Tridymite: Effect of hydrostatic pressure to 6 kbar on temperatures of two rapidly reversible transitions

Abstract

Trajectories of two reversible phase transitions in a low-Na synthetic tridymite have been determined to 6 kbar by differential thermal analysis (DTA) in hydrostatic apparatus using Ar or CO₂. The temperature of the lower transition increases from ≈111 ° C at 1 bar linearly with pressure with slope ∼1₅ deg kbar⁻¹. Pressure raises the temperature of the upper transition from 157 ±2 ° or 159 ° C (independently determined) at 1 bar witħ a slope of ≈53 deg kbar⁻¹, up to ∼0.7 kbar; for the data above that pressure, the initial slope is ≈64 deg kbar⁻¹. Above 2−1/2 kbar, the variation is linear with slope ≈70 deg kbar⁻¹. No evidence for other transitions was found at any of the apparent changes of slope. Hystereses for both transitions decreased at high pressures compared to 1-bar. Preferred values for the transition enthalpies, together with these slopes and the Clausius-Clapeyron equation, yield estimates for the volume changes at the transitions of ≈0.01 (lower) and 0.15 to 0.25 (upper) cm³ gfw⁻¹. These calculated volume changes are not consonant with many of the high temperature volumetric data on tridymites of varying origins.