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The chemical stability of mimetite and distribution coefficients for pyromorphite–mimetite solid-solutions

Published online by Cambridge University Press:  05 July 2018

Adedayo I. Inegbenebor ,
John H. Thomas  and
Peter A. Williams
Adedayo I. Inegbenebor
Affiliation:
Department of Chemistry, University College, P.O. Box 78, Cardiff CF1 1XL
John H. Thomas
Affiliation:
Department of Chemistry, University College, P.O. Box 78, Cardiff CF1 1XL
Peter A. Williams
Affiliation:
Department of Chemistry, University College, P.O. Box 78, Cardiff CF1 1XL
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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 stabilitymimetitedistribution coefficientspyromorphite-mimetitesolidsolutions
Type
Experimental Studies
Information
Mineralogical Magazine , Volume 53 , Issue 371 , June 1989 , pp. 363 - 371
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1989

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Footnotes

*

Author for correspondence

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