VOLUME MEASUREMENTS OF ZOISITE AT SIMULTANEOUSLY ELEVATED PRESSURE AND TEMPERATURE

Unit-cell parameters of zoisite, Ca2Al3Si3O12(OH), have been measured at simultaneously high pressures and temperatures (up to 6.1 GPa and 8 00 degrees C) in a Walker-style multi-anvil apparatus at the synchrotr on radiation source at Daresbury Laboratory, U.K. Measurements were ma de in a series of heating cycles at increasing loads. Sample pressure, measured using an internal NaCl standard, increased during heating. C ell parameters vary smoothly with pressure and temperature; individual expansivities and compressibilities vary in the order c > b > a. Isot hermal bulk moduli were calculated from the volumes measured at 30, 20 0, 400, 600, and 800 degrees C by fitting the Murnaghan equation of st ate to each isothermal data set. This assumes K' = 4. Ambient-pressure volumes calculated from previous measurements of thermal expansivity of zoisite were included in the Murnaghan fits. A linear fit of the bu lk moduli with temperature gave values for the bulk modulus at 298 K, K-298 = 125(3) Gpa, and its variation with temperature, partial deriva tive K-T/partial derivative T = -0.029(6) GPa K-1. K-298 is slightly h igher than the recent value for a single crystal in a diamond-anvil ce ll, indicating a lower maximum pressure stability of zoisite than woul d be calculated using that value. Our data allow zoisite volumes to be calculated at P-T conditions relevant to the Earth and show that, in a typical subduction zone, zoisite becomes more dense as subduction pr oceeds, helping to stabilize it to high pressures.