DETAILED STUDY OF SMECTITE GENESIS IN GRANITIC SAPROLITES BY ANALYTICAL ELECTRON-MICROSCOPY

Smectite genesis in two granitic saprolites was studied, using qualita tive and quantitative mineralogical methods and high resolution transm ission electron microscopy (HRTEM). The primary minerals (plagioclase and mica) give rise to the formation of different kinds of smectites a ccording to the weathering conditions (pH and drainage). Under a relat ively basic, drained environment, a neoformed montmorillonite with a s pheroidal organization exists in fillings of plagioclase cores. In the same conditions biotites are transformed into beidellites with a quas i-crystal organization. Under hydromorphic conditions, muscovites evol ve incompletely towards smectites, whereas biotites lead to a montmori llonite with a tactoid organization.