EXPERIMENTAL-STUDY OF HYDROTHERMAL ALTERATIONS OF MG, MG-FE, AND CA-MG SILICATES

Asbestiform band silicates containing various anionic chain groups (al kali triple-chain silicates and various double-chain amphiboles) and s heet silicates (micas: Na-mica and KMgIV-phlogopite) are hydrothermall y prepared from native minerals pyroxenes, olivines, talc, serpentine, and sepiolite with NaOH and KOH solutions at elevated temperatures an d pressures. The parameters and fields of crystallization of these sil icates are determined. For alkaline media, the laws guiding the format ion of band silicates are established: the extent of polymerization of their anionic complexes decreases with increasing temperature, alkali nity of hydrothermal solutions, or iron and calcium abundances of the starting minerals. In mica synthesis, temperature elevation or an incr ease in alkalinity favors the formation of monomineral perfect crystal phases. High-resolution electron microscopy was applied to elucidate the mechanism of formation of band silicates and to reveal the sequenc e of the structural transformation of initial minerals via a series of intermediate mixed-chain banded structures.