HIGH-LOW CRISTOBALITE TRANSITIONS IN SIO2, AIPO4 AND GAPO4 INVESTIGATIONS BY DIFFERENTIAL THERMAL-ANALYSIS UNDER HYDROSTATIC PRESSURES LESS-THAN-OR-EQUAL-TO-6 KBAR

High-low cristobalite transition temperatures in SiO2 and A1P04 were deter mined by differential thermal analysis (DTA) under hydrostatic pressures of Ar and of CO2 to 6 kbar, using ‘well-crystallized’ materials with known (at 1 bar) enthalpies and volumes. The high-low GaPO4 cristobalite transitions were also investigated at pressures ∼ 3 kbar, conversion to quartz being unavoidable; these low → high and high → low transition temperatures increase with pressure at ∼34° kbar−1. For the low → high and high → low transitions in SiO2 and A1P04, there are regions of anomalous curvature (- d2T/dp2<0) up to ∼ 1-2 kbar, the pressure-induced variations of transition temperatures then being linear and the hystereses decreasing with increasing pressures. Slopes for the phase transitions are compatible with the thermodynamic data at 1 bar. Since the high-pressure data on the cristobalite transition do not seem to be explained fully by current theories, further theoretical attention is needed. ACKNOWLEDGMENTS © 1979 Taylor & Francis Group, LLC.