Cuprite

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Copper oxide
Chemical Formula: Cu2O
Method(s) of Verification: the distinctive colour and form of cuprite combined with its typical association with other copper species allows visual identification to be used with reasonable certainty. Material from Llechweddhelyg Mine has been confirmed by XRD (NMW X-1126).

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
  • Supergene: minerals on an artificial substrate
  • Supergene: in situ natural oxidation & weathering deposits
Octahedral cuprite crystals (up to 0.09 mm across) from Dolyhir Quarry. © D.I. Green.
Cubic cuprite crystals, to 0.2 mm on edge, from Lodge Park copper trial, in the Central Wales Orefield. J.S. Mason Collection (no. JMLP529). Photo M.P. Cooper, © National Museum of Wales.
Introduction: cuprite is a secondary mineral found in the oxidized zone of copper deposits.
Occurrence in Wales: cuprite is uncommon in Wales, particularly as well-formed crystals. Phillips (1823) described ferruginous red oxide of copper, or tile ore from Llanymynech hill on the border between Powys and Shropshire. No further details are known, but many other copper minerals have been reported from Llanymynech and the likelihood is that cuprite could occur albeit in small amounts. In North Wales Greenly (1919) noted cuprite from Parys Mountain on Anglesey based on a list compiled by Prof. Harold Hilton. Parys Mountain was, at one time the largest copper mine in Europe, but surprisingly old-time specimens of anything other than anglesite are rare and no examples of cuprite are known. Plush red, acicular cuprite, probably in the form of the variety chalcotrichite is however, recorded by Williams (1927) in association with biotite in vesicles within spilitic dolerite at the head of Cwm Llan on Snowdon. The majority of Welsh cuprite occurrences are from the Central Wales Orefield, where locally intense alteration of primary chalcopyrite-bearing veinstone has resulted in occasional cuprite-bearing gossan and rare post-mining developments. Jones & Moreton (1977) described the first three discoveries, at Snowbrook, Geufron and Eaglebrook mines. Bevins (1994) listed many more sites including an important discovery at Lodge Park copper trial where, cubes to several millimetres have formed on the mine walls (Mason & Green, 1996). Recent discoveries at Dolyhir Quarry in the Welsh Borderlands have produced a number of rich handspecimens of calcite veined limestone liberally coated with small, but well formed octahedral crystals. Cuprite appears to be totally absent from South Wales.

Key Localities:

  • Camdwrbach Mine, Nant-y-moch Reservoir, Ponterwyd, Ceredigion: a small dump on the north-west shore of Nant-y-moch Reservoir contains highly oxidized dressed copper ore consisting of massive cuprite containing inclusions of native copper and coated with earthy malachite.
  • Cwm Tregalan, Snowdon, Gwynedd: Williams (1927) describes needles and ragged grains of plush-red cuprite in association with biotite in vesicles in so-called ‘copper dolerites’ (pillow lavas) (Bevins, 1994) at the head of Cwm Llan.
  • Cwmavon Valley Copper Smelting Slags, Aberavon, South Wales: cuprite is common within copper smelter slag. It typically forms minute lustrous wine-red cubes or octahedra. The rare acicular variety, chalcotrichite, occurs within vesicular cavities (Plant, 2003).
  • Dolwen Mine, Devil’s Bridge, Ceredigion: this locality yields fine paragenetic material. Massive cuprite with inclusions of native copper occurs coated by malachite.
  • Dolyhir Quarry, Old Radnor, Powys: cuprite is uncommon, but has been found as minute octahedral crystals scattered on calcite in fractures within limestone, close to the main sulphide vein (D.I. Green, unpublished data). It also occurs as inclusions, replacing native copper dendrites, within calcite.
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: cuprite was described by Jones & Moreton (1977) from this locality as forming small ruby red octahedral crystals associated with native copper and malachite. Cuprite is very rare and typically associated with dark brown resinous goethite (Jones, 1983).
  • Esgairhir Mine, Tal-y-bont, Ceredigion: first reported by Rust (1985) occurring rarely as minute octahedral crystals within oxidized dump material derived from Penybwlch shaft. Cuprite is a very rare component of the secondary assemblage and forms veinlets up to 0.6 mm wide cutting massive copper sulphides including, intergrown chalcocite and tenorite with minor malachite. It also forms sparse deep red patches, up to 1.5 mm across, within veins of cerussite and more rarely as octahedral crystals (up to 0.4 mm) (Rust & Mason, 1988).
  • Frongoch Mine, Devil's Bridge, Ceredigion: cuprite is uncommon at this locality. Green et al. (1996) describe massive cuprite with native copper and basic copper sulphates, while octahedral crystals up to 0.3 mm in size have been observed on schulenbergite.
  • Geufron Mine, Llanidloes, Powys: cuprite occurs as an alteration product of chalcopyrite (Jones & Moreton, 1977) associated with malachite and native copper on the upper dumps below the lode outcrop (Mason, 1992).
  • Glogfawr Mine, Ysbyty Ystwyth, Ceredigion: cuprite was reported by Rust & Rust (1986) as small masses, up to 2 mm across, associated with other secondary species.
  • Great Orme Copper Mines, Llandudno, Gwynedd: reported from the Great Orme's Head by R. A. Ixer in Bevins (1994), but no further details are known.
  • Lletty Evan-Hen Mine, Bont-goch, Ceredigion: rich patches of massive cuprite occur within typical weathered chalcopyrite-bearing quartz veinstone on National Museum of Wales specimen NMW 87.43G.M.73.
  • Lodge Park Copper Trial, Tre’r-ddol, Ceredigion: described by Mason & Green (1996) as 'the finest specimens from the area (Central Wales) to date'. Although small (less than 2 mm), the crystals are by far the best examples of cuprite from Wales. At Lodge Park, cuprite is restricted to fractures in shales on the footwall side of the lode where it forms crusts of scattered, euhedral, transparent, wine-red crystals up to 0.5 mm in size and distorted darker, translucent, crystals up to 2 mm across. Cubic crystals are most common although these are often modified by the octahedron (Mason & Green, 1996).
  • Parys Mountain, Anglesey: cuprite is mentioned by Greenly (1919), although no specific details are recorded.
  • Snowbrook Mine, Llanidloes, Powys: reported by Jones & Moreton, (1977) as an alteration product of chalcopyrite.
  • Ystrad Einion Mine, Furnace, Ceredigion: recorded both underground and from surface dumps by Mason & Rust (1997). Underground, cuprite replaces dendritic native copper of post-mining formation. At surface, traces of massive ‘tile-ore’ cuprite within cerussite with earthy malachite from the upper dumps are posulated as representing in situ mineralization (Mason & Rust, 1997).

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Green, D.I., Rust, S.A. & Mason, J.S., 1996. Classic British mineral localities: Frongoch Mine, Dyfed. UK Journal of Mines & Minerals, 17, 29-38.
  3. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  4. Jones, A.D., 1983. Nant-y-Cagl. Mineral Realm, 3, 42-76.
  5. Jones, J.A. & Moreton, N.J.M., 1977. The Mines and Minerals of Mid-Wales 40pp.
  6. Mason, J.S. & Green, D.I., 1996. Supergene copper mineralisation in situ at Lodge Park Copper trial, Dyfed. UK Journal of Mines and Minerals. 17. 19-23.
  7. Mason, J.S. & Rust, S.A., 1997. The Mineralogy of Ystrad Einion Mine, Dyfed, Wales. U.K. Journal of Mines and Minerals, 18, 33-36.
  8. Phillips, W., 1823. An elementary introduction to the knowledge of Mineralogy. 3rd edition. London. 406pp.
  9. Plant, S., 2003. Secondary minerals from the Cwmavon Valley Copper Smelting Slags, Glamorgan, South Wales. Journal of the Russell Society, 8(1), 9-15.
  10. Rothwell, M. & Mason, J.S., 1992. Wulfenite in the British Isles. UK Journal of Mines & Minerals, 11, 30-41.
  11. Rust, S., 1985. British Micro Localities. No. 12: Esgair Hir Mine (SN 734913), Ceulanymaesmawr, Dyfed, Wales British Micromount Society Newsletter. 13. 13-14.
  12. Rust, S., & Rust, D., 1986. British Micro Localities. No. 12 (continued): Dyfngwm Mine, Penegoes, Powys (SN 850931). British Micromount Society Newsletter. 16, 15-16.
  13. Rust, S.A. & Mason, J.S., 1988. The minerals of Esgair-Hir mine, Dyfed, Wales. UK Journal of Mines & Minerals, 5, 35-43.
  14. Williams, H., 1927. The geology of Snowdon (North Wales). Quarterly Journal of the Geological Society of London, 83, 346-431.