Trimethylsilyl derivatives for the study of silicate structures. Part 4. The conversion of hemimorphite into willemite

Abstract

The method of direct trimethylsilylation described previously has been used to follow the change in constitution of hemimorphite, Zn₄(OH)₂(Si₂O₇)·H₂O, on heating at various temperatures up to 1 300 °C. The conversion has also been followed by differential thermal analysis, thermal gravimetric analysis, and X-ray diffraction measurements. An abrupt change in the chromatographic pattern occurs at 600–630 °C, corresponding to the disappearance of the Si₂O₇ and emergence of the SiO₄ derivative as the main product of trimethylsilylation. This coincides with the loss of hydroxide groups and the formation of β-Zn₂SiO₄. The initial stage of the reaction at 627 °C shows first-order kinetics. The transformation of β-Zn₂SiO₄ into willemite, α-Zn₂SiO₄, commences at 835 °C and is unaccompanied by further changes in the yield or chromatographic pattern of the products. The results illustrate the use of the method as an adjunct to conventional procedures for studying the change in anionic constitution of silicates having discrete silicate structures.