No abstract

ct-Ca3(PO4) 2 crystallizes in the monoclinic space group P2Ja with a= 12.887(2), b=27.280(4), c--15.219 (2) ,~,/~ = 126.20(1) °, and Z = 24. An approximate subcell with b' = b/3 exists. The structure was solved by direct methods of phase determination and refined to R = 0.051 and R,= 0.049 with 7002 reflections collected from a single crystal by counter methods. The structure consists of columns of cations, and columns of cations and anions with compensating cation vacancies, and is closely related to the structure of K3Na(SO4) 2, the mineral glaserite. The calcium ions in ct-Caa(PO4) 2 exhibit a wide range of coordination numbers and geometries.

Structural features of some calcium phosphates of biological interest are described. Structure of hydroxyapatite (OHAp), considered as the prototype for the inorganic component of bones and teeth is discussed with respect to the kinds and locations of ionic substitutions. Octacalcium phosphate (OCP), is a probable precursor in biological mineralization. OCP has a layer type structure, with one layer quite similar to that of OHAp and the other, a hydrated layer consisting of more widely spaced Ca, and PO4 ions and the water molecules. The closeness of fit in the apatitic layers of OCP and OHAp accounts for the epitaxial, interlayered mixtures formed by these compounds and the in situ conversion of OCP to OHAp. Possible roles of OCP in biological mineralization are discussed.