Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Copper chloride hydroxide
Chemical Formula: Cu2Cl(OH)3
Method(s) of Verification: Aberavon - XRD (Natural History Museum, film no. 11987); Abersoch - XRD (Manchester Museum, MANCH:XRD368).
- Supergene: post-mining oxidation & weathering deposits
- Supergene: minerals on an artificial substrate
Deep emerald-green atacamite microcrystals coating weathered chalcopyrite-rich veinstone from Penrhyn Du, on Ll?n. National Museum of Wales specimen (NMW 2002.31G.M.3). Photo T.F. Cotterell, © National Museum of Wales.
Scanning electron micrograph of pseudocubic atacamite crystals from Penrhyn Du, Ll?n. National Museum of Wales specimen (NMW 2002.31G.M.3). Image T.F. Cotterell, © National Museum of Wales.
Introduction: atacamite is a supergene copper mineral typically found in areas where unusually high concentrations of chloride ions are present. Associated minerals are other copper halides and sulphates. Atacamite is restricted in occurrence in the UK due to its stringent geochemical/environmental requirements, which tends to limit its presence to areas where copper-rich primary mineralization is exposed to seawater, supplying the chloride ions required for its formation. The only Welsh occurrences are consistent with this theme. Researchers should be aware that most UK occurrences of atacamite-like minerals have proved to be the commoner polymorph paratacamite or the newly described clinoatacamite (Jambor et al., 1996).
Occurrence in Wales: only two Welsh localities have produced atacamite, one of natural origin in North Wales (Dossett & Green, 1998) and one of anthropogenic origin (weathered smelter slag) in South Wales (Plant, 2003).
- Aberavon, South Wales: specimens of slag from this locality contain green amorphous masses in vesicles; dark green tabular crystals; interlocking rosettes of thin plates and sprays of slender prismatic needles (Plant, 2003). According to the latest definition of what constitutes a mineral (Nickel, 1995), such occurrences on a matrix of manmade (i.e. therefore not natural) origin, are not minerals: however, the occurrence is included because so-called "slag minerals" remain of great interest to many mineral collectors.
- Abersoch, Gwynedd: atacamite, with associated botallackite and cumengeite, occurs in fractures in pebbles of vein-breccia cemented by quartz and containing Cu-Pb sulphides, on the beach between Abersoch and Penrhyn Du, Gwynedd (Dossett & Green, 1998). It forms dark green encrustations of minute drusy crystals and is undoubtedly the result of reactions between seawater and chalcopyrite.
- Dossett, I. & Green, D.I., 1998. Atacamite, botallackite and cumengeite from Abersoch, Gwynedd, Wales. Journal of the Russell Society 7(1), 38-39.
- Jambor, J.L., Dutrizac, J.E., Roberts, AC., Grice J.D. & Szymaski, J.T., 1996. Clinoatacamite, a new polymorph of Cu2(OH)3Cl, and its relationship to paratacamite and 'anarakite'. Canadian Mineralogist, 34, 61-72.
- Nickel, E.H., 1995. Definition of a mineral. Mineralogical Magazine, 59, 767-768.
- Plant, S., 2003. Secondary minerals from the Cwmavon Valley Copper Smelting Slags, Glamorgan, South Wales. Journal of the Russell Society, 8(1), 9-15.