Les vermiculites hydroxy-alumineuses du Massif Armoricain (France) approches mineralogique,microanalytique et thermodynamique

Hydroxy-aluminous vermiculites have been identified in samples of soils and weathering products from the Armorican Massif of France by a combination of mineralogic, microanalytic and thermodynamic techniques.The hydroxy-aluminous vermiculites behave originally as intergrades between vermiculites sensu stricto and chlorites. The interlayered aluminum responsible for the intergrade behaviour cannot be removed by means of ammonia oxalate but can be by sodium citrate. After such a treatment the mineral loses its intergrade behaviour and acquires the behaviour of sensu stricto vermiculite. The extracting solution contains 12 mg of aluminum per gramme of clay in the studied sample.Further evidence on the nature of the aluminous vermiculites follows from microanalysis in situ. Such in situ quantitative microanalysis makes it possible to establish the structural formulae of minerals. This technique has been applied to weathering products of micas. The data obtained indicate that these weathering products are vermiculites and that one part of the aluminum must be in interlayer positions.The thermodynamic study of equilibrium between waters and aluminous vermiculites needs to be set in a multidimensional space Rⁿ. In such space the chemical composition of a water is represented by one point. Waters in equilibrium with a given mineral are situated in a (n ? 1) dimension hyperplan. For a given population of waters and minerals a program can calculate water-hyperplan distances. The use of this program shows that one part of the examined waters is compatible with the equilibrium with aluminous vermiculites.