initial dehydration stages, attributable to the presence of phlogopite and hydrobiotite.
The less intense and broader peaks of sample U with prolonged mechanochemical treatment would indicate progressive material destructuring. The formation of new peaks and displacement of existing ones in DTG curves, especially notable in C3 after 24 h, would coincide with the formation of crystalline phases such as quartz, moganite, and Fe3Si. The additional thermal events exhibited by samples C1, C2, and C3, particularly notable in the high-temperature region, would be related to recrystallization and formation of new mineral phases.
The correlation with mineralogical composition revealed that thermal evolution was intimately linked to structural transformations induced by mechanochemical treatment. The purer vermiculite, sample U, maintained its characteristic thermal behavior, although it was modified by hydrobiotite formation. Samples C1, C2, and C3 showed more dramatic changes, consistent with the formation of new crystalline phases and modification of the original structure.
The remarkable increase in adsorption capacity of sample U-mq30m would suggest significant porosity development in the early stages of treatment. The hysteresis loop widening of U-mq30m would indicate greater heterogeneity in the pore size distribution. U samples treated for longer periods (8 h and 24 h) followed a similar trend but with varying magnitudes. The lower adsorption capacity of sample U-mq24h would suggest that prolonged treatment might lead to a slight decrease in total porosity, possibly due to pore coalescence or partial structural collapse. Optimal treatment time varies for each vermiculite type, beyond which counterproductive effects occur. Samples C1 and C2 exhibited distinct behaviors, with C1 reaching optimization at 8 h and C2 at 30 min of treatment. Sample C3 showed a dramatic adsorption capacity increase after 30 min (2.47 mmol/g, over 27 times the untreated sample), though extended treatments (24 h) led to reduced adsorption, likely due to particle reaggregation or structural collapse.
The observed changes in adsorption properties are strongly correlated with mineralogical transformations and crystallinity loss. For sample U, crystallinity decreased from 66.37% to 3.47% after 24 h, coinciding with hydrobiotite formation at 30 min of treatment and vermiculite structural changes. This loss of crystallinity correlated with an initial increase in adsorption capacity, though prolonged treatment led to a decline. In C1, gradual crystallinity reduction (from 33.25% to 6.30%) accompanied mineralogical simplification, where the initial vermiculite-phlogopite-hydrobiotite mixture evolved into simpler phases, culminating in phlogopite and quartz after 24 h of treatment. This transformation coincided with peak adsorption at 8 h of treatment (4.0 mmol/g). C2 experienced rapid crystallinity loss (from 55.86% to 5.11%) and new phase appearances, such as Fe3Si. Its maximum adsorption capacity at 30 min of treatment (2.26 mmol/g) coincided with a mineralogical transition phase retaining vermiculite. C3, despite its initially low crystallinity (12.43%), exhibited significant mineralogical transformations, with moganite and Fe3Si formation correlating with a dramatic adsorption capacity increase at 30 min.
Porosity evolution also correlated with composition and crystallinity. Sample U, with the highest initial crystallinity (66.37%) experienced drastic porosity reduction, corresponding to strong crystallinity loss (41.09% at 30 min, 3.96% at 8 h) and hydrobiotite formation. Sample C1, with lower initial crystallinity (33.25%) and a complex composition (vermiculite, phlogopite, hydrobiotite), showed gradual porosity reduction, correlating with moderate crystallinity loss (27.36% at 30 min, 23.13% at 8 h) and new phase formation at 24 h of treatment (quartz and Fe3Si). In sample C2, high initial crystallinity (55.86%) and complex composition led to porosity evolution influenced by rapid crystallinity loss (25.71% at 30 min of treatment), progressive phase transformation, and final low crystallinity (5.11%). Sample C3, with the lowest initial crystallinity (12.43%), exhibited stable crystallinity until 30 min of treatment (12.42%), followed by progressive quartz, moganite, and Fe3Si formation and final crystallinity reduction (3.82%).
In summary, all samples exhibited progressive pore structure degradation with mechanochemical treatment. U samples underwent the most dramatic changes, with near-complete pore collapse, while C samples (C1–C3) demonstrated greater resistance to structural changes. Treatment time directly impacted pore volume reduction. Initial mineral complexity correlated with structural stability, with mixed-phase samples (C1–C3) showing better resistance to collapse. New phase formation (especially Fe3Si) coincided with major structural changes, and the presence of phlogopite enhanced stability. Initial crystallinity did not predict structural stability, as mixed-phase samples maintained better integrity despite lower crystallinity.
The initial increase in SBET observed after 30 min and 8 h of treatment, particularly in sample C1, can be attributed to the generation of structural disorder, exfoliation, and partial amorphization, which created additional surface area and mesoporosity. However, the subsequent decrease in SBET after 24 h is not merely due to pore blockage but rather appears to be the result of multiple concurrent processes. SEM analyses revealed a substantial reduction in particle size, but also a tendency toward particle agglomeration, which can reduce accessible surface area. Moreover, the XRD data indicated the formation of new crystalline phases such as Fe3Si, quartz, and moganite at longer treatment times. These phases typically exhibit denser and more compact structures compared to vermiculite, thereby contributing to the reduction in overall porosity. Additionally, the BJH pore size distribution analysis showed narrowing of pore sizes and partial disappearance of mesopores in samples treated for 24 h, suggesting pore collapse or coalescence rather than sintering, which is less likely under the low-temperature milling conditions employed. Thus, the reduction in SBET after prolonged treatment is best explained by a combination of pore collapse, structural densification due to new phase formation, and particle reaggregation, rather than by simple pore blockage or thermal sintering.
The SBET-crystallinity relationship showed that in the initial phase of the treatment (30 min), the higher SBET coincided with a moderate reduction in crystallinity. After 8 h of treatment, the maximum SBET in C1-mq8h corresponded with moderate crystallinity retention (23.13%). After 24 h, crystallinity dropped below 7%, coinciding with a significant reduction in specific surface area. For practical applications, the optimal treatment to maximize SBET was 30 min for U, C2, and C3, and 8 h for C1. For processes requiring greater average pore volume, C1-mq8h showed the best performance, while the untreated U sample retained the widest average pores (40 nm). The composition effect on textural properties indicated that the maximum SBET (C1-mq8h) occurred when vermiculite was still present, and the formation of Fe3Si coincided with a decrease in surface area in prolonged treatments.
From a practical standpoint, initial crystallinity loss benefits adsorption capacity, but a critical threshold exists beyond which adsorption deteriorates. Mineralogical composition significantly influences mechanochemical treatment response, necessitating prior characterization for process optimization. The formation of new mineral phases (Fe3Si, moganite) could impact the physicochemical properties of final materials. The ability to control particle size and mineralogical composition through mechanochemical treatment could enable applications in catalysis, composite materials, ceramics, and refractory applications. Understanding optimal treatment times could improve process efficiency and cost-effectiveness in industrial applications.
The mechanochemical transformations observed in this study directly impact the potential applications of the treated vermiculites. Short-term treatments (30 min) optimize adsorption properties, with sample C3 showing a remarkable 27-fold increase in adsorption capacity, making these materials valuable for environmental remediation processes. The controlled reduction to submicron particles (0.32–0.75 μm) enables their use as functional fillers in composite materials, while the formation of phases like Fe3Si provides energy-efficient routes to specialized ceramic precursors. The modified interlayer spacing in transformed phyllosilicates could be exploited in energy storage applications, and the predictable thermal behaviors allow for the development of materials with tailored thermal responses for insulation or fire retardancy. These application-specific benefits are directly linked to our identified optimal treatment parameters: 30 min for samples U, C2, and C3; 8 h for C1. The greater structural stability observed in mixed-phase samples suggests that blended starting materials may be preferable for applications requiring extended processing. By understanding these structure-property-application relationships, manufacturers can maximize performance while minimizing energy consumption, enhancing both the economic and environmental sustainability of vermiculite-based materials.

5. Conclusions

The XRD results demonstrate the capability of mechanochemical treatment to alter silicate mineralogy, promoting the decomposition of phyllosilicates and the formation of more stable phases under mechanical stress. All samples showed significant crystallinity reduction due to mechanochemical treatment, with the purest sample U experiencing the most drastic reduction. Mechanical energy induced chemical reactions and phase transformations, including mineral decomposition, redox reactions, ion exchange, and hydration/dehydration.
Samples mechanochemically treated for 30 min and 8 h showed greater variability in particle size, especially in samples C1 and C2, suggesting greater agglomeration and particle growth at these treatment times. Mechanochemical treatment for longer times (24 h) significantly reduces particle size in all samples, with a tendency towards a more uniform distribution of small particle sizes.
Thermogravimetric analysis revealed that mechanochemical treatment induced significant transformations in the structure and composition of materials, reflected in systematic changes in their thermal behavior. Sample U maintained more predictable behavior, while the C1, C2, and C3 samples underwent more complex transformations, resulting in materials with modified thermal properties and new crystalline phases.
The findings obtained from the correlation among the porosity, composition, and crystallinity suggested that the selection of materials for mechanochemical applications should consider both initial composition and desired final properties, with mixed-phase materials potentially offering better stability for extended treatment times.
The ability to induce specific phase transformations and reduce crystallinity can be used to tailor material properties for various applications. Understanding these mechanisms can lead to more energy-efficient processes in material synthesis and treatment. The findings highlight the potential for using mechanical energy to drive chemical reactions, which could be applied in various industrial processes.

Author Contributions

Conceptualization, C.M.; methodology, C.M., P.Á.-L. and J.F.R.; software, C.M. and P.Á.-L.; validation, C.M.; formal analysis, C.M.; investigation, C.M., P.Á.-L. and J.F.R.; resources, C.M., P.Á.-L. and J.F.R.; data curation, C.M.; writing—original draft preparation, C.M.; writing—review and editing, C.M., P.Á.-L. and J.F.R.; visualization, C.M.; supervision, C.M., P.Á.-L. and J.F.R.; funding acquisition, P.Á.-L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Ministry of Science, Innovation and Universities, Spain, project references PCI2019–111931-2 and PID2024-161146OB-C22.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors would like to acknowledge the technical assistance provided at the Scientific-Technical Services, University of Oviedo, Spain, executing X-ray diffraction, thermal gravimetric analyses, and textural parameters.

Conflicts of Interest

The authors declare no conflicts of interest.

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Minerals 15 00383 g001
Figure 1. Appearance of the investigated vermiculites in hand sample: (a) U, (b) C1, (c) C2, (d) C3.
Figure 1. Appearance of the investigated vermiculites in hand sample: (a) U, (b) C1, (c) C2, (d) C3.
Minerals 15 00383 g001
Minerals 15 00383 g002aMinerals 15 00383 g002b
Figure 2. XRD of untreated and treated samples of U (a), C1 (b), C2 (c), and C3 (d). Note: Vrm = vermiculite, Phl = phlogopite, Bt = biotite, HBt = hydrobyotite, Qz = Quartz, Mog = moganite.
Figure 2. XRD of untreated and treated samples of U (a), C1 (b), C2 (c), and C3 (d). Note: Vrm = vermiculite, Phl = phlogopite, Bt = biotite, HBt = hydrobyotite, Qz = Quartz, Mog = moganite.
Minerals 15 00383 g002aMinerals 15 00383 g002b
Minerals 15 00383 g003aMinerals 15 00383 g003b
Figure 3. Histograms of the maximum length of the particles of the samples U, C1, C2, and C3 mechanochemically treated for 30 min (a), 8 h (b), and 24 h (c). Note: The figures in the insets correspond to SEM images of some of the samples as an example.
Figure 3. Histograms of the maximum length of the particles of the samples U, C1, C2, and C3 mechanochemically treated for 30 min (a), 8 h (b), and 24 h (c). Note: The figures in the insets correspond to SEM images of some of the samples as an example.
Minerals 15 00383 g003aMinerals 15 00383 g003b
Minerals 15 00383 g004aMinerals 15 00383 g004b
Figure 4. Curves TG (a) and DTG (b) and SDTA (c) for the untreated and treated samples U.
Figure 4. Curves TG (a) and DTG (b) and SDTA (c) for the untreated and treated samples U.
Minerals 15 00383 g004aMinerals 15 00383 g004b
Minerals 15 00383 g005aMinerals 15 00383 g005b
Figure 5. Curves TG (a) and DTG (b) and SDTA (c) for the untreated and treated samples C1.
Figure 5. Curves TG (a) and DTG (b) and SDTA (c) for the untreated and treated samples C1.
Minerals 15 00383 g005aMinerals 15 00383 g005b
Minerals 15 00383 g006aMinerals 15 00383 g006b
Figure 6. Curves TG (a) and DTG (b) and SDTA (c) for the untreated and treated samples C2.
Figure 6. Curves TG (a) and DTG (b) and SDTA (c) for the untreated and treated samples C2.
Minerals 15 00383 g006aMinerals 15 00383 g006b
Minerals 15 00383 g007aMinerals 15 00383 g007b
Figure 7. Curves TG (a) and DTG (b) and SDTA (c) for the untreated and treated samples C3.
Figure 7. Curves TG (a) and DTG (b) and SDTA (c) for the untreated and treated samples C3.
Minerals 15 00383 g007aMinerals 15 00383 g007b
Minerals 15 00383 g008aMinerals 15 00383 g008bMinerals 15 00383 g008c
Figure 8. Adsorption isotherms of samples: (a) U samples (U-untreated, U-mq30m, U-mq8h, U-mq24h), (b) C1 samples (C1-untreated, C1-mq30m, C1-mq8h, C1-mq24h), (c) C2 samples (C2-untreated, C2-mq30m, C2-mq8h, C2-mq24h), (d) C3 samples (C3-untreated, C3-mq30m, C3-mq8h, C3-mq24h).
Figure 8. Adsorption isotherms of samples: (a) U samples (U-untreated, U-mq30m, U-mq8h, U-mq24h), (b) C1 samples (C1-untreated, C1-mq30m, C1-mq8h, C1-mq24h), (c) C2 samples (C2-untreated, C2-mq30m, C2-mq8h, C2-mq24h), (d) C3 samples (C3-untreated, C3-mq30m, C3-mq8h, C3-mq24h).
Minerals 15 00383 g008aMinerals 15 00383 g008bMinerals 15 00383 g008c
Minerals 15 00383 g009aMinerals 15 00383 g009b
Figure 9. Pore size vs dV/dlog(w) graphs: (a) U samples (U-untreated, U-mq30m, U-mq8h, U-mq24h), (b) C1 samples (C1-untreated, C1-mq30m, C1-mq8h, C1-mq24h), (c) C2 samples (C2-untreated, C2-mq30m, C2-mq8h, C2-mq24h), (d) C3 samples (C3-untreated, C3-mq30m, C3-mq8h, C3-mq24h).
Figure 9. Pore size vs dV/dlog(w) graphs: (a) U samples (U-untreated, U-mq30m, U-mq8h, U-mq24h), (b) C1 samples (C1-untreated, C1-mq30m, C1-mq8h, C1-mq24h), (c) C2 samples (C2-untreated, C2-mq30m, C2-mq8h, C2-mq24h), (d) C3 samples (C3-untreated, C3-mq30m, C3-mq8h, C3-mq24h).
Minerals 15 00383 g009aMinerals 15 00383 g009b
Table 1. Labels of the mechanochemically treated vermiculite samples.
Table 1. Labels of the mechanochemically treated vermiculite samples.
Untreated SamplesMechanochemical Treatment
30 Min8 h24 h
UU-mq30mU-mq8hU-mq24h
C1C1-mq30mC1-mq8hC1-mq24h
C2C2-mq30mC2-mq8hC2-mq24h
C3C3-mq30mC3-mq8hC3-mq24h
Table 2. Mineral phases and its corresponding JCPDs card of the investigated samples.
Table 2. Mineral phases and its corresponding JCPDs card of the investigated samples.
SamplesMineral Phases
UntreatedMechanochemical Treatment
30 Min8 h24 h
UVermiculite (JCPDs 34-166) Vermiculite (JCPDs 34-166),
Hydrobiotite (JCPDs 10-362)		Vermiculite (JCPDs 16-613)Vermiculite (JCPDs 16-613)
C1Vermiculite (JCPDs 34-166), Hydrobiotite (JCPDs 10-362), Phlogopite (JCPDs 10-495)Vermiculite (JCPDs 34-166),
Hydrobiotite (JCPDs 10-362), Calcite (5-586), Sekaninaite (17-525), KMg3BSi3O10(OH)2 (24-868)		Vermiculite (JCPDs 34−166), Hydrobiotite (JCPDs 10-362), Phlogopite (JCPDs 16-344), Biotite (JCPDs 42-603)Phlogopite (JCPDs 2-53),
Quartz (JCPDs 2-53),
Fe3Si (JCPDs 45-1207)
C2Vermiculite (JCPDs 34-166),
Biotite (JCPDs 46-1440),
Hydrobiotite (JCPDs 10-358)		Vermiculite (JCPDs 5-518), Phlogopite (JCPDs 10-495), Hydrobiotite (JCPDs 10-362)Hydrobiotite (JCPDs 10-363),
Cuarzo (JCPDs 33-1161),
Fe3Si (JCPDs 45−1207)		Phlogopite (JCPDs 16-344),
Hydrobiotite (JCPDs 10-362),
NaMgH3 (JCPDs 42-1143),
Fe3Si (JCPDs 45-1207)
C3Vermiculite (JCPDs 2-21),
Phlogopite (JCPDs 10-495), Hydrobiotite (JCPDs 10-362)		Vermiculite (JCPDs 2-21),
Phlogopite (JCPDs 10-493), Biotite (JCPDs 42-1339) Hydrobiotite (JCPDs 10-362), Cuarzo (JCPDs 5-490), MgO4S (JCPDs 1-540)		Quartz (JCPDs 33-1161)
Moganite (JCPDs 33-1161)
Fe3Si (JCPDs 45-1207)		Quartz (JCPDs 3-444), Phlogopite (JCPDs 14-466), Fe3Si (JCPDs 45-1207)
Table 3. Crystallinity (%) of the investigated samples.
Table 3. Crystallinity (%) of the investigated samples.
SamplesCrystallinity (%)
UntreatedMechanochemical Treatment
30 Min8 h24 h
U66.3741.093.963.47
C133.2527.3623.136.30
C255.8625.715.505.11
C312.4312.424.703.82
Table 4. Maximum, minimum, mean, and the standard deviation (std.) length of the maximum length of the particles mechanochemically treated.
Table 4. Maximum, minimum, mean, and the standard deviation (std.) length of the maximum length of the particles mechanochemically treated.
SampleLength (μm)
MaximumMinimumMeanStd.
U-mq30m85.313.7420.9317.54
C1-mq30m146.615.0439.4735.64
C2-mq30m109.593.9331.5225.52
C3-mq30m94.906.6428.5016.08
U-mq8h122.745.3023.8324.19
C1-mq8h123.244.6124.9523.81
C2-mq8h130.014.9623.4823.46
C3-mq8h44.673.7412.227.77
U-mq24h0.840.090.320.14
C1-mq24h0.650.190.390.11
C2-mq24h1.410.230.540.25
C3-mq24h2.550.260.750.41
Table 5. Specific surface area (SBET), adsorption capacity (Qm), average pore volume (Vp) and the average pore width, BET constant (C), and correlation coefficient (R2) of nitrogen adsorption-desorption measurements for untreated and treated vermiculites.
Table 5. Specific surface area (SBET), adsorption capacity (Qm), average pore volume (Vp) and the average pore width, BET constant (C), and correlation coefficient (R2) of nitrogen adsorption-desorption measurements for untreated and treated vermiculites.
SampleSBET
(m2/g)		Qm
(mmol/g)		Vp
(cm3)		Pore Width
(nm)		CR2
U11.68 ± 0.10.120.00340.00550.9999
U-mq30m19.33 ± 0.00.200.0528.841970.9999
U-mq8h7.10 ± 0.1 0.070.00323.06900.9997
U-mq24h10.13 ± 0.00.100.0627.261350.9999
C117.20 ± 0.10.180.0417.50930.9999
C1-mq30m24.40 ± 0.10.250.0622.051930.9999
C1-mq8h31.83 ± 0.10.320.1022.522210.9999
C1-mq24h9.04 ± 0.00.090.0427.751680.9999
C215.40 ± 0.10.160.0426.401370.9998
C2-mq30m26.50 ± 0.10.270.0620.661980.9999
C2-mq8h5.95 ± 0.00.060.0424.801350.9999
C2-mq24h12.20 ± 0.00.120.0424.372030.9999
C315.30 ± 0.10.160.0527.151000.9997
C3-mq30m21.65 ± 0.10.220.0626.361450.9998
C3-mq8h12.46 ± 0.10.130.0422.321460.9999
C3-mq24h6.57 ± 0.00.070.0325.951450.9999
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Marcos, C.; Reynes, J.F.; Álvarez-Lloret, P. Mechanochemical Treatments of Commercial Vermiculites. Minerals 2025, 15, 383. https://doi.org/10.3390/min15040383

AMA Style

Marcos C, Reynes JF, Álvarez-Lloret P. Mechanochemical Treatments of Commercial Vermiculites. Minerals. 2025; 15(4):383. https://doi.org/10.3390/min15040383

Chicago/Turabian Style

Marcos, Celia, Javier F. Reynes, and Pedro Álvarez-Lloret. 2025. "Mechanochemical Treatments of Commercial Vermiculites" Minerals 15, no. 4: 383. https://doi.org/10.3390/min15040383

APA Style

Marcos, C., Reynes, J. F., & Álvarez-Lloret, P. (2025). Mechanochemical Treatments of Commercial Vermiculites. Minerals, 15(4), 383. https://doi.org/10.3390/min15040383

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