Low temperature heat capacity of the high-pressure-phase of SiO2, coesite, and calculation of the α-quartz-to-coesite equilibrium boundary

The high-pressure-phase of SiO2, coesite, was synthesized from alpha-quartz under high pressure using a multi-anvil apparatus, and the heat capacity was measured by adiabatic calorimetry between T = (13 and 300) K. The smoothed values of the heat capacity were obtained from the primary data, and the thermodynamic functions of coesite were calculated. The heat capacity, enthalpy, entropy, and Gibbs energy at T = 298.15 K were (45.12 +/- 0.26) J.K-1 mol(-1), (6.771 +/- 0.045) kJ.mol(-1) (39.28 +/- 0.27) J.K-1 mol(-1), and (-4.939 +/- 0.035) kJ.mol(-1), respectively. The results were compared with the values reported previously. The equilibrium boundary of the alpha-quartz-to-coesite transition was also calculated. The dp/dT slope of the boundary was about 5.8.10-4 GPa.K-1. (C) 2000 Academic Press.