Progress and new directions in high temperature calorimetry

Calorimetry at 600-900° C, using Calvet-type microcalorimeters, has provided data in several areas of interest to geology. The best-developed application is solution calorimetry in molten oxide solvents (lead borate, sodium molybdate) to determine the enthalpies of formation of anhydrous silicates and related minerals, the enthalpies of phase transformations and order-disorder reactions, and the enthalpies of mixing in molten salts, glasses, and solid solutions. An attractive feature of the technique is the ability to use rather small samples; ∼ 300 mg total often being sufficient for the study of a phase synthesized at high pressure. Current developments include the improvement of precision by careful control of solvent composition and water content, the development of alkali borate or borosilicate solvents for use under atmospheres of controlled oxygen fugacity, and the study of compounds containing fluorine as well as oxygen. In addition, direct-reaction calorimetry at high temperature has been used to study rapid phase transformations and gas-solid reactions. The latter include the oxidation of metals and the thermodynamics of oxides having wide homogeneity ranges, for example wüstite. A recent development is calorimetry at pressures up to 2 kb, using a cold-seal pressure vessel inserted in the calorimeter. Applications include the study of geothermal fluids and, eventually, the study of equilibria involving hydrous phases and water-containing melts and glasses.