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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 [Fe.sup.3+], some of them in moderate amounts (up to 17% [Fe.sub.2][O.sub.3]) distributed between the tetrahedral and octahedral sheets (maximum values: [sup.IV]Fe = 0.32, [sup.VI]Fe = 0.68, per [O.sub.10][[OH].sub.2]). Octahedral Fe abundance correlates with the presence of molecular water that is lost in a well-defined dehydration event above 200[degrees]C. This water does not cause layer expansion and is interpreted to be present within the pseudo-hexagonal cavity, next to [Fe.sup.3+] ions that generate a local charge imbalance. The presence of octahedral [Fe.sup.3+] is accompanied by vacancies in the octahedral sheet to balance the excess positive charge. Ah infrared band at ~790 [cm.sup.-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 tale, 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. Keywords: Crystal chemistry, infrared spectroscopy, kerolite, talc, talc-smectite, thermal analysis, trioctahedral smectite, X-ray diffraction

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 [E.sub.a] = 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 ([E.sub.a] = 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 ([E.sub.a] = 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 ([E.sub.a] = 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. Keywords: Crystal structure, smectite-water, diffusion, water in smectite interlayer, kinetics, smectite dehydration, mixed-layering, montmorillonite

The dehydration dynamics of the of the Ca-saturated Keywords: Crystal structure, smectite-water, diffusion, water in smectite interlayer, order-disorder, mixed-layering, XRD data, smectite, montmorillonite

A smectite and a rhyolitic obsidian specimen were subjected to hydrothermal alteration for 4 months at 150 [degrees]C in two solutions having different A1[Cl.sub.3] concentrations. The solids were analyzed by means of X-ray diffraction, thermogravimetry, electron microscopy (with microanalysis), and Fourier transform infrared spectroscopy. The solutions were chemically analyzed and the ion activities were plotted on activity diagrams. They are broadly coherent with the reaction products. The smectite specimen produced mixed-layer kaolinite-smectite (K-S), with a higher kaolinite proportion for the solution with higher Al content. The glass specimen produced glass + boehmite + kaolinite in the low-Al solution, and kaolinite in the high-Al solution. Thus, K-S probably forms only from smectite precursors. The formation of K-S follows the same mechanism of stripping of one of the tetrahedral sheets from smectite described for natural specimens by Dudek et al. (2006). Comparison of the synthetic and natural K-S indicates great similarity, although the arrangement of the kaolinite layers in the synthetic specimens is more ordered and some of their kaolinite infrared bands are more intense. The results of Dudek et al. (2006) and this study show a wide range of chemical and structural 'maturity' of kaolinite in K-S, as kaolinization proceeds, that varies depending on the reaction conditions. Keywords: Crystal synthesis, kaolinite, kaolinite-smectite, DTA, TGA, mixed-layering, electron microscopy, IR spectroscopy, XRD data

This study aims to contribute to a better understanding of the nature and evolution mechanism of interstratified clay minerals. We examined the Keywords: Crystal structure, kaolinite-smectite, solid-state transformation, TGA, XRD

We studied mixed-layer illite-smectite (I/S) (< 1 or 0.5 [[micro]meter] size fractions) in 28 diagenetic bentonite samples, Pliocene to Ordovician in age, from different locations in North America and Europe to investigate the smectite-to-illite transformation mechanism. XRD-measured illite contents ranged from 3 to 100%, and layer ordering varied continuously from R = 0 to R [greater than or equal to] 3. XRD polytype analysis showed that with progressive illitization, US changes from a predominantly cis-vacant to a trans-vacant structure, although considerable scatter is present in the data. There is no evolution of illite in the I/S from 1[M.sub.d] to 1M polytypes with progressive illitization. Microprobe chemical analysis and combustion analysis for N showed that [Mathematical Expression Omitted] comprised 9-22% of the total fixed cation content. We calculated an interlayer charge for the illite end-member of 1.5 per [O.sub.20](O[H.sub.4]). Sample morphology was studied by atomic force microscopy, which showed a constant aspect ratio and particle diameter over the whole I/S series. Particle thickness increased with percent illite. Oxygen isotope compositions of the I/S clays did not display any systematic trends. Our results suggest that illitization of bentonite US proceeds via a solid-state mechanism, with local lattice rearrangements that permit structural (octahedral cis/trans occupancy) and chemical changes in 2:1 layers and cause interlayer coalescence.

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 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]