Compressibility and thermal expansivity of synthetic apatites, Ca-5(PO4)(3)X with X = OH, F and Cl

The room-temperature unit-cell volumes of synthetic hydroxylapatite, Ca-5(P O4)(3)OH, fluorapatite, Ca-5(PO4)(3)(F1-x,OHx) with x = 0.025, and chlorapa tite, Ca-5(PO4)(3)(Cl-0.7,OH0.3), have been measured by high-pressure (diam ond 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 K-0 = 97.5 (1.8) GPa, K-0 = 97.9 (1.9) GPa and K-0 = 93.1(4.2) GPa, resp ectively. The room-pressure volume variation with temperature was measured on the same hydroxyl- and fluorapatite synthetic samples using a Huber Guin ier camera up to 962 and 907 degrees C, respectively. For hydroxyl- and flu orapatite, the volume data were fitted to a second-order polynomial: V(T)/V -293 = 1 + alpha(1) (T-293) + alpha(2) (T-293)(2) with T expressed in K lea ding to alpha(1)(OH) = 2.4(+/-0.1) x 10(-5) K-1, alpha(2)(OH) = 2.7(+/-0.1) x 10(-8) K-2 and alpha(i)(F) = 3.4(+/-0.1) x 10(-5) K-1, alpha(2)(F) = 1.6 (+/-0.1) x 10(-8) K-2, respectively. A significant increase is observed in hydroxylapatite thermal expansion above ca. 550 degrees C and extra reflect ions start to clearly appear on the X-ray film above 790 degrees C. These f eatures are interpreted as the progressive dehydration of slightly Ca-defic ient hydroxylapatite (i.e. with Ca/P < 1.67). Phase relation calculations, taking these new volume data for apatite into account, show that at 1200 degrees C, in the presence of kyanite + SiO2, hy droxylapatite should dehydrate to form gamma-Ca-3(PO4)(2) + Ca3Al2Si3O12 be low 12 GPa, i.e. below the upper-pressure stability-limit of apatite that w as previously determined experimentally.