STUDY OF THE SYSTEM CA3(ASO4)2-CASO4 BY SOLID-STATE REACTION

Study of the system Ca3(AsO4)2-CaSO4 has led to the preparation of a series of solid solutions whose general formula is [(Ca21 - x square 3 + x)(AsO4)14 - 2x(SO4)2x square 2] with O less-than-or-equal-to x less-than-or-equal-to 1. These solid solutions are isostructural with tricalcium orthoarsenate Ca3(AsO4)2. The limit of solubiLity of sulfate ions in tricalcium orthoarsenate corresponds to the ionic ratio R = SO4(2-)/AsO4(3-) = 0.167 (corresponding to x = 1 in the general formula). The limit compound, calcium arsenosulfate Ca20(AsO4)12(SO4)2, is characterized by its infrared absorption spectra, crystallographic unit-cell parameter in the rhombohedral system a(r) = 10.78(2) angstrom and density [rho = 3.469(5) g cm-3]. A mechanism of SO4(2-) ion integration in the lattice of tricalcium orthoarsenate is proposed.