Chemical Transformations in the Course of Solid-Phase Synthesis of Fibrous Alkali-Free Fluoroamphibole

This paper reports on the results of investigations into the possible chemical transformations occurring in the course of the solid-phase synthesis of fibrous alkali-free fluoroamphibole from synthetic magnesium silicate dihydrate (MSDH) MgO · SiO₂ · 2H₂O in the MSDH–MgF₂–NaCl model system in the concentration range of compositions corresponding to Mg-fluorocupfferite (Mg₇Si₈O₂₂F₂) at temperatures ranging from 60 to 1000°C. It is found that the formation of Mg-fluorocupfferite from MSDH under these conditions is a complex multistage chemical process. In the temperature range 60–700°C, dehydration, dehydroxylation of MSDH, and pyrohydrolysis of fluorides (MgF₂, SiF₄, etc.) proceed near the lower temperature boundary of the fluoroamphibole formation region (T 750°C). These processes result in the formation of compounds containing monomeric and simple polymeric silicon–oxygen anions SiO₄]^4– and SiO₃]^2– (forsterite, fluoronorbergite, enstatite, etc.). In the temperature range 850–900°C, the transformation of intermediate compounds leads to the formation of the fluoroamphibole structure. The main elements of this structure are silicon–oxygen anions of the Si₄O₁₁]^6– type.