Digenite

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Copper sulphide
Chemical Formula: Cu9S5
Method(s) of Verification: occurrences cited - identification by optical properties.

Chemical Group:

  • Sulphides

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Introduction: this species, along with chalcocite, djurleite and a number of others, belongs to a group of minerals which are difficult to distinguish as they look superficially similar and each contains a subtly different copper/sulphur ratio. They are distinguishable in polished section with care and a lot of experience. Quantitative electron microprobe analysis has to be of the highest accuracy in order to obtain reliable identifications, and it is not unreasonable to recommend that any undifferentiated grey supergene copper sulphide be simply referred to as such until detailed analytical work confirms which actual species is/are present. Digenite occurs in zones of supergene enrichment overlying ore deposits in which chalcopyrite, tetrahedrite and other primary copper ores are present.
Occurrence in Wales: only three confidently identified digenite occurrences are currently listed from Wales. This may be more of a reflection of the lack of detailed work on grey supergene copper sulphides, uncharacterized occurrences of which are widespread.

Key Localities:

  • Cwmystwyth Mine, Cwmystwyth, Ceredigion: confirmed to date only from Copa Hill, Cwmystwyth (Ixer, 2003), where it is present as thin rims on chalcopyrite grains with covelline, spionkopite and yarrowite.
  • Great Orme Copper Mines, Llandudno, Gwynedd: reported by Ixer & Davies (1996) as rims up to 60 µm thick between chalcopyrite and limonite and also as thin rims on altered pyrite/marcasite.
  • Llantrisant area, South Wales: digenite was reported from mines in this district by Bowler & Kingston (1971), forming an intimate intergrowth with chalcocite replacing galena along grain margins and cleavage planes.

References:

  1. 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.
  2. 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.
  3. 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.