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The chemical stability of mendipite, diaboleïte, chloroxiphite, and cumengéite, and their relationships to other secondary lead(II) minerals

Published online by Cambridge University Press:  05 July 2018

D. Alun Humphreys ,
John H. Thomas ,
Peter A. Williams  and
Robert F. Symes
D. Alun Humphreys
Affiliation:
Department of Chemistry, University College, Cardiff CF1 1XL
John H. Thomas
Affiliation:
Department of Chemistry, University College, Cardiff CF1 1XL
Peter A. Williams
Affiliation:
Department of Chemistry, University College, Cardiff CF1 1XL
Robert F. Symes
Affiliation:
Department of Mineralogy, British Museum (Natural History), Cromwell Road London SW7 5BD
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Summary

The chemical stabilities of mendipite, Pb3O2Cl2, diaboleïte, Pb2CuCl2(OH)4, chloroxiphite, Pb3CuCl2O2(OH)2, and cumengéite, Pb19Cu24Cl42 (OH)44, have been determined in aqueous solution at 298.2 K. Values of standard Gibbs free energy of formation, ΔGf°, for the four minerals are −740, −1160, −1129, and −15163±20 kJ mol−1 respectively. These values have been used to construct the stability diagram shown in fig. I which illustrates their relationships to each other and to the minerals cotunnite, PbCl2, paralaurionite, PbOHCl, and litharge, PbO. This diagram shows that mendipite occupies a large stability field and should readily form from cold, aqueous, mineralizing solutions containing variable amounts of lead and chloride ions, and over a broad pH range. The formation of paralaurionite and of cotunnite requires a considerable increase in chloride ion concentration, although paralaurionite can crystallize under much less extreme conditions than cotunnite. The encroachment of the copper minerals on to the stability fields of those mineral phases containing lead(II) only is significant even at very low relative activities of cupric ion. Chloroxiphite has a large stability field, and at given concentrations of cupric ion, diaboleïte is stable at relatively high aCl−. Cumengéite will only form at high concentrations of chloride ion.

Type
Research Article
Information
Mineralogical Magazine , Volume 43 , Issue 331 , September 1980 , pp. 901 - 904
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1980

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