The Accuracy and Precision of Calculated Mineral Dehydration Equilibria

Abstract

The calculation of dehydration equilibria at pressures greater than 2 Kb has only become possible fairly recently with the publication of reliable free energy data for water by Burnham, Holloway and Davis (1969). The converse operation, the extraction of free energy data for minerals from P-T curves, has also received considerable attention. It is now well known that small errors in thermochemical data lead to large errors in calculated mineral equilibria, and that mineral equilibria at high P and T can be used to derive thermochemical data having uncertainties quite comparable to those obtained through calorimetry. In this paper I intend to examine the factors affecting the accuracy and precision of calculated mineral equilibria, and the more general relationship between calorimetry, phase equilibria determinations and thermochemical data. The discussion centres on dehydration equilibria, but the same principles are involved for decarbonation and solid-solid equilibria. Uncertainties introduced by order-disorder, non-stoichiometry, solid solution and so on are not considered.