Compressibility, thermal expansion and Raman scattering of synthetic whitlockite Ca9Mg(PO3OH)(PO4)6 at high pressures and high temperatures

Abstract

Abstract In-situ X-ray diffraction and 16 Raman scattering of synthetic whitlockite, Ca9Mg(PO3OH)(PO4)6, have been systematically measured at high pressures and high temperatures, respectively. The results show that whitlockite is stable up to ~ 15 GPa at ambient temperature and undergoes a temperature-induced dehydrogenation to merrillite above 973 K at ambient pressure. The obtained pressure-volume data were fitted using a third-order Birch-Murnaghan equation of state to yield the isothermal bulk modulus as K0 = 79(4) GPa with pressure derivative K0' = 4.3(6). When K0' was fixed at 4, the refined isothermal bulk modulus was 81(1) GPa. The volumetric thermal expansion coefficient (αV) is equal to 4.05(8) × 10-5 K-1. The axial thermal expansion coefficients (αa and αc) are 1.07(5) × 10-5 K-1 and 1.91(6) × 10-5 K-1. Both compressibility and thermal expansion show an axial anisotropy. The effects of pressure and temperature on the Raman spectra of whitlockite have been quantitatively analyzed. The isothermal and isobaric mode Grüneisen parameters, and the intrinsic anharmonic mode parameters of whitlockite were calculated. Some amounts of OH--bearing whitlockite may be preserved and could be discovered in meteorites if whitlockite undergoes a low temperature process.