HEAT-CAPACITY AND THERMODYNAMIC PROPERTIES FOR COESITE AND JADEITE, REEXAMINATION OF THE QUARTZ-COESITE EQUILIBRIUM BOUNDARY

The heat capacities of synthetic coesite and jadeite were measured bet ween about 15 and 850 K by adiabatic and differential scanning calorim etry. The experimental data were smoothed and estimates were made of h eat capacities to 1800 K, The following equations represent our estima te of the heat capacities of coesite and jadeite between 298.15 and 18 00 K: C-p(0)(coesite) = 141.35 - 0.01514T + 987190.7T(-2) - 1780.5T(-1 /2) + 1.029 x 10(-6)T(2) C-p(0)(jadeite) = 259.08 + 0.038032T - 251890 8T(-2) - 1332.57T(-1/2) - 8.8 x 10(-6)T(2). Tables of thermodynamic va lues for coesite and jadeite to 1800 K are presented. The entropies of coesite and jadeite are 40.38 +/- 0.12 and 136.5 +/- 0.32 J/(mol.K), respectively, at 298.15 K, The entropy for coesite derived here confir ms the value published earlier by Helm et al. (1967). We have derived an equation to describe the quartz-coesite boundary over the temperatu re range of 600 to 1500 K, P(GPa) = 1.76 + 0.001T(K). Our results are in agreement with the enthalpy of transition reported by Akaogi and Na vrotsky (1984) and yield -907.6 +/- 1.4 kJ/mol for the enthalpy of for mation of coesite from the elements at 298.15 K and 1 bar, in agreemen t with the value recommended by CODATA (Khodakovsky et al. 1995). Seve ral sources of uncertainty remain unacceptably high, including: the he at capacities of coesite at temperatures above about 1000 K; the heat capacities and volumetric properties of alpha quartz at higher pressur es and at temperatures above 844 K; the pressure corrections for the p iston cylinder apparatus used to determine the quartz-coesite equilibr ium boundary.