Heat Capacity of High Pressure Silicates from Fast Scanning Calorimetry

Thermodynamic properties of high-pressure silicates are important for understanding the dynamics of planetary interiors. Many high pressure phases are quenchable to ambient conditions, but synthesis requires small sample volumes which preclude conventional calorimetric analysis. Multi-component systems may also yield mixed-phase aggregates at high P-T conditions. While it may be possible to separate individual phases, they may not be present in sufficient amounts, even in a larger sample, to measure their thermal properties separately by normal scanning calorimetry. This limitation can be addressed with the Mettler Toledo Flash DSC 2+, a chip calorimeter which heats a small target at controllable rates up to several thousand degrees per second. Quantitative measurements of heat capacities and enthalpies of transformation using it require overcoming two challenges: accurate calibration of sensors and knowledge of sample masses too small to measure on a microbalance. We have developed protocols addressing these issues for accurate measurements on samples of 0.1 to 5 μg. We have calibrated our Flash DSC at 200-350 °C based on heat capacity to determine the mass of sub-microgram samples, such as can be recovered from a diamond anvil cell or multianvil device. We are now studying heat capacities and transformation energetics of high pressure silicates. Stishovite is a form of SiO2 stable above 8-10 GPa. It is metastable at ambient conditions, and becomes a dense amorphous phase when heated to ~500 °C. The heat capacity of this dense glass has not been reported. Mg-Si-Ti weberite is a high-pressure titanium silicate with a stability field overlapping those of stishovite and other Mg,Ti,Si phases. Only a few syntheses of this weberite composition have been reported. The heat capacity of nominal composition Mg0.99Si1.67Ti1.34O7 synthesized at 18 GPa and 1600 °C in a multianvil press will be reported. The measured CP for stishovite agrees within 4.9 % of the reported literature values and ~2.9 % of the values measured using standard DSC. The heat capacity for the dense glass is within 6.6 % of the CP of stishovite, and 3.5 % of the CP of standard silica glass. Experiments on other materials are planned and a high T chip, operational to 1000°C, is being calibrated, which will enable studies on smaller samples to higher temperatures.