The crystal structures of cesanite and its synthetic analogue—A comparison

Abstract

Single crystals of a synthetic apatite-like phase with composition Na_6.9Ca_3.1(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 Na_7.0Ca_3.0(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.