Fluor-chlorellestadite solid solutions Ca-10(SiO4)(3)(SO4)(3)Cl2-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 angstrom) displaces chlorine (IR = 1.81 angstrom). FTIR spectra indicate little or no OH- in the solid solutions. All compositions conform to P6(3)/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 (<2 atom per formula unit), particularly Cl as a result of CaCl2 volatilization, with charge balance achieved by the creation of Ca vacancies (Ca2+ + 2Cl(-) ->square(Ca) + 2 square(Cl)) leading to the formula Ca10-y(SiO4)(3)(SO4)(3)Cl2-x-2yFx. For F-rich compositions the vacancies are found at Ca(2), while for Cl-rich ellestadites, vacancies are at Ca(1). It is likely the loss of CaCl2 which leads tunnel anion vacancies promotes intertunnel positional disorder, preventing the formation of a P2(1)/b monoclinic dimorph, analogous to that reported for Ca-10(PO4)(6)Cl-2. Trends in structure with composition were analyzed using crystal-chemical parameters, whose systematic variations served to validate the quality of the Rietveld refinements.