The high pressure-high temperature behavior of bassanite

The pressure evolution of bassanite (CaSO4 center dot 1/2H(2)O) was investigated by synchrotron X-ray powder diffraction along three isotherms: at room temperature up to 33 GPa, at 109 degrees C up to 22 GPa, and at 200 degrees 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 K-0 = 86(7) GPa with K' =2.5(3) was obtained. The axial compressibility values are beta(a) = 3.7(2), beta(b) = 3.6(1), and beta(c) = 2.8(1) GPa(-1) (x 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 degrees 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 K-109 = 79(4) GPa and K-200 = 63(7) GPa, respectively. The axial compressibility values for isotherm 109 degrees C are beta(a) = 2.4(1), beta(b) = 3.0(1), beta(c) = 2.5(1) (x 10(-3)) GPa(-1), and for isotherm 200 degrees C they are beta(a) = 3.5(3), beta(b) = 3.4(3), beta(c) = 2.6(4) (x 10(-3)) GPa(-1). These two bulk moduli and the 20 degrees C bulk modulus, K-0.20 = 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 tip to 200 degrees C. The sire 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 tip to 33 GPa and 200 degrees C.