Crystal structure refinements of vanadinite and pyromorphite

Abstract

The crystal structures of vanadinite tpb5(VO)3Cll and pyromorphite [Pbs@Od3Cl] have been refined to R : 0.022,0.021, respectively, using three-dimensional X-ray data. The atomic arrangement of pyromorphite is con_ firmed as an apatite isostructure. In the structures the pb polyhedra show little response to the tetrahedral substirution, yith average Pb-O bond lengths that agree to within 0.02 A. The XOa tetrahedra yield bond lengths typical of VOa and POa tetrahedra, and X-O bond angles with the crystallographic axes are similar to within 2. The atomic arrangement responds to the tetrahedral substitution of p for V by contraction along a due to variations in X-O(|,2) bondlengths, yielding a predictive relationship between the a axial parameter and tetrahedral occupancy in the two minerals. The distortion of the VO4 tetrahedron in vanadinite reported in an earlier film study was not confirmed, suggesting that immiscibility between vanadinite and other Pb-bearing apatites results from differential size of tetrahedral cation and not differential distortion in the tetrahedra.