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-Kα1). Annealing (900-1100 °C) in air of chlorellestadite with the unit cell content: Ca9(SiO4)3(SO4)3⋅(1−x)CaCl2(x≈0) produced a nonstoichiometric phase (0⩽x⩽0.6), which then upon further heating decomposed into ternesite Ca5 (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 P63/m(0⩽x⩽0.6); and a=9.677Å, c=6.858Å for x≈0; as compared to a=9.561Å, c=6.895Å for x≈0.6. The results indicated that the chlorellestadite structure can accommodate an unusually high level of Schottky defects.