In situ X-ray diffraction and Raman spectroscopy of a synthetic whitlockite, Ca9Mg(PO3OH)(PO4)6, have been conducted at high pressures or high temperatures. The results show that whitlockiteis stable up to ~15 GPa at ambient temperature and undergoes a thermally induced dehydrogenationto merrillite above 973 K at ambient pressure. The obtained pressure-volume data were fitted using athird-order Birch-Murnaghan equation of state, yielding an isothermal bulk modulus of K0 = 79(4) GPawith a pressure derivative of K′0 = 4.3(6). When K′0 was fixed at 4, the refined isothermal bulk moduluswas 81(1) GPa. The volumetric thermal expansion coefficient (αV) is 4.05(8) × 10–5 K–1, and the axialthermal 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 theRaman spectra of whitlockite have been quantitatively analyzed. The isothermal and isobaric modeGrüneisen parameters and the intrinsic anharmonic mode parameters of whitlockite were derived.Some amounts of OH–-bearing whitlockite may be preserved in meteorites if whitlockite undergoesa low-temperature process.