Mineral Database (Saesneg yn unig)

Mineral Database (Saesneg yn unig)
Mineralogy of Wales

Siegenite

Crystal System: Cubic
Formula: (Co,Ni)3S4
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Cobalt nickel sulphide
Method(s) of Verification: verified by electron microprobe analyses (see localities below).
Chemical Group:
  • Sulphides
Geological Context:
  • Hydrothermal : Mississippi Valley Type veins
  • Hydrothermal : mesothermal polymetallic veins
Introduction: siegenite occurs in a range of mineral associations but is chiefly known from polymetallic mineral veins, where it occurs with base-metal sulphides and other Co and Ni minerals. It may also occur in diagenetic septarian nodules associated with coal-bearing strata, where siderite and millerite are common associates. Siegenite is not easy to identify in hand specimen or polished section due to its similarity to linnaeite. Without analysis, it is better to refer to such samples as 'thiospinel', which is the name of the mineral group to which siegenite and linnaeite belong. All thiospinels have the basic formula M3S4 in which M can be Co, Ni, Fe, Cu, Cr or In and S is sulphur but can in some cases be Se or Te.
Occurrence in Wales: the occurrence of siegenite in Wales was determined relatively recently from three areas, namely the South Wales Coalfield, the Central Wales Orefield and Great Orme, Llandudno. In all three cases it had previously been identified as linnaeite, by Des Cloizeaux (1880), North & Howarth (1928) and Firth (1971) from South Wales, by Mason (in Rust & Mason, 1988) from Central Wales and noted, on the basis of an old specimen label, by Bevins (1994) from Great Orme. The corrections were published, respectively, by Bevins & Horak (1985), Mason (1994, 1997, 1998) and by Ixer & Stanley (1996). Siegenite, while generally uncommon or rare in Wales, is present in significant quantities at some sites within the Central Wales Orefield.
Key Localities:
  • Brynyrarian Mine, Tre-Taliesin, Ceredigion: siegenite is present as a member of the early polymetallic vein assemblage (A1-c of Mason, 1994, 1997), forming fine-grained pinkish-grey masses in quartz, up to 2 cm across.
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: occasionally visible as intergrowths with chalcopyrite in hand specimens, siegenite is more frequently observed in polished sections where it is associated chiefly with chalcopyrite and millerite.
  • Erglodd Mine, Talybont, Ceredigion: formerly common, to the extent that Jones (1922) commented that 'some pyritous lode-matter occurs on the tip' - pyrite is in fact rare at this locality so that he must have noticed, but not correctly identified, siegenite. Typically it occurs as disseminated crystals (exceptionally up to 3 mm) in quartz-cemented breccia, chiefly in association with chalcopyrite. Some rich specimens have been collected in past years, showing extensive disseminations of siegenite. In polished section, intergrown lamellar cobalt pentlandite is a frequent associate.
  • Esgair Fraith Mine, Tal-y-bont, Ceredigion: present as inclusions in galena and as fine-grained disseminations in quartz with chalcopyrite and millerite. More frequently observed in polished section than in hand specimens.
  • Esgairhir Mine, Tal-y-bont, Ceredigion: present as inclusions in galena and as fine-grained disseminations in quartz with chalcopyrite and millerite. More frequently observed in polished section than in hand specimens.
  • Great Orme Copper Mines, Llandudno, Gwynedd: a small number of specimens collected from the spoil heaps have revealed disseminated polymetallic sulphides in argillaceous dolomitized limestones. Siegenite occurs as a component of this assemblage, forming euhedral to subhedral crystals (up to 4 mm across - e.g. National Museum of Wales specimen NMW 83.41G.M.1489) in association with pyrite, gersdorffite and chalcopyrite: shattered grains to several millimetres in size are cemented by pyrite and marcasite. This is a distinctively different assemblage compared to the main, chalcopyrite-saddle dolomite ore; at Great Orme (Ixer & Stanley, 1996).
  • Henfwlch Mine, Ceulanymaesmawr, Ceredigion: observed in polished sections where it occurs as microscopic intergrowths with chalcopyrite, galena, millerite and tucekite.
  • Loveden Mine, Tre'r-ddol, Ceredigion: occasionally found in polished sections as intergrowths with cobalt pentlandite embedded in chalcopyrite. Very rarely is it noted in hand specimens, and further finds are unlikely as the site is virtually obliterated.
  • South Wales Coalfield: siegenite is widespread in small quantities in the siderite-lined septarian clay-ironstone nodules that occur in the Coal Measures in the area and it also occurs in the ankerite-quartz dominated assemblage developed on joints in associated massive sandstone beds. In both settings the chief associated sulphides are millerite, galena, chalcopyrite and sphalerite. Crystals of siegenite tend to be small (approx. 0.5 mm) although examples up to 1.5 mm have been recorded from a number of coal-tips, notably that at Gelli Colliery in the Rhondda Valley. Other localities include Wyndham Colliery, Ogmore Vale (Bevins & Horak, 1985); International Colliery, Blaengarw; Oakdale Colliery, Caerphilly; Universal Colliery, Senghenydd; Ferndale Colliery, Rhondda; Park Slip West Opencast, Bridgend and Nant Helen Opencast, Ystradgynlais (National Museum of Wales specimens).
  • Ystrad Einion Mine, Furnace, Ceredigion: rare, as intergrowths with cobalt pentlandite associated with chalcopyrite in quartz.
References:
  • Bevins, R.E., 1994 A Mineralogy of Wales  National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  • 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.
  • Firth, J.N.M., 1971 The Mineralogy of the South Wales Coalfield.  Unpublished Ph.D. thesis, University of Bristol.
  • 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. & 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.