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 (Al 2 O 3 ) 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) = 219.1±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 K T [ V T,r −7 ‐ V T,r −5 ][1 ‐ 0.75 (4 ‐ K ′ T ) V T,r −2 ‐ 1], V T,r ≡ ( V / V T ,0 ) 1/3 , K T = K + (∂ K T /∂ T ) P ( T ‐ 298.15), V T ,0 = 117.84 Å 3 exp (∫a+b T d T ), resulting in K = 133.6±4.0 Gpa, K ′ T = 4.1±1.5, (∂ K T /∂ 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 −8 K −2 .