The Chemical Stability of Mimetite and Distribution Coefficients for Pyromorphite—Mimetite Solid-Solutions

Adedayo I. Inegbenebor, John H. Thomas and Peter A. Williams
Department of Chemistry, University College, P.O. Box 78, Cardiff CF1 1XL

Abstract: The equilibrium solubility of mimetite has been determined in aqueous solution at 298.2K. For the reaction Pb5(ASO4)3Cl(s,mimetite)+6H+(aq)⇌5Pb2+(aq)+3H2AsO4−(aq)+Cl−(aq) at this temperature log KH+, extrapolated to zero ionic strength, is equal to −27.9(4). This value is equal, within experimental error, to that corresponding to pyromorphite, Pb5(PO4)3Cl, derived from the literature, and redetermined here under analogous conditions. Distribution coefficients in terms of both HXO42− and H2XO4−(aq) ions (X = P,As) have also been determined for solid phases of the pyromorphite-mimetite solid solution series containing from 5 to 95 mol. % mimetite. Although the two end-members are isostructural without being strictly isomorphous, the solid solution series behaves ideally over the whole compositional range; that is, the composition of the solid phase reflects the ratio of arsenate to phosphate species in aqueous solution at pH values corresponding to naturally-occurring aqueous solutions generally associated with the oxidized zones of base metal orebodies. Some relationships between mimetite and other secondary lead(II) and copper(II) arsenate minerals have been explored.

Keywords: chemical stability • mimetite • distribution coefficients • pyromorphite-mimetite • solid-solutions

Mineralogical Magazine; June 1989 v. 53; no. 371; p. 363-371; DOI: 10.1180/minmag.1989.053.371.12
© 1989, The Mineralogical Society
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