Tridymite: Effect of hydrostatic pressure to 6 kbar on temperatures of two rapidly reversible transitions |
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| Authors: | Lewis H Cohen William Klement Jr |
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| Institution: | (1) Department of Earth Sciences, University of California, 92521 Riverside, California, USA;(2) School of Engineering and Applied Science, University of California, 90024 Los Angeles, California, USA |
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| 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 wit 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. |
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