Melting and Subsolidus Phase Relationships for CaSiO_3 to 35 Kilobars Pressure

Abstract

Phase transitions between pseudowollastonite, wollastonite I, and wollastonite II have been reversed in piston-cylinder apparatus, and the melting curve for these CaSiO, polymorphs has been determined between 10 and 35 kbars. The pseudowollastonite melting curve rises from 1544°C at 1 bar with an unusually steep slope (dP/dT = 580 bar/°C) to a triple point at 1588°C and 23 kbar, where the solidus meets the transition from pseudowollastonite to wollastonite I, rising from 1125°C at 1 bar. The transition boundary between wollastonite I and wollastonite II has a shallow, negative slope (dP/dT = -8.4 bar/ °C); the triple point where it reaches the solidus is at 1598°C and 27.5 kbar. The slope of the fusion curve decreases at each triple point, and dP/dT for wollastonite II, 192 bar/°C, is much closer to that of other silicates, such as diopside, enstatite, and albite, than to wollastonite I. Refractive indices of glasses quenched from high pressure CaSiO, and CaMgSi_2O_8 liquids vary less as a function of pressure than do indices of glasses quenched from feldspars and feldspathic liquids. Either the densities of liquids of inosilicates are changed less than those of tektosilicates by pressure, or their high pressure densities are not quenched into the glasses.