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(Cl_0.7,OH_0.3), 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 < 1.67). Phase relation calculations, taking these new volume data for apatite into account, show that at 1200 °C, in the presence of kyanite + SiO₂, 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.