Single crystals of a synthetic apatite-like phase with composition Na6.9Ca3.1(SO4)6(OH)1.1 were grown under hydrothermal conditions. This compound crystallizes in the hexagonal space group P6̅ (a = 9.4434(13) Å, c = 6.8855(14) Å, Z = 1). The structure was solved by direct methods, and subsequently refined using 655 independent reflections (R1 = 0.0542). The chemical composition and the unit cell parameters indicated a close structural relationship with the mineral cesanite. A reinvestigation of the mineral showed that the natural and the synthetic phases are isostructural. Small differences result from the incorporation of both H2O and (OH) into the structure of cesanite. Observed systematic absences revealed that the space group P63/m allocated to cesanite in earlier studies is incorrect. The crystal structure of a cesanite with composition Na7.0Ca3.0(SO4)6(OH)1.0(H2O)0.8 was successfully refined in space group P6̅ (a = 9.4630(8) Å, c = 6.9088(5) Å, Z = 1, R1 = 0.0468 for 720 independent reflections [I > 2σ (I)]). The symmetry reduction can be attributed to ordering of the Na and Ca atoms among four symmetrically independent cation sites.

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