Monoclinic Hydroxyapatite

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Abstract
The existence of a monoclinic phase of hydroxyapatite, Ca2(PO4)4OH, has been confirmed, by single-crystal structure analysis (weighted "reliability" factor = 3.9 percent on |F|2). The structure has space group P21/b, a = 9.4214(8) angstroms, b = 2a, c = 6.8814(7) angstroms, and γ = 120°, and is analogous to that of chlorapatite. The distortions from the hexagonal structure with which the monoclinic structure is pseudosymmetric are similar to those in chlorapatite, including enlargement of that triangular array of oxygen atoms in which the chlorine ion or, in hydroxyapatite, the hydroxyl hydrogen ion is approximately centered. The hydroxyapatite specimen was prepared by the conversion of a single crystal of chlorapatite in steam at 1200°C, was mimetically twinned, and was approximately 37 percent monoclinic.

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