Calorimetry Under Extreme Conditions

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The interior of the Earth is at high temperature and high pressure, leading to structural transformations in the minerals present. The thermodynamic properties of the materials involved must be measured to properly constrain and model the chemical processes leading to plate tectonics, volcanism, and earthquakes. This project develops and applies new methodology for measuring the heat effects associated with mineral reactions, with emphasis in three areas: the study of very small samples prepared at pressures of several hundred thousand atmospheres, the direct measurement of heats of reactions at temperatures where rocks melt (1000 - 2500 degrees Centigrade), and the measurement of energetics of sulfides, important to metal ores and to the deep Earth and meteorites.The experiments rely on specialized and unique calorimetric (heat measuring) equipment, available in Navrotsky's Thermochmistry Facility at UC Davis and in the laboratories of Frances Hellman, a collaborator at UC Berkeley. The heat capacities of microgram sized samples of high pressure phases will be measured using micromachined calorimeters on a silicon chip at Berkeley, which have been used in the solid state physics community for thin films and small crystals. Their application to high pressure mineral phases requires some modification and development of techniques. The work at high temperature will rely on a suite of custom built and commercial calorimeters at Davis. The heat capacities and magnetic transitions in high pressure silicate phases (spinel, wadsleyite, perovskite, garnet, and others) will be measured by calorimetry on a chip. The entropies derived from these measurements will be combined with other thermodynamic data to provide tight constraints on the phase chemistry of the Earth's mantle. The heats of formation of phases in the iron-sulfur and iron-nickel sulfur system will be measured. High temperature phase transitions and melting in a number of oxides and silicates will be characterized. Together, these various data will be used to understand high temperature processes occurring in the Earth. The data also have applications in high temperature materials processing, ceramics, smelting and refining. A fundamental understanding of thermodynamic parameters links structure on the molecular level with macroscopic properties and provides benchmarking for theoretical calculations.