Low temperature heat capacity of the high-pressure-phase of SiO2, coesite,and calculation of the alpha-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 w as measured by adiabatic calorimetry between T = (13 and 300) K. The smooth ed values of the heat capacity were obtained from the primary data, and the thermodynamic functions of coesite were calculated. The heat capacity, ent halpy, 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 wit h the values reported previously. The equilibrium boundary of the alpha-qua rtz-to-coesite transition was also calculated. The dp/dT slope of the bound ary was about 5.8.10-4 GPa.K-1. (C) 2000 Academic Press.