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.

There are no key localities for this specimen.

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.

There are no references for this specimen.