Pressure-induced metastable phase transformations of calcium metasilicate (CaSiO3): A Raman spectroscopic study

Pressure-induced phase transformations of two kinds of calcium metasilicate (chain-type wollastonite-2M and ring-type pseudowollastonite) were characterized by in-situ Raman spectroscopy at room temperature. Wollastonite transforms to two high-pressure polymorphs, i.e., wollastonite Ha and Hb at ca. 8.5 and 19 GPa, respectively and then slowly changes into amorphous phase above 26 GPa. Pseudowollastonite transforms to its high-pressure polymorph (pseudowollastonite II) at ca. 10 GPa, and then rapidly vitrifies at ca. 22-23 GPa. The three polymorphic transformations of wollastonite and pseudowollastonite at pressures are all reversible, but the amorphization of both wollastonite Hb and pseudowollastonite II are irreversible. A Raman analysis on the samples recovered from high pressure shows that the pressure-induced amorphs of wollastonite and pseudowollastonite are alike, but have different structures from the melt-derived CaSiO3 glasses regardless of compression treatment, i.e., far more Q2 but less Q3 species. The presence of the non-Q2 species in both wollastonite- and pseudowollastonite-derived amorphous phases indicates that disproportional polymerization and depolymerization of silicate chains and rings at the expense of the Q2 species characteristic to the two crystalline phases occurred during pressure-induced amorphization. Pressure-induced disproportionation also occurs in the melt-derived CaSiO3 glass under compression to show less Q3 yet more Q1 species after pressure release.