The Crystal Chemistry of Ca10–y(SiO4)3(SO4)3Cl2–x–2yFx Ellestadite

Fluor-chlorellestadite solid solutions Ca10(SiO4)3(SO4)3Cl2–xFx, serving as prototype crystalline matrices for the fixation of hazardous fly ash, were synthesized and characterized by powder X-ray and neutron diffraction (PXRD and PND), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). The lattice parameters of the ellestadites vary linearly with composition and show the expected shrinkage of unit cell volume as fluorine (IR = 1.33 Å) displaces chlorine (IR = 1.81 Å). FTIR spectra indicate little or no OH– in the solid solutions. All compositions conform to P63/m symmetry where F– is located at the 2a (0, 0, 1/4) position, while Cl– is displaced out of the 6h Ca(2) triangle plane and occupies 4e (0, 0, z) split positions with z ranging from 0.336(3) to 0.4315(3). Si/S randomly occupy the 6h tetrahedral site. Ellestadites rich in Cl (x ≤ 1.2) show an overall deficiency in halogens (