Altération des biotites dans les arènes des pays tempérés, tropicaux et équatoriaux

The study of the weathering of biotites from different origins (France, Africa and Malagasy) shows two ways of evolution of this mineral. The first way leads to kaolinite with an intermediate stage of vermiculite, more or less ephemeral. This degradation takes place in a leached and well drained environment usually typical of humid tropical regions. If drainage becomes excessive, the final product of the evolution of biotite can be gibbsite. Under humid and cool temperate climates, the intermediate stage of vermiculite is prolonged. The second way leads to montmorillonite. This transformation, pointed out under arid climates, can take place only in the environments where solutions are rich in silica. Some approaches of structural formula calculations show that, during this transformation, aluminium migrates from tetrahedral sites towards octahedral ones and that silica can occupy the vacant tetrahedral sites.