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 Q(2) but less Q(3) species. The presence of the non-Q(2) 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 Q(2) 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 Q(3) yet more Q(1) species after pressure release. (C) 2016 Elsevier B.V. All rights reserved.