No abstract

W E have recently reported (Humphreys et al., 1980;Abdul-Samad et al., 1982) the free energies of formation of a variety of chloride-bearing minerals of Pb(II) and Cu(II) together with carbonate and sulphate species of the same metals including leadhillite, Pb,SO4(COa)2(OH)2, caledonite, PbsCu2CO3(SO4)3(OH)6, and linarite, (Pb,Cu)2 SO4(OH)2. By using suitable phase diagrams it has proved possible to reconstruct, in part, the chemical history of the development of some complex secondary mineral assemblages such as those at the Mammoth-St. Anthony mine, Tiger, Arizona, and the halide and carbonate suite of the Mendip Hills, Somerset.A celebrated locality for the three sulphatebearing minerals above is the Leadhills-Wanlockhead district of Scotland (Wilson, 1921; Heddle, I923, 1924) from which the minerals leadhillite and caledonite were first described, and in which several other rare species have been noted including susannite, the hexagonal dimorph of leadhillite, hydrocerussite, Pba(CO3)2(OH)2 and lanarkite, Pb2OSO ,. Preliminary attempts to incorporate this last species into our thermodynamic model for secondary mineral formation were frustrated because of the wide variation in the reported values for its free energy of formation. We have therefore redetermined this quantity, and use it to outline possible conditions for the crystallization oflanarkite from aqueous solution.

SynopsisFREE 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.