Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Basic copper chloride
Chemical Formula: Cu2Cl(OH)3
Method(s) of Verification: Abersoch - XRD at Manchester Museum (X-ray reference no. MANCH:XRD376).
- Supergene: post-mining oxidation & weathering deposits
- Supergene: minerals on an artificial substrate
Tabular botallackite crystals in slag from Neath Valley, South Wales. Crystals to sub mm on edge. Dave Evans Collection (no. 363). Photo T.F. Cotterell, © National Museum of Wales.
Introduction: botallackite is a supergene copper mineral typically found in areas where unusually high concentrations of chloride ions are present and the host matrix periodically dries out, allowing solutions to evaporate. This rather demanding geochemical/environmental requirement explains why, in the UK, it is a rare mineral, typically occurring where chalcopyrite-bearing mineralization outcrops in coastal areas. A number of other copper and lead-bearing chloride minerals may be present in association. Botallackite is the least thermodynamically stable of the basic copper chlorides, and experimental studies have shown that it is an intermediate, metastable phase under most conditions (Pollard et al., 1989).
Occurrence in Wales: the single confirmed, natural occurrence of botallackite in Wales is a very recent discovery, on the Llŷn, within a geochemical environment typical for the species (Dossett & Green, 1998). A previously noted occurrence from Halkyn Mountain, Clwyd (Bevins, 1994) is in a slag matrix, and thus does not represent a natural mineral, according to the latest International Mineral Association definition (Nickel, 1995).
- Abersoch, Gwynedd: botallackite is rare at Abersoch beach, the only natural locality in Wales (Dossett & Green, 1998). It forms acicular to lath-like crystals up to 0.4 mm long, varying from deep emerald green to pale green. Associated minerals are atacamite and rare cumengeite. The host is pebbles of vein-breccia cemented by quartz and containing chalcopyrite and galena. It is suggested that the occurrence is explained by periodic drying out of the pebbles in hot weather at low tide, thus, critically, permitting both high concentrations of chloride ions and evaporation of these solutions.
- Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
- Dossett, I. & Green, D.I., 1998. Atacamite, botallackite and cumengeite from Abersoch, Gwynedd, Wales. Journal of the Russell Society 7(1), 38-39.
- Nickel, E.H., 1995. Definition of a mineral. Mineralogical Magazine, 59, 767-768.
- Pollard, AM., Thomas, R.G. & Williams, PA., 1989. Synthesis and stabilities of the basic copper(II) chlorides atacamite, paratacamite and botallackite. Mineralogical Magazine, 53, 557-563.