Abstract
Single crystals of a synthetic apatite-like phase with composition Na 6.9 Ca 3.1 (SO 4 ) 6 (OH) 1.1 were grown under hydrothermal conditions. This compound crystallizes in the hexagonal space group P 6 ( a = 9.4434(13) A, c = 6.8855(14) A, Z = 1). The structure was solved by direct methods, and subsequently refined using 655 independent reflections ( R 1 = 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 H 2 O and (OH) − into the structure of cesanite. Observed systematic absences revealed that the space group P 6 3 / m allocated to cesanite in earlier studies is incorrect. The crystal structure of a cesanite with composition Na 7.0 Ca 3.0 (SO 4 ) 6 (OH) 1.0 (H 2 O) 0.8 was successfully refined in space group P 6 ( a = 9.4630(8) A, c = 6.9088(5) A, Z = 1, R 1 = 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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