The Chemistry of Formation of Some Secondary Arsenate Minerals of Cu(II), Zn(II) and Pb(II)

Abstract

Solution studies have been carried out on natural and synthetic arsenate minerals, which are often found in the oxide zones of base metal orebodies. Solubility products and free energy of formation data have been derived for the minerals olivenite, cornubite, clinoclase, adamite, legrandite, euchroite, duftite, conichalcite, austinite, bayldonite, and schultenite at 298.2 K (25 ~ The data have been used in turn to construct stability field diagrams illustrating the chemical conditions under which the various species may crystallize from aqueous solution. This equilibrium model is then compared with several natural occurrences of the arsenate suites and it is demonstrated that it can be used to explain a number of observed paragenetic sequences. Descriptions of solution conditions which describe the stabilities of the arsenate minerals with respect to more commonly found secondary minerals of Pb(II), Cu(II), and Zn(II) involve more complex calculations and estimates of the likely levels of various dissolved species, but it is shown how these may be taken into account in the development of a more complex equilibrium model.

Hydrogen bonding and structural complexity of the Cu3(AsO4)(OH)3 polymorphs (clinoclase, gilmarite): a theoretical study

“…Usually, they form rather diverse mineral associations that may include metastable as well as stable mineral species (e.g., euchroite is metastable relative to olivenite, yet both minerals have been found in the same association - Magalhães et al 1988;Majzlan et al 2017). Clinoclase and gilmarite are two polymorphs of Cu 3 (AsO 4 ) (OH) 3 that are essentially different in their natural occurrences.…”

Crystal structure of austinite, CaZn(AsO4)OH

“…They occur as secondary minerals in the oxide zone of base metal ore bodies (Magalh~.es et al, 1988;Crowley, 1975). From crystallographic data, they belong to the conichalcite/descloizite structure group with space group P212121 (Richmond, 1940;Radcliffe and Simmons, 1971).…”

Philipsburgite from the Yamato mine, Yamaguchi Prefecture, Japan

“…Olivenite, which does not occur with philipsburgite at the Yamato mine, crystallizes in lower copper ion activity and pH than cornwallite and clinoclase. Magalhães et al (1988) reported that the stability field of cornwallite is very narrow, explaining the rarity of cornwallite in nature. At the Yamato mine, cornwallite was commonly found at the area where philipsburgite occurred, and philipsburgite is much rarer than cornwallite there.…”

“…The paragenetic sequence of those minerals is first cornwallite, next clinoclase, philipsburgite or zalesiite. A stability field diagram for the copper(II) -arsenate minerals was reported by Magalhães et al (1988) and Williams (2005), which shows dependence on copper ion activity and pH of the aqueous solution. Olivenite, which does not occur with philipsburgite at the Yamato mine, crystallizes in lower copper ion activity and pH than cornwallite and clinoclase.…”