Nonstoichiometry in chlorellestadite

The structural development of apatite-type chlorellestadite was studied as a function of annealing time by Rietveld refinements based on X-ray powder diffraction data (Cu-K-alpha1). Annealing (900-1100 degreesC) in air of chlorellestadite with the unit cell content: Ca-9(SiO4)(3)(SO4)(3)-(1-x)CaC1(2) (x approximate to 0) produced a nonstoichiometric phase (0 less than or equal to x less than or equal to 0.6), which then upon further heating decomposed into ternesite Ca-5 (SiO4)(2)(SO4), anhydrite CaSO4 and lime CaO. Increasing the temperature caused a more rapid loss of CaCl2 and a more rapid onset of decomposition into the former phases. Crystal data of the chlorellestadite series: hexagonal symmetry, space group P6(3)/m (0 less than or equal to x less than or equal to 0.6); and a = 9.677 Angstrom, c = 6.858 Angstrom for x approximate to 0; as compared to a = 9.561 Angstrom, c = 6.895 Angstrom for x approximate to 0.6. The results indicated that the chlorellestadite structure can accommodate an unusually high level of Schottky defects. (C) 2004 Elsevier SAS. All rights reserved. (Less)