ON THE CRYSTAL STRUCTURE OF APATITE Ann-Kristin Larsson^a, Angel Landa^bc, ^aResearch School of Chemistry, Australian National University, ^bInorganic Chemistry 2, University of Lund, Sweden, ^cPresent address: Dpto. Q. Inorganica, F.CC. Quimicas, Universidad Complutense, 28040 Madrid, Spain

To investigate the bioinorganic interactions in bone and enamel, and to develop synthetic replacements, a thorough crystallographic understanding of the calcium phosphates involved is critical but far from achieved. (In accordance with "apatite" being Greek for "I deceive".) This contribution is to provide a useful crystal structure description of apatite, enlightening its relation to other calciumphosphates and CaO.

The oxygen array of hydroxyapatite is described as a cyclic translation of an fcc lattice, as has been used to describe wightmanit and fluoborite². Three-octahedra wide monolayers of edge-sharing oxygen octahedra are infinitely extended along <110>_fcc (= <001>_apatite). The central octahedral row contains empty octahedra and every second tetrahedron is filled with P. The outer octahedra are alternately empty or filled with Ca (at different heights in the different rows). The difference between this ideal and the real model can be accounted for by considering the contraction of the PO₄ tetrahedra (due to the small and highly charged P⁵⁺ ions).

HREM experiments showed the presence of slabs of CaO (O forms an fcc lattice) growing coherently in between slabs of apatite (Ca/P=1.67). The apatite/CaO crystallites were formed from crystallites of tetracalcium phosphate (Ca/P=2) inducing the phase transformation in the electron beam (JEOL 4000EX, Transmission Electron Microscope). The extra calcium was found as 2 or 3 octahedra thick intergrowths of CaO.

The stoichiometry of the reaction 3 Ca₄(PO₄)₂O Æ Ca₁₀(PO₄)₆O + 2 CaO

suggests the formation of oxyapatite instead hyroxyapatite explaining the apparent p3 symmetry observed in the <001> images of the apatite slabs.

¹Kay, M. I., Young, R. A. & Possner, A. S (1964) Nature, 204, 1050.

²Hyde, B. G. & Andersson, S. (1988) Inorganic Crystal Structures, John Wiley & Sons, New York.