APATITE STRUCTURES, J.C. Elliott, R.M. Wilson, S.E.P. Dowker, pp. 172-181

The structure of apatite, Ca10(PO4)6F2, can be described in terms of an approximate hexagonal close packed set of spheres in which each sphere represents a tetrahedral PO4 3ion. This packing has octahedral interstitial sites that form channels through the structure parallel to the hexagonal axis. The F and 4/10ths of the Ca ions are located in these channels forming columns of F and Ca ions respectively. The remaining 6/10ths of the Ca ions form Ca triangles around the F ions. The relation between the hexagonal closed packed and apatite cells is discussed. CO3 2ions are thought to replace PO4 3ions in the lattice in precipitated carbonate-containing apatite (CO3Aps). Rietveld refinements based on powder X-ray diffraction data from a series of precipitated CO3Aps containing Na with a maximum CO3 content of 17 wt% are reported. An apatite model without explicit inclusion of the CO3 2ion was used. There was clear evidence (PO4 volume, O and P occupancies) of the replacement of PO4 3by CO3 2ions in the apatite lattice. However, the observed P occupancy was much higher than expected if all the CO3 2replaced PO4 3ions. INTRODUCTION Apatites have the general formula, Ca10(PO4)6X2 where X is typically F (fluorapatite, FAp), OH (hydroxyapatite, OHAp), or Cl (chlorapatite, ClAp). The apatite lattice is very tolerant of substitutions, vacancies and solid solutions, for example, X can be replaced by 1⁄2CO3 or 1⁄2O; Ca by Sr, Ba, Pb, Na or vacancies; and PO4 by HPO4, AsO4, VO4, SiO4 or CO3 [1]. Apatites are widely distributed as accessory minerals in igneous rocks and in small quantities in most metamorphic rocks. This wide-spread occurrence is an important factor in their extensive use in fission-track chronothermometry for the study of geological thermal history. Rock phosphates (microcrystalline apatites), mostly of biological origin, are the starting material for phosphate fertilizer manufacture and a source of phosphorus for the chemical industry. The mineral of bones and teeth is an impure form of OHAp, the major departures in composition being a variable Ca/P mol ratio (1.6 to 1.7, OHAp is 1.66), and a few percent CO3 and water. The mineral is microcrystalline. The crystals are very approximately 150 Å wide by 400 Å long in bone and dentine, and 400 Å wide by from 1000 Å to 5 :m or more in dental enamel. They are much thinner than they are wide. OHAp is also used as a biomaterial, for example, for bone replacement and augmentation, and for coating metal prostheses to improve their biocompatibility. Synthetic apatites (Sb and Mn doped halophosphates, i.e. chlorfluorapatites) are used as phosphors in fluorescent light tubes. Microcrystalline OHAps are also used for chromatographic separation of polypeptides and proteins. Apatites have potential for use as lasers and microcrystalline apatites for Copyright (c)JCPDS-International Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume 45. 172