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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