Chalcophyllite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Copper aluminium arsenate sulphate hydroxide hydrate
Chemical Formula: Cu18Al2(AsO4)3(SO4)3(OH)27.33H2O
Method(s) of Verification: Dylife Mine - XRD (Natural History Museum); Bontddu - XRD (Natural History Museum); Gwaith-yr-afon Mine - XRD (Natural History Museum); Aberavon - IR & XRD (Natural History Museum, XRD film no. 1979F).

Chemical Group:

  • Arsenates
Introduction: chalcophyllite is a supergene copper mineral, typically occurring in the oxidation zones of copper-bearing orebodies in which significant primary arsenic mineralization (e.g. arsenopyrite) is also present. It may also form in post-mining assemblages derived from similar primary sources. Associated minerals are typically cuprite, other copper arsenates and carbonates and, in post-mining assemblages, sulphates. The distinctive morphology of chalcophyllite crystals - as groups of lamellar hexagonal plates - makes visual identification with the binocular microscope possible in well-crystallized examples.
Occurrence in Wales: the first occurrence of chalcophyllite in Wales was described by Rust & Rust (1987), on a single specimen collected at Dyfngwm Mine (actually the surface workings of Dylife Mine). Further occurrences, all microcrystalline in nature, have since come to light, but chalcophyllite remains a very rare mineral in Wales.

Key Localities:

  • Aberavon, South Wales: chalcophyllite, although rare on Aberavon beach, occurs with slag forming turquoise pseudohexagonal interlocking plates, mostly grouped in the form of rosettes. The crystals are generally transparent covering several square millimetres of matrix (Plant, 2003). According to the latest definition of what constitutes a mineral (Nickel, 1995), such occurrences, on a matrix of manmade (ie. 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.
  • Bontddu, Dolgellau, Gwynedd: chalcophyllite occurs within a trial level at Bontddu, occurring as extremely rare intergrown hexagonal green platy crystals in masses to 2.5 mm, as scattered single crystals, and rosettes associated with tyrolite, malachite and devilline (Saich & Rust, 1987).
  • Dylife Mine, Penegoes, Powys: chalcophyllite occurs as scattered and aggregated crystals to 0.2 mm, with a thin tabular hexagonal habit and dark green colour, in association with langite, brochantite and covelline on chalcopyrite rich veinstone (Rust & Rust, 1987). Only one specimen has been found to date.
  • Gwaith-yr-Afon Mine, Goginan, Ceredigion: rarely as minute (<0.5 mm) emerald-green hexagonal plates aggregated together in small cavities in quartz associated with tyrolite (Rust & Mason, 1994).

References:

  1. Nickel, E.H., 1995. Definition of a mineral. Mineralogical Magazine, 59, 767-768.
  2. Plant, S., 2003. Secondary minerals from the Cwmavon Valley Copper Smelting Slags, Glamorgan, South Wales. Journal of the Russell Society, 8(1), 9-15.
  3. Rust, S. & Rust, D., 1987. Micro-minerals from Dyfngwm Mine. U.K. Journal of Mines and Minerals, No. 2, 28-32.
  4. Rust, S.A. & Mason, J.S., 1994. An unusual occurrence of arsenate minerals at Gwaith-yr-Afon mine, Dyfed, Wales. Journal of the Russell Society, 5(2), 109-113.
  5. Saich, D.A. & Rust, S.A., 1987. Micro-minerals from a trial level in Wales. U.K. Journal of Mines and Minerals, No. 3, 3-4.