Crystallization in Complex Silicate and Germanate Systems

Abstract

It has been shown [1–3] that existing methods of producing single crystals of silicates and germanates must be based on processes that allow one to control the rates and extents of association (condensation) of the silicon—oxygen and germanium—oxygen radicals. The most effective means involves managing the spatial distribution of the —Si—O—Si— and —Ge —O—Ge—bridges by protonation [4], and therefore most artificial silicates have been produced under hydrothermal conditions. The dissolution of a silicate (germanate) under hydrothermal conditions may be represented as the reaction ...—Si—O—Si— ... + HOH→...—Si—OH + OH—Si—..., and the silica entering the solution tends to form large molecules, so very often the dissolution of a silicate or germanate in water is irreversible (incongruent). Also, silicon tends to show reversible condensation—decondensation reactions, so the synthesis of a silicate or germanate cannot be represented as a set of simple reactions. The same factor is responsible for the great variety of natural and artificial silicates, and it is also the reason for the prediction* that there should be no less variety among the germanates.