Mécanismes de croissance, polytypisme et polymorphisme de la muscovite hydroxylée synthétique

Abstract

Synthetic OH-muscovite has been hydrothermally synthesized at T = 600 °C and 460 °C under 1000 bar water pressure using different starting mixtures. A systematic microtopographic study of basal faces of micron-sized crystals has been performed by means of replica techniques of conventional electron microscopy. Statistically, growth is found to proceed first through nucleation and layer-by-layer growth processes, subsequently followed by a screw dislocation mechanism. Careful spiral growth pattern studies lead to the determination of the different stacking sequences involved in the exposed ledges of the screw dislocations, i.e. of both periodicities and stacking schemes of the spiral grown polytypes. A pre-ordering of 1M, 2M 1, 3T[222] and 3T[ 2 2 2 ] type is found to occur first during layer-growth before any screw dislocation appears in these crystals, thereby indicating that common polymorphic ordering is muscovite is occuring rather during perfect growth than during dislocation growth. On crystals of 2M 1 initial structure, spirals are more frequently reproducing the 2M 1 structure than generating new polytypes like 3Tc[02 2 ] and 5Tc[0( 22 ) 2]. Structural determinations of polymorphs by means of the microtopographic method are in good agreement with powder X-ray identifications. In the last part of this paper, the nature of stacking faults in 1M and 2M 1 sequences, the role of the stacking sequence on the crystal habit, as well as the nature of growth twins in synthetic muscovite are emphasized. With the present experimental data in mind, the earlier reported growth theories on mica polymorphism and polytypism are discussed.