Textural and hydration properties of a synthetic montmorillonite compared with a natural Na-exchanged clay analogue.

International audienceAn aluminous montmorillonite has been synthesized at hydrothermal conditions (623 K, 120 MPa) and compared to the natural Na-exchanged Wyoming montmorillonite (Na-SWy2). The synthetic product is characterized by a high cation exchange capacity (83 meq/100g) and ethylene glycol specific surface area (764 m2/g) similar to Na-SWy2 but with a higher purity with one crystalline phase. Compacted smectite samples were percolated with water by using an œdometer cell equipped with a separated injection system, in order to simulate the behaviour of an engineered clay barrier in the context of subsurface waste landfill. Percolation experiments showed very low hydraulic conductivities (10-12 m/s) for both synthetic and natural smectites, in addition to the non macroscopic swelling of the synthetic montmorillonite, in contrast to Na-SWy2. At the nanometric scale, this synthetic clay mineral is characterized by higher hydration states and a better organization in the distribution of water molecules in particles, as proved by X-ray diffraction technique and continuous water vapor gravimetry method. Moreover, adsorption-desorption isotherms of nitrogen revealed the classical feature of swelling clay minerals, and the Brunauer-Emmet-Teller calculation gives a nitrogen specific area of 87 m2/g for the synthetic smectite, twice higher than for the Wyoming clay, due to its higher mesoporosity. Finally, low-pressure argon adsorption coupled with the Derivative Isotherm Simulation procedure highlighted classical adsorption energy distributions of natural phyllosilicates, and thus the synthetic montmorillonite was characterized by an average of 11 layers per particle in the dry state, with basal and edge surface areas different from Na-SWy2

Geological context of the Boumnyebel talcschists (Cameroun): Inferences on the Pan-African Belt of Central Africa Le contexte géologique des talcschistes de Boumnyebel (Cameroun) : implications pour la chaîne panafricaine d'Afrique centrale

International audienceThe talcschists of the Boumnyebel area (southern Cameroon) form ≤ 30 m thick discontinuous layers within a Pan-African nappe unit (Yaoundé group), which includes, at the base, muscovite + biotite ± garnet micaschists associated with amphibolites and pyroxenites, and, at the top, muscovite + biotite + garnet + kyanite micaschists locally associated with marble and amphibolites. The metamorphic peak (not, vert, similar650 °C/9.5 kbar; ca. 620 Ma) postdates nappe emplacement. Isograds are in normal position, micaschists passing downwards to migmatites in the northwestern part of the area studied. The rock types in the lower part of this nappe suggest active margin environments with detrital input from a nearby continental crust (arc or back-arc context)

Evolution of product phase assemblages during thermal decomposition of muscovite under strong disequilibrium conditions

http://www.minsocam.org/MSA/AmMin/TOC/Abstracts/2006_Abstracts/FM06_Abstracts/Devineau_p413_06.pdfWe investigated the thermal decomposition of muscovite in natural granite powders heated to 1175°C for durations from 5 min to 68 h, at 1 bar, paying special attention to the early stages of decomposition. This study shows that muscovite is completely transformed after 5 min. Muscovite pseudomorphs consist of glass, mullite, and Al-rich oxides. For short durations (5 and 40 min), the Al-rich phase was identiÞ ed by XRD, electron diffraction, and TEM microanalysis as γ-Al2O3 containing 4.8 wt% FeO (total Fe), probably a few weight percents of MgO, and possibly up to 10 wt% SiO2. Faint superstructure spots and diffuse streaks observed in electron-diffraction patterns suggest vacancy or trace elements ordering in the γ-Al2O3 defect spinel structure. γ-Al2O3 displays an unexpected acicular morphology, elongated along three directions at 120° in the basal (001)musc planes and parallel to lateral faces of the former muscovite. Mullite forms rods elongated in the basal (001)musc planes along a direction at 90° from one set of γ-Al2O3 needles. The γ-Al2O3 structure appears to be a metastable phase that is replaced by corundum for longer durations

Étude des modifications des propriétés du talc et de la chlorite par traitement thermique

Not availableLes conséquences d'un traitement thermique entre 400 et 1100°C sur les propriétés du talc et de la chlorite du gisement de Trimouns (Pyrénées, France) ont été étudiées par analyse thermique, diffraction des rayons X, spectroscopie IRTF, UV-visible, RPE, RMN, et par adsorption de gaz. Ces analyses ont permis de déterminer les mécanismes de réaction de la chlorite, les propriétés physico-chimiques des poudres calcinées aux différentes températures ainsi que les modifications de comportement en présence de minéralisateur