Compressibility and thermal expansivity of synthetic apatites, Ca5(PO4)3X with X = OH, F and Cl

The room-temperature unit-cell volumes of synthetic hydroxylapatite, Ca5(PO4)3OH, fluorapatite, Ca5(PO4)3(F(1-x),OH(x)) with x = 0.025, and chlorapatite, Ca5(PO4)3(Cl0.7,OH0.3), have been measured by high-pressure (diamond anvil-cells) synchrotron X-ray powder diffraction to maximum pressures of 19.9 GPa, 18.3 GPa, and 51.9 GPa, respectively. Fits of the data with a second-order Birch-Murnaghan EOS (i.e. (dK/dP)(P=0) = 4) yield bulk moduli of K0 = 97.5 (1.8) GPa, K0 = 97.9 (1.9) GPa and K0 = 93.1(4.2) GPa, respectively. The room-pressure volume variation with temperature was measured on the same hydroxyl- and fluoropatite synthetic samples using a Huber Guinier camera up to 962 and 907°C, respectively. For hydroxyl- and fluorapatite, the volume data were fitted to a second-order polynomial: V(T)/V293 = 1 + α1 (T-293) + α2 (T-293)2 with T expressed in K leading to α1(OH) = 2.4(±0.1) x 10-5 K-1, α2(OH) = 2.7(±0.1) x 10-8 K-2 and α1(F) = 3.4(±0.1) x 10-5 K-1, α2(F) = 1.6(±0.1) x 10-8 K-2, respectively. A significant increase is observed in hydroxylapatite thermal expansion above ca. 550 °C and extra reflections start to clearly appear on the X-ray film above 790 °C. These features are interpreted as the progressive dehydration of slightly Ca-deficient hydroxylapatite (i.e. with Ca/P 2, hydroxylapatite should dehydrate to form γ-Ca3(PO4)2 + Ca3Al2Si3O12 below 12 GPa, i.e. below the upper-pressure stability-limit of apatite that was previously determined experimentally.