Aurichalcite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Zinc copper carbonate hydroxide
Chemical Formula: (Zn,Cu)5(CO3)2(OH)6
Method(s) of Verification: Machen Quarry - XRD at the National Museum of Wales (NMW X-776); Dylife Mine, Powys - XRD at the Natural History Museum, London (x13801).

Chemical Group:

  • Carbonates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
  • Supergene: in situ natural oxidation & weathering deposits
Acicular aurichalcite crystals lining 10 mm cavity. Collected from Bench 9, Machen Quarry, July 1993. Specimen I.E. Jones Collection. © National Museum of Wales.
Rosettes of aurichalcite from Craig Rhiwarth (Ochr-graig) Mine, Llangynog. Specimen I.E. Jones Collection (1333A), © National Museum of Wales.
Introduction: aurichalcite is a supergene mineral typically encountered in the oxidation zones of base-metal sulphide veins which include significant carbonate gangue. Associated minerals are typically hemimorphite, smithsonite and malachite.
Occurrence in Wales: aurichalcite occurs at scattered localities across Wales, many of these having been identified only within the last 20 years. Fine specimens are known from a limited number of localities; from many of the others it is only present in micromount quantities.

Key Localities:

  • Castell Mine, Ponterwyd, Ceredigion: similar specimens to those from Henfwlch are well-known from this site (e.g. Jones & Moreton, 1977).
  • Craig Rhiwarth Mine, Llangynog, Powys: this mine has produced the finest Welsh examples of aurichalcite with sky-blue aggregates to several millimetres covering areas of weathered carbonate to several square centimetres. National Museum of Wales specimens NMW 68.576.GR18 and 82.17G.M1 are important examples.
  • Henfwlch Mine, Ceulanymaesmawr, Ceredigion: small but colourful specimens of aurichalcite on rotted ferroan dolomite carrying chalcopyrite have been collected in recent years.
  • Llangynog Mine, Llangynog, Powys: smaller but still attractive specimens have been collected from the dumps above the quarry with crystal-lined pockets in weathered carbonate to 2 cm across.
  • Llanymynech copper mines, Powys: there are several fine specimens of aurichalcite from these old mines in the National Museum of Wales Collection.
  • Machen Quarry, Caerphilly, South Wales: hand specimens of well-crystallized acicular aurichalcite covering areas to several square centimetres have been collected in recent years from veins exposed in this working quarry. Plant & Jones (1995) provide a detailed description of this occurrence.

There are no key localities for this specimen.

References:

  1. Jones, J.A. & Moreton, N.J.M., 1977. The Mines and Minerals of Mid-Wales 40pp.
  2. Plant, S.P. & Jones, I.E., 1995. Minerals of Machen Quarry, Mid Glamorgan, Wales. Journal of the Russell Society, 6(1), 31-36.

There are no references for this specimen.