High-temperature instability of artificial cuprorivaite: a study using thermal analysis, X-ray powder diffractometry and polarized light microscopy
CaCuSi$_4$O$_{10}$ powder was studied by differential scanning calorimetry and thermogravimetry methods in the range from room temperature to 1450$\,^{\circ}$C at heating and cooling rates of 20$\,^{\circ}$C/min. The process of decomposition of cuprorivaite, the composition and transformations of its decomposition products during successive heat treatments were also studied by powder X-ray diffraction and polarization optical microscopy techniques. It was found that CaCuSi$_4$O$_{10}$ starts to decompose by incongruent melting at a temperature of about 1020$\,^{\circ}$C, with the minimum of the endothermic DSC peak associated with this process being at 1064.4$\,^{\circ}$C. CaCuSi$_4$O$_{10}$ decomposes irreversibly and subsequent cyclic annealings up to a temperature of 1450$\,^{\circ}$C at heating and cooling rates of 20$\,^{\circ}$C/min do not cause its re-synthesis. CaCuSi$_4$O$_{10}$ transforms into a two-phase system consisting of acicular crystals of monoclinic tridymite fused with green glass with the composition CuO$\,-\,$Cu$_2$O$\,-\,$CaO$\,-\,$SiO$_2$, with the weight ratio of tridymite to glass being about $12:13$, as a result of two successive annealings up to the temperature of 1450$\,^{\circ}$C.