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Abstract Phyllosilicate minerals have the potential to be used as indicators of past environmental conditions and climate. However, the appropriate use of this tool requires that the origin of the phyllosilicates be ascertained to correspond to the age under investigation, as mineralogical changes postdating it would lead to wrong conclusions. The soft sandstone of the Ashdown Formation (south-central and southeaster UK) contains fine-grained Al-phyllosilicates of limited interlayer expandability that have been described by other authors as vermiculite, smectite, interstratified illite-vermiculite and interstratified illite-smectite. The origin of these mineral phases is not yet agreed, with some authors reporting them as detrital and some as authigenic, generated by the weathering of detrital mica in the sandstone. In order to investigate whether or not these fine-grained minerals are detrital, four samples of the sandstone of variable softness were studied because they most likely had different clay content and because the different clay content could be possibly due to differential weathering. Investigation of mineralogy (X-ray diffraction), chemistry (bulk and SEM-EDS microanalysis) and rock fabric (image analysis of SEM micrographs) indicated that the sandstone consists of quartz, microcline, albite, anorthite, muscovite/illite, kaolinite, interstratified illite-smectite and smectite, where smectite layers expand only partially. Kaolinite is mainly detrital. Rock hardness was controlled by quartz abundance in the original sediment, not by later weathering. Alteration of the investigated sandstone samples after deposition was limited but reaction models are compatible with partial (<10 wt%) alteration of muscovite/illite to illite-smectite and smectite, and of microcline and plagioclase to kaolinite. Analysis of mineralogical data of the fraction <2 μm of the Ashdown Formation from the literature is compatible with this interpretation and indicates that the range of muscovite/illite alteration to interstratified illite-smectite and smectite varied widely (0–45% in the <2 μm size fraction) depending on location and depth. Highlights • Hardness of Sandstone from the Ashdown Formation (UK) follows quartz/clay ratio. • Quartz/clay ratio changes are controlled first by sedimentation, then by diagenesis. • The sandstone contains expandable clays of disputed origin. • Mineralogy and reaction models suggest that the expandable clays are authigenic. [ABSTRACT FROM AUTHOR]

Abstract: Seafloor hydrothermal sites generate abundant Mg- and Fe-rich clays. These clays are structurally and compositionally interesting because these environments are characterized by large, dynamic temperature and chemical gradients in their deposition environment, which promote the formation of chemically and structurally complex clays, including interstratified phases. The system is also interesting as a proxy for the study of the large Mg- and Fe-rich phyllosilicate deposits on Mars, which are broadly characterized as smectitic clay of hydrothermal, volcanic or sedimentary origin. Thirty submarine samples and four terrestrial ones, for comparison, were studied by means of X-ray diffraction (XRD), thermogravimetry (TG), mid-IR and Mössbauer spectroscopies and chemical analysis. The samples include nontronite and the mixed-layer phases glauconite–nontronite, talc–nontronite and talc–saponite. Some of the talc–saponite samples have Fe contents well above those typical for these Mg-rich, trioctahedral phases (up to 1.69 Fe per O10[OH]2, in the tetrahedral and octahedral sheets). Tetrahedral Fe ranges from 0 to 0.66atoms per O10[OH]2 across the samples. As found in previous studies of similar specimens, Fe promotes the retention of molecular water that is released upon heating above 200C, and is mainly emplaced in non-expandable layers (talc and glauconite layers). In talc–nontronite and talc–saponite octahedral Fe (both di- and trivalent) appears to be bound to this trapped molecular water, whereas in glauconite–nontronite the bond appears to be with tetrahedral Fe. Samples typically show more than one dehydroxylation event in the TG analysis. The weight loss at each dehydroxylation event is broadly consistent with the proportion of individual layers as determined by means of XRD, but there is no good correlation between both. By contrast, the weight loss at each dehydroxylation event correlates with the chemistry of the layers, where certain cations promote chemical domains in the octahedral sheet (e.g., trioctahedral, nontronite-like, and montmorillonite-like) that dehydroxylate at the several temperatures. The correlations found for talc–nontronite and glauconite–nontronite samples suggest that the distribution of cations in the octahedral sheets of most, but not all, samples is intermediate between total dispersion and total segregation, perhaps random. The talc–nontronite samples have talc layers with cation-deficient octahedral sheets. The above results are best explained by layers of polar character, where the octahedral sheets are composed of domains with different chemical characteristics corresponding to the two types of layers in the interstratified clay. The existence of molecular water (bands at 1635 and 3300–3500cm−1, or ~6.1 and 2.9–3.1μm, respectively) bound in non-expanding layers is relevant to the mineral characterization of Martian Fe-rich clays with spectroscopic methods, where molecular water is so far assumed to be linked to smectite. The variety of mixed-layer clays in which Fe is found in the submarine samples suggests that mixed-layering may be important in Martian clays, which are spectroscopically variable but generally Fe-rich. [Copyright &y& Elsevier]

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 extent of natural weathering of phiogopite crystals in a karstic environment and the composition of its weathering products have been examined through a multi-method investigation carried out on a volcanoclastic deposit discovered in the Grotta del Cervo cave, Pietrasecca, central Italy. Phlogopite crystals are a mixture of 1M and 2M1 polytypes and show mixed- layering with a low amount (1-4%) of smectite in the glass groundmass. Phiogopite crystals vary in composition from phlogopite to ferroan phiogopite. Their average structural formula is (K0.92Na0.03Ba0.03Ca0.02) (Mg2.05Al0.01Fe2+0.19Fe3+0.04Mn0.01Ti0.15 0.19 (Si2.63Al1.37)O10.16(OH1.57F0.27). Micromorphological, X-ray diffraction (XRD), Fourier Transform Infrared (FTIR), scanning electron microscopy (SEM), and microprobe analyses (EMPA) showed that the mineralogical evolution of phiogopite crystals is consistent with in-situ pseudomor- phic weathering of mica to Al-rich dlioctahedral (rather than Mg-rich) clay minerals (beideffite-like) via phlogopite-smectite mixed-layer phases. The main mechanism of transformation is a layer-by-layer modification in which water composition, enriched in Si and Al from glass dissolution, is a major control. Mg loss, Ca and Na exchange for K, enrichment in Si and Al are the main processes required for beidellitization of phiogopite. Glass alteration produced sequentially smectite, chabazite, phillipsite and analcime, as leaching progressed and the interstitial fluids became increasingly alkaline. The type of zeolite found in the several samples (in the chabazite, phillipsite, analcime series) indicates that the alteration extent increases from wall to the ceiling and to the floor of the cave. [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]

Abstract: Amino acid adsorption on smectite is relevant to prebiotic processes involving possible catalytic reactions in the early Solar System, as implied by the clay-organic correlation found in meteorites, and the generation and modification of organic components essential for the origin of life. Here we report the results of a study investigating the adsorption of l-lysine (0.025–0.4M) onto montmorillonite. The reaction products were studied using X-ray diffraction, chemical analysis and infrared spectroscopy. We find that lysine adsorption is first dominated by cation exchange and then by adsorption of electrically neutral lysine (as a zwitterion), as indicated by chemical and FTIR evidence. At the maximum concentration, lysine displaces only ∼1/3 of the original interlayer cations. The d-spacing of the smectite–lysine complex increases from 1.2 to 2.1nm as more lysine enters the interlayer space, and water is expelled. We propose a structural model of lysine sorption in the interlayer in which lysine is oriented at >45–90° to the plane of siloxane O atoms. [Copyright &y& Elsevier]

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 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]

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]