Crystal System: Cubic
Status of Occurrence: Discredited Occurrence
Chemical Composition: Cobalt sulphide
Chemical Formula: Co3S4
Method(s) of Verification: occurrence discredited by EMPA analyses which indicate siegenite composition.
Siegeneite, previously identified as linnaeite, on millerite. © National Museum of Wales.
Introduction: linnaeite occurs with other cobalt and nickel-bearing minerals in hydrothermal veins and other ore-deposits. It is a member of the thiospinel group of minerals which have the basic formula M3S4 in which M can be cobalt (Co), nickel (Ni), iron(Fe), copper (Cu), chrome (Cr) or indium (In) and S is sulphur but can in some cases be selenium (Se) or tellurium (Te).
Occurrence in Wales: back in the late 19th Century, small (0.25-0.75 mm) silvery-white octahedral crystals, on a specimen from 'Rhonda Valley, Glamorganshire' belonging to 'Mr Terrill of Swansea' were identified by Des Cloizeaux (1880) as linnaeite. A further South Wales report was added by North & Howarth (1928), this time more specifically provenanced to No. 1 Pit, Ferndale, Mid Glamorgan. But doubt had already been cast on the first report, by North (1916) who noted that an analysis by Des Cloizeaux had detected the presence of nickel. Further descriptions of 'linnaeite' from South Wales were provided by Howarth (1954), who noted that the original Terrill specimen had been transferred, along with much of the rest of the Terrill Collection, to the NMW in 1937. The specimen in question (NMW 37.239.GR1) was investigated by Bevins & Horak (1985) who confirmed that it was in fact an example of the cobalt-nickel member of the same mineral group, siegenite. A misidentification of siegenite as linnaeite in Central Wales was made in the 1980s (Rust & Mason, 1988) but was subsequently rectified following electron microprobe analyses (Mason, 1994, 1997, 1998). Once again the role of chemical analyses of thiospinel minerals, as the best means of identification, was demonstrated. Another confirmed siegenite occurrence in which the sample was originally labelled as linnaeite was at the Great Orme mines near Llandudno in Gwynedd: in this case the correction was made by Ixer & Stanley (1996). According to available data, linnaeite is thus discredited as a Welsh species.
There are no key localities for this specimen.
- Bevins, R.E. & Horak, J.M., 1985. Siegenite in clay-ironstone nodules from the South Wales Coalfield. Journal of the Russell Society, 1, 83-85.
- Cloizeaux, A.L. Des., 1880. Bulletin de la Société Française de Minéralogie, 3, 170-171.
- Howarth, W.E., 1928. On the occurrence of linnaeite in the Coal Measures of South Wales. Geological Magazine, 91, 407.
- Ixer, R.A. & Stanley, C.J., 1996. Siegenite-bearing assemblages found at the Great Orme Mine, Llandudno, N. Wales. Mineralogical Magazine, 60, 978-982.
- Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
- 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.
- 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.
- North, F.J., 1916. The minerals of Glamorgan. Transactions of the Cardiff Naturalists' Society, 49, 16-51.
- North, F.J. & Howarth, W.E., 1928. On the occurrence of millerite and associated minerals in the Coal Measures of South Wales. Proceedings of the South Wales Institute of Engineers, 44, 325-348.
- Rust, S.A. & Mason, J.S., 1988. The minerals of Esgair-Hir mine, Dyfed, Wales. UK Journal of Mines & Minerals, 5, 35-43.