Abstract

The room-temperature unit-cell volumes of synthetic hydroxylapatite, Ca 5 (PO 4 ) 3 OH, fluorapatite, Ca 5 (PO 4 ) 3 (F (sub 1-x) ,OH x ) with x = 0.025, and chlorapatite, Ca 5 (PO 4 ) 3 (Cl (sub 0.7) ,OH (sub 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) (sub P = O) = 4) yield bulk moduli of Ko = 97.5 (1.8) GPa, K 0 = 97.9 (1.9) GPa and K 0 = 93.1(4.2) GPa, respectively. The room-pressure volume variation with temperature was measured on the same hydroxyl- and fluorapatite synthetic samples using a Huber Guinier camera up to 962 and 907 degrees C, respectively. For hydroxyl- and fluorapatite, 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 leading to alpha 1OH = 2.4(+ or -0.1)X10 (super -5) K (super -1) , alpha 2OH = 2.7(+ or -0.1)X10 (super -8) K (super -2) and alpha 1F = 3.4(+ or -0.1)X10 (super -5) K (super -1) , alpha 2F = 1.6(+ or -0.1)X10 (super -8) K (super -2) , respectively. A significant increase is observed in hydroxyl-apatite thermal expansion above ca. 550 degrees C and extra reflections start to clearly appear on the X-ray film above 790 degrees 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 degrees C, in the presence of kyanite+SiO 2 , hydroxylapatite should dehydrate to form gamma -Ca 3 (PO 4 ) 2 +Ca 3 Al 2 Si 3 O 12 below 12 GPa, i.e. below the upper-pressure stability-limit of apatite that was previously determined experimentally.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.