Covellite

Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Copper sulphide
Chemical Formula: CuS
Method(s) of Verification: Dolyhir Quarry - EMPA & XRD (NMW X-1446 & X-1472); Central Wales, Dolgellau Gold-belt, Great Orme & Parys Mountain - all optical identifications from polished sections.

Chemical Group:

  • Sulphides

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
  • Supergene: in situ natural oxidation & weathering deposits
Well-crystallized covellite from Dolyhir Quarry, Old Radnor, Powys. P. Todhunter specimen. Photo D.I. Green, © D.I. Green.
Photomicrograph of covellite (characteristic blue shades in plane polarized light) in polished ore section. Dolyhir Quarry, Old Radnor, Powys. National Museum of Wales Collection. Image T.F. Cotterell, © National Museum of Wales.
Introduction: covellite is a widespread mineral in the supergene zones of copper ore deposits. It typically replaces hypogene sulphides and is commonly associated with other copper sulphides such as digenite. Its typical occurrence is as a thin deep indigo-blue film on chalcopyrite but it may also occur as a coating on other sulphides. Covellite is sometimes difficult to identify in hand specimen but in polished section it is a very distinctive mineral.
Occurrence in Wales: the occurrence of covellite in Wales, though widespread, has only been reported in the literature in recent years. It is present in all mining districts where primary chalcopyrite occurs.

Key Localities:

  • Central Wales Orefield: present in microscopic amounts at all mines where chalcopyrite is present. Rust & Mason (1988) describe hexagonal plates to 0.1mm from Esgairhir Mine, but generally it is to be found as thin coatings and aggregates (Mason, 1994) as in the example illustrated above.
  • Dolgellau Gold-belt, Gwynedd: covellite is widespread and occurs at all mines in this area. Written accounts are available from Gilbey (1968) and, with reference to Clogau Mine, from Naden (1988).
  • Dolyhir Quarry, Old Radnor, Powys: covellite occurs as minute iridescent to metallic purple hexagonal plates in cavities in massive sulphide and is commonly seen to surround galena in polished section. Well formed metallic blue covellite crystals to 1 mm were recently (2002) found in carbonate veins occurring in Precambrian strata in the lower part of the quarry (D.I. Green, unpublished data).
  • Great Orme Copper Mines, Llandudno, Gwynedd: recorded by Ixer & Davies (1996) as a microscopic alteration product of primary sulphides.
  • Llangynog Orefield, NW Powys: thin films of covellite are common at Llangynog and nearby mines wherever slightly weathered chalcopyrite is present (Bevins & Mason, 1997).
  • Llantrisant area, South Wales: Bowler & Kingston (1971) noted thin films of covellite as an interim phase in the oxidation of chalcopyrite to limonite.
  • Parys Mountain, Anglesey: covellite is recorded as a common alteration product of chalcopyrite (Pointon & Ixer, 1995).
  • Snowdonia, Gwynedd: in the copper mines of the Snowdon caldera covellite is frequently seen as thin coatings on weathered chalcopyrite and other sulphides in this area (Bevins & Mason, 1998).

There are no key localities for this specimen.

References:

  1. Bevins, R.E. & Mason, J.S., 1998. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Gwynedd. National Museums of Wales, Cardiff.
  2. Bevins, R.E. & Mason, J.S., 1997. Welsh metallophyte and metallogenic evaluation project: Results of a minesite survey of Dyfed and Powys. CCW Contract Science Report No. 156. National Museums & Galleries of Wales.
  3. Bowler, C.M.L. & Kingston, G.A., 1971. Mineralisation in the Triassic rocks of the Llantrisant area, Glamorgan. 1970 Gregynog Mineral Exploitation Colloquium Report. University College Cardiff, 30-34.
  4. Gilbey, J.W., 1968. The mineralogy, paragenesis and structure of the ores of the Dolgellau Gold Belt, Merionethshire, and associated wall rock alteration. Unpublished Ph.D thesis, University of London, UK.
  5. Ixer, R.A. & Davies, J., 1996. Mineralisation at the Great Orme Copper Mines, Llandudno, North Wales. U.K. Journal of Mines and Minerals, 17, 7-14.
  6. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  7. Naden, J., 1988. Gold mineralisation in the Caledonides of the British Isles with reference to the Dolgellau Gold Belt and the Southern Uplands of Scotland. Unpublished Ph.D thesis, University of Aston, UK.
  8. Pointon, C.R. & Ixer, R.A., 1980. Parys Mountain mineral deposit, Anglesey, Wales: geology and ore mineralogy. Transactions of the Institution of Mining and Metallurgy (Section B: Applied earth science), 89, B143-B155.
  9. Rust, S.A. & Mason, J.S., 1988. The minerals of Esgair-Hir mine, Dyfed, Wales. UK Journal of Mines & Minerals, 5, 35-43.

There are no references for this specimen.