Crystal Structure of Synthetic Mg-Whitlockite, Ca₁₈Mg₂H₂(PO₄)₁₄

Abstract

Crystals of synthetic Mg-whitlockite, Ca 18 Mg 2 H 2 (PO 4 ) 14 , have been grown by hydrothermal techniques and their structure refined by full-matrix least-squares methods, using 1626 unique reflections measured with a Syntex [Formula: see text] automatic diffractometer, to a final R value of 0.046. The crystals have space group R3c with Z R = 1, a = 13.765(8) Å and α = 44.25(5)° with the equivalent hexagonal parameters a = 10.350(5), c = 37.085(12) Å and Z H = 3. The structure, although similar to that of βCa 3 (PO 4 ) 2 , differs significantly. The structure contains interconnected infinite chains of polyhedra paralleling the hexagonal c axis with links in the chains consisting of three CaO 8 polyhedra separated by two PO 4 tetrahedra. Six of these chains surround a chain of MgO 6 octahedra and PO 4 H groups which lie on the three-fold axes. These chains however contain only half the formula unit density of the calcium containing ones and a proton, presumably disordered, is attached to the oxygen atom on the triad axes.