Aluminosilicate Clay Research Articles

Pressure-induced structural modifications of imogolite nanotubes and of their methylated analogues

Structural modifications of single-walled aluminosilicate clay nanotubes have been studied under hydrostatic pressure by in situ synchrotron X-ray scattering. Imogolite nanotubes (INT) of nominal composition (OH)3Al2O3SiOH, and methyl-modified imogolite nanotubes (m-INT), (OH)3Al2O3SiCH3, have hydrophilic and hydrophobic internal cavities, respectively. Nanotube chiralities also differ, with zigzag (INT) and armchair (m-INT) chirality. In this work, pressure-induced changes in nanotube morphology and atomic structure are studied as a function of chirality, affinity of the inner cavity, and the pressure-transmitting medium used. Radial deformation and collapse of nanotubes are evidenced below 3 GPa, followed by the formation of a lamellar phase at higher pressures. In the case of INT, the collapse pressure value depends on the pressure transmitting medium chosen. Axial compressibility is measured, and a pseudo Young's modulus Y is determined to be equal to ∼265 GPa for INT and below 80 GPa for m-INT, underpinning the role of nanotube chirality in mechanical properties.

Comparison of the Properties and Structure of Montmorillonites Modified with Polyvinylpyrrolidone

The modification method of aluminosilicate clays and the modifier content have a decisive influence on the structure and properties of the obtained nanocomposites. Therefore, the development and research of new techniques of nanoclays modification are of great importance for polymer nano-chemistry. The work studied the modification of montmorillonite (MMT) using polyvinylpyrrolidone (PVP) in an aqueous colloidal solution with the subsequent separation of three separate fractions of the solution according to the previously developed method. The influence of the initial mass ratio of components (MMT/PVP) on the structure and thermal properties of the modified MMT in each solution layer was determined. Scanning electron microscopy (SEM), X-ray diffraction (XRD), thermal gravimetry (TG), differential scanning calorimetry (DSC), and Fourier-transform infrared (FTIR) spectroscopy were used for research. It was established that the mass ratio of components in the initial solution has almost no effect on the structure of MMT in the top and middle solution layers, but significantly affects the yield of final products. Different structures can be obtained in the bottom layer of the solution. Still, intercalated MMT was obtained exclusively at the mass ratio of MMT/PVP = 1:2. In addition, the work investigated the structure and thermal properties of modified montmorillonites with the same PVP content, and it has been proven that such composites have dissimilar structures and properties when obtained by different methods.

Preparation of Ceramic Press Powder Based on Aluminosilicate Clay Raw Materials and Ash and Slag Waste from Thermal Power Plants Synthesized by Vitreous Microspheres

The possibility of using 3-component charges in the production of wall ceramics by semi-dry pressing from aluminosilicate loams in a composition with ash and slag waste from CHP plants, cullet and silica gel obtained by the sol-gel method is considered. The physicochemical processes and phase formations occurring in the production of ceramic materials using WCO at the firing stage have been studied. It was found that the introduction of fluxes in the form of cullet and silica gel reduces the temperature of heat treatment and are intensifiers of mineral neoplasms that increase mechanical strength compared with samples made from 2-component compositions «loam + ASW».

Mineralogical, geochemical and physical properties assessment of clay deposits in Umuoke Obowo Southeastern Nigeria for industrial applications

This study assessed the suitability of clay deposits from Umuoke, Obowo in southeastern Nigeria as local raw materials for industrial applications. Ten samples were collected from different mining pits across the locality. X-ray fluorescence, X-ray diffraction and physical property tests characterized the clays based on geochemical composition, mineralogy and key attributes. The clays showed high silica (56.41%) and alumina (32.82%) contents typical of aluminosilicate clays. Iron oxide levels were moderately elevated (3.34% Fe2O3). Clay minerals kaolinite (18.9-37.0%) and illite (0.5-4.15%) occurred predominantly alongside non-clay minerals like quartz, feldspars and metal oxides. The clays exhibited high plasticity (avg. plasticity index 26.61%) enabling easy moulding and shaping. Porosity averaged 21.33% appropriate for refractories. Firing shrinkage (6.5-19.2%) and density (1.54-1.76 g/cm3) were in acceptable ranges. Strength post-firing reached the 15 N/mm2 minimum standard. Estimated refractoriness was 1680.22°C. Overall, the Umuoke clays demonstrate favourable chemistry, mineralogy and physical properties for refractories and structural ceramics applications pending some processing adjustments. Locally exploiting these deposits can promote import substitution, rural industrialization and sustainable development in Nigeria. Further pilot testing can optimize formulations and processes for targeted ceramic products. Comprehensive nationwide clay deposit prospecting is also recommended. The clays are suitable for refractory bricks, ceramic tableware, architectural ceramics, wall tiles, pottery items.

Carrier capability of halloysite nanotubes for the intracellular delivery of antisense PNA targeting mRNA of neuroglobin gene

Peptide nucleic acid (PNA) is a DNA mimic that shows good stability against nucleases and proteases, forming strongly recognized complementary strands of DNA and RNA. However, due to its feeble ability to cross the cellular membrane, PNA activity and its targeting gene action is limited. Halloysite nanotubes (HNTs) are a natural and low-cost aluminosilicate clay. Because of their peculiar ability to cross cellular membrane, HNTs represent a valuable candidate for delivering genetic materials into cells. Herein, two differently charged 12-mer PNAs capable of recognizing as molecular target a 12-mer DNA molecule mimicking a purine-rich tract of neuroglobin were synthetized and loaded onto HNTs by electrostatic attraction interactions. After characterization, the kinetic release was also assessed in media mimicking physiological conditions. Resonance light scattering measurements assessed their ability to bind complementary single-stranded DNA. Furthermore, their intracellular delivery was assessed by confocal laser scanning microscopy on living MCF-7 cells incubated with fluorescence isothiocyanate (FITC)-PNA and HNTs labeled with a probe. The nanomaterials were found to cross cellular membrane and cell nuclei efficiently.Finally, it is worth mentioning that the HNTs/PNA can reduce the level of neuroglobin gene expression, as shown by reverse transcription-quantitative polymerase chain reaction and western blotting analysis.

Study on the sintering behavior of the ceramic precursor Daqiuni clay

Daqiuni clay is an aluminosilicate clay with unique properties that make it a promising ceramic precursor. Understanding its sintering behavior is crucial for quality control in manufacturing. This study aimed to investigate the phase evolution, densification process, and reaction kinetics during sintering of Daqiuni clay. A combination of X-ray diffraction (XRD), simultaneous thermogravimetric-differential scanning calorimetry (TG-DSC), scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FTIR) techniques were employed to comprehensively analyse the phase transformations. The densification behavior, apparent activation energies, and reaction mechanisms were elucidated through thermodilatometric analysis conducted at varying heating rates. Four stages were identified during sintering including dehydration, dehydroxylation, mullite formation, and partial melting. The maximum shrinkages at each stage were measured. Kissinger analysis yielded apparent activation energies of 122.90, 258.40, 361.98 and 348.69 kJ/mol for the dehydroxylation, mullite nucleation, mullite growth, and partial melting stages, respectively. The dehydroxylation and partial melting processes followed the mechanisms of 3rd order reaction and 3D diffusion. Mullite formation was a two-step nucleation and growth process following the JMA model. This study clarified the phase evolution of Daqiuni clay during sintering.

Study of the metal ion adsorption capacity of palygorskite by computer simulation

Palygorskite is a magnesium-rich aluminosilicate clay mineral with a unique chain-layered structure. This structure gives palygorskite a large specific surface area and interesting physical properties. Many researchers have investigated the applications of palygorskite in various fields, including heavy metal adsorption, petroleum and chemical industries, building materials, medicine and agriculture. In this study, molecular dynamics simulations were used to explore the heavy metal adsorption ability of palygorskite. The results showed that polyacrylic acid (PAA) had a heavy metal adsorption ability. In terms of the ability of the substrate to adsorb Pb2+, Ni2+ and Cr3+, palygorskite (attapulgite, ATP) was more effective than SiO2 or clay. Based on this study, the same phenomenon reported in the literature was confirmed, and it was demonstrated that molecular dynamics could properly simulate the filtration of heavy metal ions in water using novel materials. Moreover, H+ was found to play an essential role in assisting PAA/ATP in capturing heavy metal ions. Using this method, we were able to observe the details of heavy-ion adsorption.

Injectable Methacrylated Gelatin Hydrogel for Safe Sodium Hypochlorite Delivery in Endodontics.

Keeping sodium hypochlorite (NaOCl) within the root canal is challenging in regenerative endodontics. In this study, we developed a drug delivery system using a gelatin methacryloyl (GelMA) hydrogel incorporated with aluminosilicate clay nanotubes (HNTs) loaded with NaOCl. Pure GelMA, pure HNTs, and NaOCl-loaded HNTs carrying varying concentrations were assessed for chemo-mechanical properties, degradability, swelling capacity, cytocompatibility, antimicrobial and antibiofilm activities, and in vivo for inflammatory response and degradation. SEM images revealed consistent pore sizes of 70-80 µm for all samples, irrespective of the HNT and NaOCl concentration, while HNT-loaded hydrogels exhibited rougher surfaces. The hydrogel's compressive modulus remained between 100 and 200 kPa, with no significant variations. All hydrogels demonstrated a 6-7-fold mass increase and complete degradation by the seventh day. Despite an initial decrease in cell viability, all groups recovered to 65-80% compared to the control. Regarding antibacterial and antibiofilm properties, 12.5 HNT(Double) showed the highest inhibition zone on agar plates and the most significant reduction in biofilm compared to other groups. In vivo, the 12.5 HNT(Double) group displayed partial degradation after 21 days, with mild localized inflammatory responses but no tissue necrosis. In conclusion, the HNT-NaOCl-loaded GelMA hydrogel retains the disinfectant properties, providing a safer option for endodontic procedures without harmful potential.

Sensitizing effect of montmorillonite on the gas sensing property of CuO particles to n-butanol

Individual montmorillonite (MMT) platelets as an electronic additive were inserted into CuO particle matrix to derive CuO-MMT nanocomposites via a 2-methylimidazole ligand-assisted hydrolysis method in combination with thermal decomposition. The analyses by XRD, FTIR, and XPS confirmed the presence of monoclinic copper oxide and aluminosilicate in the nanocomposites. MMT platelets were observed to be surrounded among quasi-spherical CuO particles closely, even generating some large-sized aggregates. While CuO particles had a decrease in surface area after the loading of MMT, their electrical resistance was modulated to gradually increase. The CuO-MMT(0.050) based sensor showed the highest response of ∼31 (Rg/Ra) to 200 ppm n-butanol at 260 °C. The limit of detection was estimated to be ∼0.75 ppm for CuO-MMT(0.050), which was lower than that of CuO (∼3.12 ppm). Additionally, the nanocomposite-base sensor was also relatively tolerant against humidity. The improved sensing property could be attributed to the resistance modulation, nanoscale junction, and interface oxygen vacancies upon the addition of MMT. This work suggests that aluminosilicate clays can serve as a promising additive with the ability to enhance the gas sensing property of p-type metal oxides.

Investigating Solvent-Induced Aggregation in Edge-Functionalized Layered Silicates via All-Atom Molecular Dynamics Simulations.

Molecular dynamics simulations can provide the means to visualize and understand the role of intermolecular interactions in the mechanisms involved in molecular aggregation. Along these lines, simulations can allow the study of how surface chemical modifications can influence nanomaterial assembly at the molecular level. Layered silicate clays have been of significant interest for some time, particularly with regard to their use in organic/inorganic nanocomposites. However, despite numerous reports on the covalent linkage of organic moieties via silanol condensation, the theoretical understanding of these systems has heretofore been limited to noncovalent interactions, specifically ionic interactions at the charged basal surfaces. Herein, a model for edge-functionalized layered aluminosilicate clay, based on the siloxane linkage, is presented. In addition to reproducing experimentally observed degrees of molecular aggregation of clay-linked perylene diimide derivatives with different terminal functional groups as a function of solvent composition, a molecular-level understanding of the role of van der Waals interactions and hydrogen bonding of the different end-groups on the aggregation state in different water/N,N-dimethylformamide mixtures is obtained. The reported model provides a means to simulate organic moieties covalently bound to the layered silicate edge, which will enable future simulations of nanocomposites and organic/inorganic hybrids based on this system.

Novel Electrospun Composite Membranes Based on Polyhydroxybutyrate and Poly(vinyl formate) Loaded with Protonated Montmorillonite for Organic Dye Removal: Kinetic and Isotherm Studies.

The use of biodegradable polyesters derived from green sources and their combination with natural abundantly layered aluminosilicate clay, e.g., natural montmorillonite, meets the requirements for the development of new sustainable, disposable, and biodegradable organic dye sorbent materials. In this regard, novel electrospun composite fibers, based on poly β-hydroxybutyrate (PHB) and in situ synthesized poly(vinyl formate) (PVF), loaded with protonated montmorillonite (MMT-H) were prepared via electrospinning in the presence of formic acid, a volatile solvent for polymers and a protonating agent for the pristine MMT-Na. The morphology and structure of electrospun composite fibers were investigated through SEM, TEM, AFM, FT-IR, and XRD analyses. The contact angle (CA) measurements showed increased hydrophilicity of the composite fibers incorporated with MMT-H. The electrospun fibrous mats were evaluated as membranes for removing cationic (methylene blue) and anionic (Congo red) dyes. PHB/MMT 20% and PVF/MMT 30% showed significant performance in dye removal compared with the other matrices. PHB/MMT 20% was the best electrospun mat for adsorbing Congo red. The PVF/MMT 30% fibrous membrane exhibited the optimum activity for the adsorption of methylene blue and Congo red dyes.

Solarization of the zeolite production: Calcination of kaolin as proof-of-concept

The current production of zeolites from natural sources requires high temperatures to achieve a phase change in the feedstock materials. Thermal processes such as calcination are energy-consuming and industry relies on fossil fuels to generate the necessary heat, which is neither sustainable nor environmental. This study presents for the first time the treatment of aluminosilicate clay kaolin in a solar reactor to produce metakaolins that are eligible as precursors for zeolite synthesis. Kaolin was loaded in a rotary kiln and a solar simulator was used for calcination at temperatures of 700–1000 °C for 2 h. After the solar calcination process, solar-produced metakaolins were used to synthesize zeolite 4 A and zeolite 13X by hydrothermal route. The kaolin, metakaolin, and product samples obtained were analyzed by XRD, XRF, FTIR, and FE-SEM. Results show that solar-calcined kaolin was as good and efficient as the conventionally calcined one in terms of supplying the metakaolin for zeolite synthesis. Often disregarded by researchers, high-temperature calcination of the kaolin was proved to be an alternative way for the synthesis of zeolite 13X without using an extra silica source, which was achieved for the first time. The overall results suggest that solar calcination is a good alternative to mass produce meta forms of the minerals to be used in the synthesis of value-added products such as zeolites.

Aluminosilicate Clay Minerals: Kaolin, Bentonite, and Halloysite as Fuel Additives for Thermal Conversion of Biomass and Waste

The current focus on renewable energy sources and the circular economy favors the thermal conversion of low-quality fuels, such as biomass and waste. However, the main limitation of their usability in the power sector is the risk of slagging, fouling, ash deposition, and high-temperature corrosion. These problems may be avoided or significantly mitigated by the application of aluminosilicate clay minerals as fuel additives. In this paper, the three most commonly occurring aluminosilicates are reviewed: kaolin, halloysite, and bentonite. Their application has been proven to minimize combustion-related problems by bonding alkalis in high-melting compounds, thus increasing ash melting temperatures, reducing ash deposition tendencies, and decreasing the particulate matter emission. Due to excellent sorption properties, aluminosilicates are also expected to fix heavy metals in ash and therefore decrease their emissions into the atmosphere. The application of aluminosilicates as fuel additives may be a key factor that increases the attractiveness of biomass and other low-quality fuels for the power sector.

Solid-State Characterization of Heteropolyacid-Phyllosilicate Complexes: Structural, Morphological and Thermal Properties

We have previously described the complex formation from a synthetic smectite clay, Sumecton SA (SSA), and the heteropolyacid (HPA), 12-phosphotungstic acid (PTA). Three new synthetic layered aluminosilicate clays of the smectite- [Lucentite SWN (SWN)] and (fluoro)mica-type [Tetrasilicic mica (TSM); Somasif ME100, (SME)] have been self-assembled with PTA, at a specific PTA-clay weight ratio of 5. New protogenic inorganic complexes have been prepared for the future fabrication of proton exchange membrane for fuel cell devices (PEMFC). Complex formation, thermal and structural properties of PTA-phyllosilicate complexes have been investigated using EDX, XRD, SEM, and DRIFT. EDX indicates that complexes incorporate a substantial amount of PTA (50-60% w/w) and that a moderate depletion of Mg2+ ions from octahedral layers of micas clays occurs during the process compared to smectite ones. In contrast, a significant amount of F is removed from the octahedral framework of the mica clays. DRIFT experiments indicate that the Keggin structure is preserved within all complexes and that complex formation predominantly involves the external oxygen (W-Od) of PTA. According to Patterson functions, coherence lengths - which reflect for the virgin clays the extent of the longitudinal platelet stacking (Lc) - of the two smectite clays (Lc ~25 Å) are significantly lower than for their analogous of the mica type (Lc = 180-225 Å). Clays in complexes are characterized by the loss of in-plane organization but are still partially ordered. The nature of the order is still speculative but could be related to the formation of a new inorganic PTA species formed during the complex formation. Compared to the virgin clays, the order of smectite complexes is maintained in the same range (Lc = 27-42 Å) while for mica complexes, coherence lengths are dramatically reduced to ~ 60 Å. All PTA-activated complexes contain a substantial amount of amorphous silica, which tends to increase when heated at temperatures above 280 oC. However, the Keggin structure within the complexes is at least stable up to 280 oC and even, for the most robust TSM complex, up to 450 oC.

Enhanced gas sensing property of Co3O4 matrix nanocomposites with halloysite nanotubes toward triethylamine

Despite their high surface reactivity, the gas sensor based on Co3O4 nanoparticles frequently suffers from a low response to volatile organic compounds (VOCs) due to high-temperature sintering and inefficient resistance modulation. Herein, halloysite nanotubes (HNTs) were incorporated as the additive into the Co3O4 nanoparticle matrix to enhance the sensing property toward VOCs in terms of gas response (Rg/Ra) and operation stability. Co3O4 nanoparticles intimately attached on HNTs were achieved by a facile deposition-precipitation method followed by calcination. The best-performing Co3O4/0.3HNTs-based sensor had a high gas response of 22.9 to 50 ppm triethylamine at 240 °C, which was 4.9 times than that of Co3O4 alone. The enhanced sensitivity could be attributed to the HNT incorporation that facilitated effective resistance modulation, surface property tuning, as well as surface defect formation. These results bring new insight into an enhancement of gas sensing property of a p-type metal oxide with insensitive materials. They also provide a potential to expand the application of aluminosilicate clays to gas sensors.

EVALUATION OF KAOLIN CLAY DEPOSITS IN GOMBE STATE AS ADDITIVE FOR INDUSTRIAL APPLICATIONS

The geological origin and geographical formation of kaolin coupled with the processing method influences some physical and chemical properties of the kaolin, which invariably affect its industrial application capabilities. In this study, raw kaolin clay at different point from four identified deposits around Gombe state were excavated and collected for assessment. The samples were characterized using the X-ray diffractometer (XRD) to determine the mineralogical peaks and the crystalline phases in each of the specimen. After which, the chemical composition of the samples was quantified using X-ray fluorescence (XRF) test. Firing of all specimens was conducted at varied firing temperatures of 900°C, 1000°C, 1100°C and 1200°C at the heating rate of 2.5°C/min. The result of the mineralogical analysis revealed that all the studied samples are alumino-silicate clays and were categorized under chemical classification group of 60-70% of SiO2, 23-33% of Al2O3, and 6-10% of impurities, suitable for use as low duty refractories and fire bricks in accordance with ASTM C27-98 standard.

A pH-triggered reinforcement of transient network of wormlike micelles by halloysite nanotubes of different charge

Soft viscoelastic nanocomposites based on entangled linear wormlike micelles of cationic surfactant erucyl bis(hydroxyethyl)methylammonium chloride and aluminosilicate clay halloysite nanotubes with a surface charge triggered by pH were prepared and studied by rheometry, ξ-potential measurements, thermogravimetric analysis, and cryo-TEM. It was shown that the nanotubes induce an increase of viscosity, which can be attributed to their incorporation into the network of entangled wormlike surfactant micelles via the attachment of micellar endcaps to the surfactant double layer on the surface of the nanotubes. The junctions between the micelles and the nanotubes were visualized by cryo-TEM. The soft nanocomposites demonstrate peculiar flow curves with two shear thinning regions and a plateau between them. The two slopes were attributed to the orientation of the nanotubes (at lower shear rates) and then of the micellar worms (at higher shear rates) along the direction of flow. The intermediate viscosity plateau may represent a stable flow when all nanotubes are oriented, while the micellar worms are not. The nanocomposite system was shown to be pH responsive. Its viscosity increases by 30 times with increasing pH from 4 to 9, which was explained by increasing surface charge of the nanotubes favoring the interaction with oppositely charged WLMs. Such soft materials with easily triggered rheological properties are very promising for various applications.

Near-Infrared-II Activatable Symbiotic 2D Carbon-Clay Nanohybrids for Dual Imaging-Guided Combinational Cancer Therapy.

Two-dimensional (2D) nanomaterials hold great potential for cancer theranostic applications, yet their clinical translation faces great challenges of high toxicity and limited therapeutic/diagnostic modality. Here, we have created a kind of symbiotic 2D carbon-2D clay nanohybrids, which are composed of a novel 2D carbon nanomaterial (carbon nanochips, or CNC), prepared by carbonizing a conjugated polymer polydiiodobutadiyne, and a 2D layered aluminosilicate clay mineral montmorillonite (MMT). Intriguingly, with the formation of the nanohybrids, MMT can help the dispersion of CNC, while CNC can significantly reduce the hemolysis and toxicity of MMT. The symbiotic combination of CNC and MMT also leads to a synergistic anti-cancer theranostic effect. CNC has a strong absorption and high photothermal conversion efficiency in the second near-infrared region (NIR-II, 1000-1700 nm), while MMT contains Fe3+ that can facilitate the generation of reactive oxygen species from highly expressed H2O2 in tumor microenvironment. The nanohybrids not only enable a synergy of photothermal therapy and chemodynamic therapy to suppress the extremely rapid growth of RM1 tumors in mice but also allow for dual photoacoustic and magnetic imaging to guide the drug delivery and NIR-II irradiation execution, hence establishing a highly efficient and biosafe "all-in-one" theranostic platform for precision nanomedicine.

Synthesis of porous hydroxysodalite from aluminosilicate rich clay soils: application towards fluoride and pathogen removal.

This article discussed the potential application of hydrothermally synthesized porous hydroxysodalite material synthesized from aluminosilicate clay material as a multifunctional adsorbent for fluoride and pathogen removal from groundwater. The efficiency of the material towards fluoride removal was evaluated using batch experiments while the efficacy against the E. coli strain was evaluated using well-assay diffusion method. The material showed a maximum fluoride adsorption capacity of 6.01mg/g at initial concentration range of 5 to 100mg/L when 1g/100mL adsorbent dosage was used at initial pH of 6 ± 0.5 after agitation time of 10min. The adsorption kinetics data fitted better to pseudo first order of reaction kinetics indicating the dominance of physiosorption adsorption mechanism while the adsorption isotherm data showed better fit to both Langmuir and Freundlich adsorption isotherm model confirming monolayer and multilayer adsorption. The material was successfully regenerated and reused for up to eight successive regeneration-reuse cycles. However, its efficiency was inhibited by the presence of Cl- and CO32-. The material also proven to have antimicrobial activity against E. coli strain. This study concluded that the porous hydroxysodalite material prepared in this study can be used as a multifunctional adsorbent for fluoride and pathogen removal from groundwater.

LEAVES IN IRON OXIDE: REMARKABLE PRESERVATION OF A NEOGENE FLORA FROM NEW CALEDONIA

Abstract A Neogene hematite-goethite concretionary ‘ironstone' horizon in laterized fluvial sediments in the Massif du Sud of New Caledonia yields abundant fossil dicotyledonous angiosperm leaves. The leaves are preserved in iron oxide, mainly goethite, which replicates the morphology and anatomy of the leaf tissues and comprises 73% of the matrix. Organic remains are minimal and associated with aluminosilicate clay. Leaf tissues are preserved three-dimensionally in multiple ways including casts/molds, permineralization/petrifaction, and replacement. Although the mesophyll is less well preserved, reflecting its greater susceptibility to decay, cellular details of vascular and epidermal tissues are commonly evident. Analyses of leaves from an analogous modern setting reveal the early encrustation and impregnation of tissues by amorphous iron-oxides and clays in association with a microbial biofilm. We propose a taphonomic model in which the fossil leaves, like their modern counterparts, were permeated by iron oxides due to the high availability of iron derived from weathering of ultramafic basement. In contrast to the iron-rich aluminosilicate coatings that form in relatively iron-poor settings, the unusually high concentration of dissolved iron oxides permitted rapid anatomical preservation.