Structural Variations of Apatite Solid Solutions, Ca10(PO4)6(F,Cl,OH)2

Abstract

Apatite, Ca10(PO4)6(F,Cl,OH)2, is the most abundant phosphate mineral in the world with numerous uses in geology, materials science, biology, medicine, agriculture, and dentistry. The three main natural end members are fluorapatite (Ca10(PO4)6F2), chlorapatite (Ca10(PO4)6Cl2), and hydroxylapatite (Ca10(PO4)6OH2). Quantitative chemical compositions and backscattered electron (BSE) images on twenty-four natural apatite samples, from various locations, were obtained with an electron-probe microanalyzer (EPMA). Varying cation and anion content led to distinctive zoning and unique crystal features. Eight samples were selected and studied with synchrotron high-resolution powder X-ray diffraction (HRPXRD). All eight samples observed a hexagonal structure (P63/m), consisting of a phosphate tetrahedron, a Ca1 polyhedron, and a Ca2 polyhedron. The Ca1 site is a 9-coordinated system and the Ca2 site is a 6-coordinated with an addition anion containing either F, Cl and/or OH. Five of the seven samples were found to be single- phased fluorapatites with varying features and cation/anion content. Cation substitutions, such as Mn substituting for Ca in sample 1W2, reduces the cell parameters of apatite (a, c, V). Four samples displayed unique zoning (1V9, 1E9, SM and 1V5), however only 1E9 and SM were found to be multiphase apatites, with all phases being hexagonal in structure. Sample SM exhibited three phases - with fluorapatite being the primary phase along with 2 guest phases. Sample 1E9 displayed two separate phases of fluorapatite and hydroxylapatite, with the OH residing at 0.224 Å and F placed on the mirror planes at 0,0, 1⁄4.