Pressure-volume-temperature behavior of diaspore and corundum: An in situ X-ray diffraction study comparing different pressure media

Abstract

The lattice parameters of synthetic diaspore (AlO(OH)) and synthetic and natural corundum (Al2O3) have been measured at simultaneously elevated pressure and temperature up to 7 GPa and 800°C (1000°C for corundum) using a MAX 80 cubic anvil high-pressure apparatus. Several experimental runs were carried out for both minerals, in which the samples were first compressed at room temperature and then heated; NaCl served as an internal standard for pressure calibration. In most runs the samples were mixed with Vaseline or NaCl to ensure hydrostatic pressure-transmitting conditions. Some runs, mainly at pressures < 4 GPa, were performed without using a pressure medium. A Birch-Murnaghan equation of state (EOS) was fitted to the experimental data obtained at room temperature (assuming K′ = 4), yielding bulk moduli K (diaspore) = 134.4±1.4 GPa in contrast to 230 GPa obtained from diamond anvil cell measurements without using a pressure medium recently, and K (corundum) = 219.±3.5 GPa, consistent with previous results based on X-ray diffraction measurements in conjunction with multi-anvil high-pressure techniques. The high-temperature Birch-Murnaghan EOS was fitted to the high-temperature high-pressure diaspore data, P = 1.5 KT[VT,r-7 - VT,r-5] [1 - 0.75 (4-K′T) (VT,r-2 - 1)], VT,r ≡ (V/ VT,0) KT = K + (∂KT/∂T)P (T - 298.15), VT,0 = 117.84 Å3 exp (∫a + bT dT), resulting in K = 133.6±4.0 GPa, K′T = 4.1±1.5, (∂KT/∂T)P = -0.017±0.007 GPa K-1, a = 2.22±0.40 10-5 K-1, and b = 2.23±0.79 10-8K-2. Copyright 2000 by the American Geophysical Union.