- © 2009 American Mineralogist
The pressure evolution of bassanite (CaSO4·½H2O) was investigated by synchrotron X-ray powder diffraction along three isotherms: at room temperature up to 33 GPa, at 109 °C up to 22 GPa, and at 200 °C up to 12 GPa.
The room-temperature cell-volume data, from 0.001 to 33 GPa, were fitted to a third-order Birch-Murnaghan equation-of-state, and a bulk modulus K0 = 86(7) GPa with K′ = 2.5(3) was obtained. The axial compressibility values are βa = 3.7(2), βb = 3.6(1), and βc = 2.8(1) GPa−1 (×10−3) showing a slightly anisotropic behavior, with the least compressible direction along c axis. The strain tensor analysis shows that the main deformation occurs in the (010) plane in a direction 18° from the a axis.
The bulk moduli for isotherms 109 and 200 °C, were obtained by fitting cell-volume data with a second-order Birch-Murnaghan equation-of-state, with K′ fixed at 4, and were found to be K109 = 79(4) GPa and K200 = 63(7) GPa, respectively. The axial compressibility values for isotherm 109 °C are βa = 2.4(1), βb = 3.0(1), βc = 2.5(1) (×10−3) GPa−1, and for isotherm 200 °C they are βa = 3.5(3), βb = 3.4(3), βc= 2.6(4) (×10−3) GPa−1. These two bulk moduli and the 20 °C bulk modulus, K0,20 = 69(8) recalculated to a second-order Birch-Murnaghan EoS to be consistent, as well as the axial compressibilities, are similar for the three isotherms indicating that the thermal effect on the bulk moduli is not significant up to 200 °C. The size variation of the pseudo-hexagonal channel with pressure and temperature indicates that the sulfate “host” lattice and the H2O “guest” molecule in bassanite do not undergo strong change up to 33 GPa and 200 °C.