Spionkopite

Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Copper sulphide (non-stoichiometric)
Chemical Formula: Cu39S28
Method(s) of Verification: both localities listed - high-resolution ore petrology.

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Introduction: spionkopite occurs in supergene alteration assemblages derived from primary sulphide mineralization in which copper is an important component. It typically occurs, in intergrowths with the other supergene sulphides, as replacements of the primary minerals, forming rims and veinlets along cleavage-planes and/or grain boundaries. Spionkopite is one of several non-stoichiometric copper sulphide species. By this it is meant that they have compositions which fit in between copper1+ sulphide (chalcocite), and copper2+ sulphide (covelline). Many of these minerals are recent discoveries as they occur in complex, microscopic intergrowths which have required modern analytical techniques to unravel their detailed mineralogy. As a consequence, many previously studied assemblages containing 'supergene chalcocite' or covellite-like minerals may well prove to be a lot more complex: hence, further occurrences of spionkopite are likely to be unearthed.
Occurrence in Wales: unknown in Wales until relatively recently, spionkopite has now been found at two localities, one in North and one in Central Wales. These discoveries have both resulted from high-resolution reflected light microscopy studies of supergene alteration assemblages associated with the 'ore' in ancient (Bronze Age) copper mines.

Key Localities:

  • Copa Hill, Cwmystwyth mines, Ceredigion: reported by Ixer (2003), as a component of thin alteration rims formed on chalcopyrite and galena. The rims also contain yarrowite, digenite and covelline.
  • Great Orme Copper Mines, Llandudno, Gwynedd: reported by Ixer & Davies (1996) as a bladed alteration product of digenite with crystals up to 20 ┬Ám in length and also as thin rims, with digenite and covelline, on altered pyrite/marcasite and galena.

References:

  1. Ixer, R.A., 2003. Ore Mineralogy. In: Excavations on Copa Hill, Cwmystwyth (1986 - 1999). An early Bronze Age copper mine within the uplands of Central Wales. S. Timberlake (ed.) BAR British Series, 348, 99.
  2. 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.