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Keywords: Phyllosilicates; glauconite; nontronite; peak shift EN Phyllosilicates glauconite nontronite peak shift 1080 1090 11 06/01/22 20220601 NES 220601 Introduction Raman spectroscopy found widespread application in the rapid and non-destructive characterization of silicate minerals ([26]; [8]; [35]; [34]; [44]; [31]; [1]; [45], 2015; [30]). Concluding remarks In this study, Raman spectroscopy analysis of nontronite and glauconite-nontronite mixed-layer phases from deep-sea hydrothermal fields reveals that these phyllosilicates have diagnostic Raman spectral features that include a series of strong peaks in the low wavenumber region (<700 cm SP -1 sp ) and the hydroxyl domain (3500-3700 cm SP -1 sp ). To fill these knowledge gaps, this study reports a comprehensive Raman spectral characterization of nontronite and glauconite-nontronite mixed-layer phases (i.e., vertical mixing/interstratification of glauconite and nontronite layers) from seafloor hydrothermal fields. Chemistry/structure-dependent Raman spectral features of glauconite and nontronite For the analyzed nontronite and glauconite-nontronite mixed-layer samples, the covariations between Raman spectral properties and octahedral chemistry (Table 2; Figs. [Extracted from the article]

Fe(II) only occupies octahedral sites in phyllosilicates, whereas Fe(III) can occupy both octahedral and tetrahedral sites. The controls on Fe(III) distribution between tetrahedral and octahedral sites have been a matter of great interest to understand the interplay between formation environment (Fe abundance, redox conditions) and crystal-chemical factors (stability of the crystal lattice) during crystallization of Fe-phyllosilicates. Here, for the first time, we present a model of Fe(III) distribution in 2:1 phyllosilicates. We investigated 21 samples of 2:1 phyllosilicates of submarine hydrothermal origin using XRD, chemical analysis, and Mössbauer spectroscopy (and other supporting techniques not presented here). An additional data set of 49 analyses of 2:1 phyllosilicates from the literature was also used. Overall, the data cover a wide range of dioctahedral and trioctahedral phyllosilicates, including end-member minerals and interstratified phases. Dioctahedral phyllosilicates have a steric control whereby tetrahedral Fe(III) is only allowed if at least five out of six octahedral atoms are larger than Al (typically Fe[III], Fe[II], Mg) that produces an expanded structure where tetrahedral sites can accommodate Fe(III). After this threshold, further Fe(III) atoms occupy tetrahedral sites preferentially (~73% of further Fe[III] atoms) over octahedral sites. In trioctahedral 2:1 phyllosilicates there is no steric hindrance to tetrahedral Fe(III) because the crystal dimensions are such that tetrahedral sites can accommodate Fe(III). On average, Fe(III) enters tetrahedral and octahedral sites in similar proportion, and the only apparent control on tetrahedral Fe(III) abundance is Fe(III) availability during crystallization. This model allows to predict Fe(III) distribution between structural sites, provides an avenue for further exploration of the thermodynamic stability of phyllosilicates using cationic size, and provides a tool to better describe stability/reactivity of Fe-rich phyllosilicates, the most reactive of phyllosilicates and very relevant in geochemical and biological processes. [ABSTRACT FROM AUTHOR]

Earth analogs are indispensable to investigate mineral assemblages on Mars because they enable detailed analysis of spectroscopic data from Mars and aid environmental interpretation. Samples from four sites in the Iberian Pyrite Belt (El Villar, Calañas, Quebrantahuesos, and Tharsis) were investigated using mineralogical, chemical, and spectroscopic techniques, with a focus on clay minerals and alteration environments. They represent Earth analogs of areas on Mars that underwent acidic alteration. X‑ray diffraction and transmittance mid-infrared data indicate that the rocks were subjected to several degrees of acid alteration corresponding to assemblages characterized by the following mixtures: (1) illite, chlorite, interstratified chlorite-vermiculite, kaolinite-smectite, and kaolinite; (2) illite, kaolinite, and alunite; and (3) jarosite and goethite. According to mineral stability data, these three assemblages correspond to pH values 7–5, 5–3, and <3, respectively. The lack of goethite in the illite-kaolinitealunite assemblage suggests an alteration in reducing conditions. Illite was progressively dissolved by acidic alteration but is sufficiently resilient not to be diagnostic of the intensity of the alteration. Illite and kaolinite were the two most abundant phyllosilicate minerals observed, and the main reaction involving phyllosilicates was the alteration of illite to kaolinite. Mixed-layer phases appeared mainly in the mildest degree of acid alteration, with few exceptions. This suggests a transition from a mechanism dominated by transformation to a mechanism dominated by dissolution-precipitation as the intensity of the acid alteration increases. Our results highlight the sparse kaolinite–alunite occurrences on Mars as worthy of specific investigation. Acid alteration on Mars is expected to be patchy and/or consisting of fine alteration rims. Alunite occurrences on Mars in the absence of goethite may indicate an acid alteration in reducing conditions. Kaolinite produced through acid alteration on Mars is expected to exist mainly as an end-member phase of low crystallinity, which would enhance IR absorption and increase its visibility. [ABSTRACT FROM AUTHOR]

Mineral weathering processes in soils are important controls on soil characteristics and on bio- and geochemical cycling. Elucidation of these processes and their mechanisms is crucial for understanding soil environments and their influence globally. An Umbric Podzol from the Falkland Islands was studied while investigating possible ways to counteract soil degradation and loss. The soil had lost the O, E, and Bs horizons through erosion, thus revealing the transitional B/C horizon, which grades into the underlying parent material. Samples were taken from the B/C surface and 5 cm below the surface, then analyzed with X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray spectroscopy, organic C and N analysis, analysis of extractable Fe and Al with the dithionitecitrate- bicarbonate and ammonium oxalate methods, and Fourier-transform infrared analysis. The soil fabric and mineralogy were compatible with derivation from sandstone rock. Clasts of heterogeneous mineral composition as well as loose material from disaggregated clasts were present. The soil had large proportions of quartz and albite, and minor amounts of muscovite, chlorite, plagioclase, feldspar, kaolinite, and non-diffracting Fe oxide (goethite and/or ferrihydrite). The most peculiar characteristic was a large component (~7 wt% of the bulk soil) of an amorphous (non-X-ray diffracting) silicate phase of small particle size (<1 µm), non-extractable, with heterogeneous composition. The average composition of this phase is similar to that of the bulk soil and approaches that of Al-Fe-rich smectite. The amorphous phase is not allophane or imogolite by any of the analyses carried out. The amorphous silicate phase is formed partly by the translocation of metals from O, E, and Bs horizons and partly by dissolution of the primary minerals of the B/C horizon, both of which precipitated in combination with low water mobility causing rapid saturation of the interstitial water. There are no reports of amorphous silicate phases with these characteristics or abundances from soils or other weathering environments. Thus, our observations indicate the existence of complex, successive weathering steps not yet identified that could be investigated in materials subjected to slow weathering such as the soil described here. [ABSTRACT FROM AUTHOR]

With the arrival of Curiosity on Mars, the MSL has started its ground validation of some of the phyllosilicate characterization carried out with remote sensing near-IR spectroscopy from orbital instruments. However, given the limited range of action of the rover, phyllosilicate identification and characterization will have to rely mainly on orbital near-IR data. Investigation of Earth analogs can greatly assist interpretation of martian spectra and enable more robust analyses. In this contribution, Mg/Fe-rich clays from submarine hydrothermal origin that had been thoroughly characterized previously were investigated with near-IR reflectance spectroscopy. The clays are mixed-layer glauconite-nontronite, talc-nontronite, talc-saponite, and nontronite samples. The hydroxyl bands in the range 2.1-2.35 μm were decomposed into their several individual components to investigate correlations between the octahedral chemistry of the samples and the normalized intensity of several bands. Good correlations were found for the samples of exclusive dioctahedral character (glauconite-nontronite and nontronite), whereas poor or no correlations emerged for the samples with one (talc-nontronite) or two (talc-saponite) trioctahedral layer components, indicating a more complex spectral response. Because these bands analyzed are a combination of the fundamental OH stretching and OH bending vibrations, the response of these fundamental bands to octahedral chemistry was considered. For 2:1 dioctahedral phyllosilicates, Fe and Mg substitution for Al displaces both fundamental bands to lower wavenumbers (longer wavelengths), so that their effect on the position of the combination band is coherent. In contrast, for trioctahedral clays, Al and Fe3+ substitution of octahedral Mg displaces the OH stretching band to lower wavenumber values, and the OH bending band to higher wavenumber values, resulting in partial or total mutual cancelation of their effects. As a result, clays with near-IR spectra indicating Mg-dominated octahedral compositions may in fact contain abundant Fe and some Al substitution. Thus, remote-sensing near-IR mineralogical and chemical identification of clays on Mars appears relatively straightforward for dioctahedral clay minerals but more problematic for trioctahedral clays, for which it may require a more detailed investigation of their near-IR spectra. [ABSTRACT FROM AUTHOR]

Soil formation occurs through numerous physical and chemical weathering processes acting to alter the parent rock on the Earth's surface. Samples of surface soils were collected over a range of elevations (2000-3600 m) from profiles directly overlying basaltic to more felsic parent rocks, over a region in NW Ethiopia. The soils were investigated to determine their chemical composition and X-ray diffraction was used to identify and quantify individual mineral phases. The data set was analyzed using non-parametric statistics (Spearman's Rank and Mann-Whitney U tests) to compare the soils forming over the two parent rocks. Principal component analysis (PCA) was used to identify the mineral alteration assemblage and formation during pedogenesis. The extent of alteration was quantified using several chemical weathering indices (Chemical Index of Alteration = CIA; Chemical Index of Weathering = CIW), including an index calculated by multivariate analyses of the soil chemical composition data (weathering "W" index). Further to this we devised and tested a new weathering index (Wmin) using multivariate analysis of the soil mineralogy, to estimate the extent of weathering and physico-chemical proprieties of the parent rock from which the soil formed. The soils present a fair to advanced stage of alteration, with abundant iron (Fe) oxides (up to 40 wt%) and phyllosilicates (up to 57 wt%), including kaolinite-smectite (K-S) mixed-layer phases. The K-S was composed of either 30-50% kaolinite or 94-98% kaolinite layers. Discrete kaolinite was also present. The bimodal K-S mineralogical composition is likely due to two precursor phases: feldspar for the kaolinite-rich K-S and volcanic glass for the smectite-rich K-S. K-S with intermediate composition (50-94% kaolinite) was rare, due to its instability. Statistical analysis showed significant differences between the chemical compositions of the soils developed on the two different parent volcanic compositions. The soils overlying the more felsic parent rocks were less altered than those overlying the flood basalt. When comparing the weathering indices calculated in this study, we conclude that while the CIA and CIW may be more readily determined, the W and Wmin indices can elucidate information on the composition of the original rock from which they formed. The W index is more sensitive to certain variables when compared with the newly derived mineralogical Wmin index; however the Wmin index takes into account mineral phases within the sample, which provides a more detailed interpretation of weathering rates than chemistry alone. In addition the Wmin index correlated with meteorological variables, such as elevation (and consequently temperature and precipitation), known to influence the degree of pedogenesis. The Wmin index can be used to enhance our understanding of the processes that occur during weathering processes to supplement information gained from traditional chemical weathering indices. [ABSTRACT FROM AUTHOR]

Biological activity plays a substantial role in the geochemistry of the Earth's surface. Particularly interesting are effects on clay formation because clays are abundant and have high surface-to-volume ratio, resulting in clays making up a large fraction of the overall mineral-fluid interface and having an effective control of mineral reactions. Thus, biological control on clay composition would affect element budget globally and the mineralogy of subsequent diagenetic processes. Biological acceleration of clay production would result in enhanced clay control of mineral reactions and faster organic C sequestration, by adsorption on clay minerals, with implications for the C and related cycles. We investigated the combined effect of microbial activity and water chemistry on the composition of neoformed clay by reacting volcanic glass with natural waters covering a large composition range (fresh water from a lake and a spring, seawater, and hypersaline water). The microbes (bacteria, fungi, and algae) were totally or partially identified using molecular and microscopy techniques. The solid alteration products were analyzed using cryo-SEM to investigate the mineral-microbe interface and TEM-AEM to study the composition of the neoformed clay. The solution chemistry was also investigated. We found that clay composition was controlled mainly by glass chemistry, rather than biological activity, through a mechanism of in situ transformation. The resulting clay was Al-rich (dioctahedral composition). In one case (inorganic experiment, freshwater lake), the specific inorganic conditions of pH and Mg and Si concentration promoted formation of Mg-rich (trioctahedral clay). Microbes, however, did influence clay composition by confining glass grains in biofilms where water chemistry is significantly different from the bulk solution. Alteration in such conditions generated significant amounts of trioctahedral, Mg-rich clay in the hypersaline water experiment, whereas it favored production of dioctahedral, Al-rich clay in the freshwater lake experiment. It is thus demonstrated that biofilms can exert an effective control on clay mineralogy. [ABSTRACT FROM AUTHOR]

The hydrothermal reactivity of the <1 µm fraction, K-saturated SWy-2 Wyoming low-charge montmorillonite was studied in the 250-400 °C temperature range with reactions lasting between 5 and 120 days, with a solid/solution mass ratio of 1:10, and in 1 mol/L KCl solution. From X-ray diffraction (XRD) profile modeling results on K-saturated and ethylene-glycol solvated samples the illitization process appears to occur as a progressive replacement of expandable layers by layers with illitic behavior, in a single illite-smectite phase. However this treatment overestimates the amount of illite layers because of the presence of smectitic non-expandable layers. This was revealed by calcium exchange of the products, which causes re-expansion of the apparent illite layers. The illitization model then obtained consists of four phases with increasing illite content: discrete smectite, a randomly interstratified mixed-layered mineral (MLM) and two ordered MLMs that progressively replace the phases with less illite content. This polyphase model is consistent with results from transmission electron microscopy and chemical microanalysis, which show several types of particle morphology with different interlayer K content and Al-for-Si substitution that seem to correspond to the several phases detected by XRD. Thus, the actual illitization mechanism of smectite in our experiments is not a progressive reaction but a dissolution-precipitation process following the Ostwald step rule in which metastable smectite transforms into illite through a series of metastable illite-smectite phases. [ABSTRACT FROM AUTHOR]

The model of Fe3+ distribution between octahedra and tetrahedra in dioctahedral smectites by Decarreau and Petit (2014) used data from infrared analysis. From their own and other general evidence, the resulting data are likely to be affected by significant uncertainty. This aside, their model has limited application because it is based on synthetic smectites containing only Si, Al, and Fe3+. [ABSTRACT FROM AUTHOR]

Clay samples of hydrothermal origin from several oceanic spreading centers were studied using XRD, microprobe, infrared, and thermal analysis. They are talc, smectite, and mixed-layer talc-smectite (T-S) where the talc layers have several degrees of crystalline order. The smectite is trioctahedral in most cases, but there is also dioctahedral smectite both as mixed-layer and as a separate phase. All specimens contain Fe3+, some of them in moderate amounts (up to 17% Fe2O3) distributed between the tetrahedral and octahedral sheets (maximum values: IVFe = 0.32, VIFe = 0.68, per O10[OH]2). Octahedral Fe abundance correlates with the presence of molecular water that is lost in a well-defined dehydration event above 200 °C. This water does not cause layer expansion and is interpreted to be present within the pseudo-hexagonal cavity, next to Fe3+ ions that generate a local charge imbalance. The presence of octahedral Fe3+ is accompanied by vacancies in the octahedral sheet to balance the excess positive charge. An infrared band at ~790 cm-1 is assigned to OH bending in the group Fe-Mg-〉-OH. Analysis of this band suggests a range of short-range Fe-〉 distributions, from random to ordered. Our sequence talc, T-S, trioctahedral smectite is defined by an increasing Al for Si substitution in the tetrahedra and increasing crystal disorder. The presence of Fe also causes crystal defects. This mixed-layer series can be considered as a continuum generated by the combination of chemical and crystal defect variability. Kerolite was used to designate disordered, hydrated talc. We find that there is no clear line delimiting talc from kerolite as a single phase or in mixed-layer minerals and that it is better to use a descriptive term for the latter such as "disordered talc." Dioctahedral smectite is also a possible end-member of the mixed-layer sequence, which implies an Al + 〉 for Mg substitution in the octahedral sheet. If T-S consists of polar TOT layers, the existence of dioctahedral smectite in T-S raises the question of the actual composition of the octahedral sheets within polar TOT layers. [ABSTRACT FROM AUTHOR]

Highlights a study which investigated the smectite-to-illite transformation mechanism through an analysis of mixed-layer illite-smectite from different locations in North America and Europe. Significance of this transformation mechanism; Details on the samples which were studied; Details on optical microscopy, scanning electron microscopy and X-ray diffraction of bulk rocks; Information on atomic force microscope analysis; Results of the study.

The Curiosity rover on Mars, landed in 2012, is capable of mineralogical investigation using X-ray diffraction, complementing the abundant infrared remote sensing data already available on clay minerals. We can, however, expect that the in situ X-ray diffraction information will convey a more complex picture than that inferred from infrared spectroscopy alone. CheMin has landed. [ABSTRACT FROM AUTHOR]

The mechanism of kaolinite transformation into dickite has been investigating using 13 samples from the Frøy and Rind oil fields (Broad Fourteens basin, North Sea), 3 kaolinite specimens with different crystal order (KGa-2, Kaolinite API 17, Keokuk kaolinite), and 2 dickite-rich samples (Natural History Museum collection). Detailed analysis of XRD, thermal analysis, and SEM data show that: (1) as dickite content increases, there is also an increase of the crystal order of kaolinite; (2) in dickite-rich specimens kaolinite and dickite have crystals (or XRD-coherent domains) of the same size; (3) there is no specific dehydroxylation temperature for each polytype, rather particle size and crystal order control dehydroxylation temperature independently of polytype; (4) with progressive dickite content, the development of both particle size and the size of the coherent crystal domains within particles is greater in the c direction than in the a-b plane; (5) the growth of defect-free segments in the c direction is not connected with the growth in the a and b directions, as would be expected in crystallization from solution; (6) textural features indicate coalescence of kaolin plates with burial; (7) there is a very weak positive correlation between particle dimensions and relative kaolinite-dickite content. These results are interpreted as resulting from a double reaction taking place in the solid state with burial. Some kaolinite domains grow in size and crystal order while other domains are transformed into dickite. Presumably, also the dickite domains formed early in the transformation grow in crystal order. The transformation into dickite stops at 90-95% dickite because the remaining kaolinite domains are so large and stable that the stability increase produced by the polytype transformation would be negligible. [ABSTRACT FROM AUTHOR]

The dehydration dynamics of the of the Ca-saturated <1 µm size fraction of SWy-1 (low-charge montmorillonite) were studied at the crystal scale under isothermal conditions using X-ray diffraction with a position-sensitive detector (XRD-PSD) in the 30.170 °C temperature range. A total of 630 XRD patterns were modeled between 30 and 125 °C using a trial-and-error approach based on the direct comparison of experimental and calculated XRD patterns. The proportion of layers with different hydration states (bihydrated, mono-hydrated, and dehydrated) were determined in the temperature-time space as well as small variations of layer thickness within each hydration state. The results showed that dehydration produces complex structures with heterogeneous hydration states, some of which are stable (not transient) and remain at the end of the experiment. The evolution of other structural parameters (interlayer water content, layer thickness fluctuation) was consistent with previous reports of smectite hydration. For bihydrated layers, the amount of water molecules per interlayer cation indicated the presence of water molecules both coordinated and non-coordinated to the interlayer cation. The transition from bi- to mono-hydrated layers produced the maximum structural heterogeneity, with (1) strong interlayer thickness fluctuation (in individual layers), and (2) the presence of several elementary mixed-layer structures. In contrast, the transition from mono-hydrated to dehydrated layers occurs homogeneously within layers. Finally, the decrease in thickness of mono-hydrated layers only implied the removal of some water molecules forming the hydration shell of the interlayer cation. [ABSTRACT FROM AUTHOR]

A kinetic study of Ca-montmorillonite dehydration was performed based on information derived from X-ray diffraction (Ferrage et al. 2007, this issue) and, thus, focusing on interlayer water only. The dehydration was quantified following the two processes that were observed in the X-ray pattern modeling: the transitions between the different hydration states and small thickness decrease observed in the bi- and mono-hydrated layers. The thickness decrease of bihydrated layers with dehydration (activation energy Ea = 16 kJ/mol) was found to be controlled by a mechanism of two-dimensional diffusion of water molecules through the interlayer space, whereas for mono-hydrated layers the variation of thickness (Ea = 18 kJ/mol) occurred as a mechanism of slight local layer collapse and collapse propagation, attributed to a rearrangement of the configuration of the interlayer cation hydration shell. For the transition between the bi- and mono-hydrated state (Ea = 84 kJ/mol), the mechanism of reaction was found to evolve gradually with increasing temperature from local layer collapse and collapse propagation to a two-dimensional diffusion mechanism, as the forced diffusion of water molecules produced by the layer collapse transfers the control of the process to diffusion mechanism. This phenomenon causes the coexistence of two hydration states in a given interlayer. Finally, the transition between mono-hydrated and dehydrated layers (Ea = 132 kJ/mol) indicated the concomitance of water diffusion and local layer collapse and propagation mechanisms, although the structures were found to be homogeneous during this transition. The determination of both mechanisms and the activation energy for these processes were used to establish a model of smectite dehydration at the crystal scale. This model can be used to calculate crystal shrinkage and interlayer water content upon dehydration, and to predict the evolution of the system. [ABSTRACT FROM AUTHOR]

This study aims to contribute to a better understanding of the nature and evolution mechanism of interstratified clay minerals. We examined the <2µm or <0.2µm size fraction of interstratified kaolinite- smectite (K-S) formed by hydrothermal and hydrogenic alteration of volcanogenic material from a Tortonian clay deposit (Almería, Spain), a weathered Eocene volcanic ash (Yucatan, Mexico), and a weathered Jurassic bentonite (Northamptonshire, England). The methods used were X-ray diffraction analysis (XRD) of random and oriented preparations, thermogravimetry, chemical analysis, and 29Si MAS nuclear magnetic resonance. The proportions of kaolinite and smectite in K-S (%K) were determined by fitting the XRD patterns of ethylene-glycol-saturated samples with patterns calculated with the NEWMOD computer program. The obtained range of compositions is 0.85%K. A comparison of the results from the various techniques showed non-linear relationships, indicating that the layers in K-S are complex and hybrid in nature. The smectite-to-kaolinite reaction is a solid-state transformation proceeding through formation of kaolinite-like patches within the smectite layers. The process consists of several non-simultaneous stages: (1) removal of parts of the tetrahedral sheet, resulting in formation of kaolinite-like patches; (2) layer collapse to ∼7 Å where the kaolinite-like patches are sufficiently large; (3) Al for Mg substitution in the octahedral sheet, simultaneous or slightly delayed with respect to layer collapse, causing a layer-charge decrease and loss of interlayer cations; (4) Si for Al replacement in the tetrahedral sheet and further loss of interlayer cations. Iron remains in the kaolinite or is lost at the latest stages of the process. [ABSTRACT FROM AUTHOR]