Hydrothermal diamond anvil cell studies: a possible new calibration mineral and applications to the hydrous-carbonate mineral, nesquehonite.

In this study, modifications are made to the experimental setup of the Bassett-type hydrothermal diamond anvil cell facilities at the University of Tennessee. Several modifications to the HDAC setup were found to increase the number of successful experimental runs by reducing fluid loss: ·

Gaskets were no longer polished or only lightly polished using 1µm diamond polishing compound. This prevented the formation of a wedge-shaped gasket. ·

Gasket diameter was reduced to equal the diameter of the diamond anvil surface to prevent “tipping” of the gasket and the possible introduction of foreign material between the gasket and diamond anvils.

Sample chamber was heated and cooled in small intervals (approximately 300 °C) rather that one large heating and cooling cycle. This reduced the amount of water loss in the sample chamber and, helped to maintain constant or nearly constant fluid pressure.

The sodium niobate structural transition most applicable to hydrothermal diamond anvil cell studies occurs at 373 °C at atmospheric pressure. This transition represents a structural change from monoclinic (P) to orthorhombic (R). Unlike other transitions reported in sodium niobate, this one in particular appeared in nearly all experiments, both fluid-absent and fluid-present. The Clapeyron slope (dp/dT) of this monoclinic to orthorhombic transition appears to be positive. Also, it was found that transitions that occurred during the heating cycle gave a better approximation of current known transition temperatures.

Currently, all thermodynamic data for nesquehonite apply to low- temperature, low-pressure conditions (<70 °C and 1 atm). Recently nesquehonite was observed as a quench phase in high-temperature, high-pressure experiments (750 °C and 50 MPa) as a result of dolomite breakdown reactions. Although nesquehonite experiments were relatively inconclusive, it is apparent from the evaluation of current thermodynamic data and tentative experimental results from this study that the high-temperature, high-pressure stability of nesquehonite needs to be re-evaluated. Nesquehonite appears to be stable up to 205 °C at high pressure in hydrothermal diamond anvil cell experiments conducted in this study.