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Chemical studies on the stabilities of boleite and pseudoboleite

Published online by Cambridge University Press:  05 July 2018

Fawzy A. Abdul-Samad ,
D. Alun Humphries ,
John H. Thomas  and
Peter A. Williams
Fawzy A. Abdul-Samad
Affiliation:
Department of Chemistry, University College, Cardiff, CF1 1XL
D. Alun Humphries
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
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Synopsis

FREE energies of formation of boleite, Pb26Cu24 Ag9Cl62(OH)47·H2O, and pseudoboleite, Pb5Cu4 Cl10(OH)8·2H2O, have been determined from solution studies at 298.2 K. ΔG°f values for the minerals are −19097.9±4.1 and −3705.4±5.5 kJ mol−1 respectively. These values, together with results of earlier studies (Humphreys et al., 1980) have been used to construct the stability field diagram shown in fig. 1. The boundaries for boleite and pseudoboleite are shown separately. If fields for the two minerals are plotted together, boleite has no thermodynamic stability at the silver ion activity chosen. At higher activities of Ag+(aq) the boleite field is negligible in extent.

The results can, however, be rationalized in terms of kinetics of mineral formation, rather than thermodynamic considerations alone. Since pseudoboleite is never found without boleite upon which it is observed to grow epitaxially (Winchell, 1963), it is clear that boleite must form metastably prior to any pseudoboleite deposition. Accordingly, boleite has a large range of solution compositions, from which it may precipitate. The hatched area of fig. 1 shows this at the Cu2+(aq) and Ag+(aq) activities chosen. The field is terminated at high aCl− by the AgCl line, above which silver is precipitated as AgCl(s), chlorargyrite.

It is also evident from the chemical studies that the deposition of several assemblages in the leadcopper-chloride group of minerals is simply related to variations of chloride activity. With decreasing aCl− the associations cumengeite + boleite + pseudoboleite, diaboleite + boleite + pseudoboleite, and diaboleite + chloroxiphite are expected in turn. This relationship is apparently borne out by field observations of occurrences of the minerals.

Type
Research Article
Information
Mineralogical Magazine , Volume 44 , Issue 333 , March 1981 , pp. 101 - 104
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1981

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