Chemical studies on the stabilities of boleite and pseudoboleite

Abstract

Synopsis FREE energies of formation of boleite, Pb 26 Cu 24 Ag 9 Cl 62 (OH) 47 ·H 2 O, and pseudoboleite, Pb 5 Cu 4 Cl 10 (OH) 8 ·2H 2 O, 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 Cu 2+ (aq) and Ag + (aq) activities chosen. The field is terminated at high a Cl − 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 a Cl − 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.