Science. 1973 Jun 8;180(4090):1055-7. doi: 10.1126/science.180.4090.1055.
The existence of a monoclinic phase of hydroxyapatite, Ca(2)(PO(4))(4)OH, 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 gamma = 120 degrees , 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 degrees C, was mimetically twinned, and was approximately 37 percent monoclinic.
通过单晶结构分析（加权“可靠性”因子为 3.9%），确认了羟基磷灰石 Ca(2)(PO(4))(4)OH 的单斜相的存在。该结构具有空间群 P21/b，a = 9.4214(8)埃，b = 2a，c = 6.8814(7)埃，γ = 120 度，与氯磷灰石类似。单斜相与六方结构的伪对称所产生的扭曲与氯磷灰石中的相似，包括扩大了三角形的氧原子阵列，其中氯离子或在羟基磷灰石中，羟基氢离子大致位于中心。羟基磷灰石标本是通过在 1200°C 的蒸汽中将一颗粒状氯磷灰石单晶转化而成的，是模拟孪晶，约有 37%是单斜相。
