Cobalt pentlandite

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Cobalt sulphide with appreciable nickel and/or iron
Chemical Formula: (Co,Fe,Ni)9S8
Method(s) of Verification: Central Wales Orefield - EMPA (British Geological Survey, Nottingham, D.J. Bland, 1989).

Chemical Group:

  • Sulphides

Geological Context:

  • Hydrothermal: mesothermal polymetallic veins
Lamellae of cobalt pentlandite (pale yellow) in siegenite (pinkish-grey). Galena (blue-grey), quartz (black). Polished section from Erglodd Mine; field of view 0.15 mm high, © J.S. Mason.
Siegenite crystals, 0.25 mm (pink-grey) with 'trellis' sets of cobalt pentlandite lamellae (pale yellow) extending as flame-like bodies into chalcopyrite (dull yellow). Quartz (black). Polished section, Loveden Mine. © J.S. Mason.
Introduction: cobalt pentlandite occurs typically in polymetallic hydrothermal ore deposits, especially veins, where other cobalt-nickel minerals (e.g. siegenite, linnaeite, millerite) are present. Pure cobalt pentlandite is ideally a cobalt sulphide but most examples are cobalt/iron/nickel sulphides. It forms a series with pentlandite, iron-nickel sulphide. Its paragenetic position as a lamellar alteration product of siegenite is particularly diagnostic in terms of its identification.
Occurrence in Wales: cobalt pentlandite was one of a number of rare ore minerals discovered during an extensive study of the mineralization of Central Wales in the period 1986-1994 (Mason, 1994, 1997, 1998). It was identified by analogy to a published, paragenetically similar occurrence in Canada (Petruk et al., 1969) and confirmed by electron microprobe analysis in 1989; subsequent occurrences have been identified optically in polished section.

Key Localities:

  • Central Wales Orefield: cobalt pentlandite occurs, with siegenite, as a component of the early (A1) polymetallic vein mineralisation which is noted for its varied mineralogy. It has been recorded at the following mines: Erglodd, Brynyrarian, Loveden, Ystrad Einion, Esgairhir, Esgairfraith and Nantycagl. In all cases it is only detectable by petrological examination of polished sections: however, coarser siegenite crystals from Erglodd often reveal weathering patterns consistent with decomposition of cobalt pentlandite lamellae that are visible to the naked eye.

References:

  1. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  2. Mason, J.S., 1998. Tucekite, a mineral new to Britain, and other rare ore minerals from the Central Wales Orefield. UK Journal of Mines and Minerals, 19, 30-36.
  3. Mason, J.S., 1997. Regional polyphase and polymetallic vein mineralisation in the Caledonides of the Central Wales Orefield. Transactions of the Institution of Mining and Metallurgy (Section B: Applied Earth Science), 106, B135-B144.
  4. Petruk, W., Harris, D.C. & Stewart, J.M.,, 1969. Langisite, a new mineral, and the rare minerals cobalt pentlandite, siegenite, parkerite and bravoite from the Langis Mine, Cobalt-Gowganda area, Ontario, Canada. Canadian Mineralogist, 9, 597-616.