gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1983.047.342.10 The Crystal Structure of Cesanite, Ca1+xNa4−x(SO4)3(OH)x · (1 − x)H2O, a Sulphate Isotypic to Apatite Tazzoli V. Centro di Studio per la Cristallografia Strutturale del CNR c/o Istituto di Mineralogia dell'Universita', Via Bassi, 4-27100 Pavia, Italy 3 1983 47 342 59 63 22 2 1982 22 6 1982 Copyright © 1983, The Mineralogical Society 1983 Abstract

Cesanite, Ca1+xNa4−x(SO4)3(OH)x · (1 − x) H2O, is hexagonal, space group P63/m, a = 9.446(1) and c = 6.895(1) Å, Z = 2. The crystal structure refinement, carried out by utilizing 457 independent reflections (final R = 0.021), shows that cesanite is isotypic to hydroxyapatite. The P atom of apatite is completely replaced in cesanite by an S atom; the two independent Ca sites of apatite are occupied respectively by Na and Na + Ca, the charge balance being by partial substitution of H2O for OH− although the occupancy of the (H2O,OH−) sites is not complete. The relationship between the water vacancies and the structural features of the (Ca,Na) polyhedron is discussed. A new general formula for the apatite-wilkeite-ellestadite series is proposed.