The Crystal Chemistry\nof Ca_{10–<i>y</i>}(SiO₄)₃(SO₄)₃Cl_{2–<i>x</i>–2<i>y</i>}F_<i>x</i> Ellestadite

Abstract

Fluor-chlorellestadite solid solutions Ca₁₀(SiO₄)₃(SO₄)₃Cl_2–<i>x</i>F_<i>x</i>, serving as prototype\ncrystalline matrices for the fixation of hazardous fly ash, were synthesized\nand characterized by powder X-ray and neutron diffraction (PXRD and\nPND), transmission electron microscopy (TEM), and Fourier transform\ninfrared spectroscopy (FTIR). The lattice parameters of the ellestadites\nvary linearly with composition and show the expected shrinkage of\nunit cell volume as fluorine (IR = 1.33 Å) displaces chlorine\n(IR = 1.81 Å). FTIR spectra indicate little or no OH^– in the solid solutions. All compositions conform to <i>P</i>6₃/<i>m</i> symmetry where F^– is located at the 2<i>a</i> (0, 0, ¹/₄) position, while Cl^– is displaced out of the 6<i>h</i> Ca(2) triangle plane and occupies 4<i>e</i> (0,\n0, <i>z</i>) split positions with <i>z</i> ranging\nfrom 0.336(3) to 0.4315(3). Si/S randomly occupy the 6<i>h</i> tetrahedral site. Ellestadites rich in Cl (<i>x</i> ≤\n1.2) show an overall deficiency in halogens (<2 atom per formula\nunit), particularly Cl as a result of CaCl₂ volatilization,\nwith charge balance achieved by the creation of Ca vacancies (Ca²⁺ + 2Cl^– →□_Ca +\n2□_Cl) leading to the formula Ca_{10–<i>y</i>}(SiO₄)₃(SO₄)₃Cl_{2–<i>x</i>–2<i>y</i>}F_<i>x</i>. For F-rich compositions the\nvacancies are found at Ca(2), while for Cl-rich ellestadites, vacancies\nare at Ca(1). It is likely the loss of CaCl₂ which leads\ntunnel anion vacancies promotes intertunnel positional disorder, preventing\nthe formation of a <i>P</i>2₁/<i>b</i> monoclinic dimorph, analogous to that reported for Ca₁₀(PO₄)₆Cl₂. Trends in structure with\ncomposition were analyzed using crystal-chemical parameters, whose\nsystematic variations served to validate the quality of the Rietveld\nrefinements.