Millerite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Nickel sulphide
Chemical Formula: NiS
Method(s) of Verification: millerite identification is based on its highly distinctive appearance, both in hand specimen and in polished section.

Chemical Group:

  • Sulphides

Geological Context:

  • Hydrothermal: Mississippi Valley Type veins
  • Hydrothermal: mesothermal polymetallic veins
  • Sedimentary
Sprays of lustrous golden millerite needles (40 mm) span a siderite-lined cavity in a clay-ironstone nodule. Specimen NMW 82.22G.M1, from Treharris, South Wales. © National Museum of Wales.
Millerite spray (25 mm in length) on siderite. Powell Duffryn Colliery, Bargoed, South Wales Coalfield. National Museum of Wales Collection (NMW 28.384.GR.1). Photo M.P. Cooper, © National Museum of Wales.
Radiating mass (up to 25 mm across) of acicular millerite in a cavity in clay ironstone nodule from Gelli Colliery, Mid Glamorgan. I.E. Jones Collection. Photo M.P. Cooper, © National Museum of Wales.
Millerite from a hydrothermal vein-breccia, Brynyrafr Mine, near Ponterwyd in Central Wales: a chalcopyrite crystal in a quartz cavity is speared by twisted metallic needles of millerite to ca. 10 mm in length. Photo D.I. Green, © D.I. Green.
Introduction: millerite occurs in a variety of hydrothermal mineral veins, from medium-temperature (mesothermal) settings to lower-temperature Mississippi Valley type (MVT) associations. It is also developed in certain sedimentary rocks, in particular coal-bearing strata. Mineral associations vary, but typically include other Co-Ni sulphides, chalcopyrite, sphalerite and galena. Millerite is easily identified when crystalline by its colour and habit and it is also distinctive in polished section.
Occurrence in Wales: millerite is justifiably one of Wales' more famous minerals, a position it shares with gold, brookite, anglesite and a number of more obscure secondary species. The best specimens from the septarian clay-ironstone nodules of the South Wales Coalfield are significant in global terms and are present in important mineral collections worldwide. The first description of millerite pre-dated it being given that name. Writing in 1842, W.H. Miller, Professor of Mineralogy at Cambridge, described crystals of 'sulphide of nickel', then known as 'hair pyrites' or 'capillary pyrites', from the Coal Measures of South Wales. Millerite was subsequently named in his honour by Haidinger (1845). Subsequent reports of further millerite localities in South Wales continued through the 19th Century, with a comprehensive account of its distribution being given by North & Howarth (1928). By then there were 15 separate localities; just over 40 years later the list had increased to 23 (Firth, 1971). Today, millerite is known from over 40 coal workings in South Wales. Localities yielding it are generally temporary in nature as it tends to be found when ironstone nodules are briefly available during coal-tip reclamation: however, the increasing tendency towards opencast coal mining has led to new discoveries, including rare in situ examples. In more recent years, millerite has been discovered in other parts of Wales, in these instances occurring in hydrothermal ore deposits. Of particular note is the Central Wales Orefield where a number of localities have been identified since the 1970s, these principally being disused mines in the Talybont, Ceredigion, district. At the majority of Central Wales localities, it occurs as a component of the early (A1) assemblages (see Mason, 1994). As such, it is found either embedded in ferroan dolomite, in cavities in vuggy quartz with chalcopyrite and sphalerite or in fine intergrowths with other Co-Ni minerals, principally siegenite (Jones & Moreton, 1977; Mason, 1994, 1997, 1998). Individual locality details are given below.

Key Localities:

  • Brynyrafr Mine, Ponterwyd, Ceredigion: more abundant here than at any other Central Wales site, millerite forms tangled masses of often twisted needles, generally 5-10 mm in length but exceptionally to 20 mm, spanning quartz cavities and associated with chalcopyrite and occasionally sphalerite (Mason, 1994; 1998). It also occurs embedded in massive quartz. Some attractive specimens have been recovered from this site, although they do not compare in quality to those of the South Wales Coalfield.
  • Brynyrarian Mine, Tre-Taliesin, Ceredigion: rarely as small needles embedded in quartz (Mason, 1998).
  • Camdwrmawr Mine, Nant-y-moch Reservoir, Ponterwyd, Ceredigion: near to Brynyrafr, this small mine also produces millerite needles embedded in quartz, though these are much less well-developed than at Brynyrafr. Noted by Mason (1998).
  • Drws-y-Coed Mine, Nantlle, Gwynedd: millerite has been identified (without description) from this copper mine (N. Hubbard, unpublished data).
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: first recorded by Jones & Moreton (1977) and described in more detail by Mason (1994), millerite occurs in two associations: firstly as a microscopic replacement of early siegenite and cobalt pentlandite, only visible using reflected-light microscopy, and secondly as coarser, idiomorphic crystals with ferroan dolomite, quartz, chalcopyrite and siegenite. The latter association is readily recognised in hand specimen, with millerite needles to 15 mm being characteristic.
  • Erglodd Mine, Talybont, Ceredigion: millerite occurs rarely as needles to 8 mm embedded in chalcopyrite (Mason, 1994).
  • Esgair Fraith Mine, Tal-y-bont, Ceredigion: noted by Rust & Mason (1988), millerite is rare but occurs in association with chalcopyrite, ferroan dolomite and quartz. In polished section, it is also present as intergrowths with siegenite, both occurring embedded in chalcopyrite.
  • Esgairhir Mine, Tal-y-bont, Ceredigion: described by Rust & Mason (1988) as a replacement of siegenite; it also forms small (<2 mm) needles intergrown with siegenite and tucekite. The fine grainsize means that it is more readily observed in polished sections.
  • Frongoch Mine, Devil's Bridge, Ceredigion: very rare, as divergent sprays of slender hairlike crystals to 2 mm in length, in cavities in quartz associated with sphalerite and marcasite (T.F. Cotterell, unpublished data). This is one of two rare examples of millerite from the later (A2) Central Wales mineralization; both of these are relatively recent discoveries.
  • Great Orme Copper Mines, Llandudno, Gwynedd: Ixer & Davies (1996) note small (to 60 ┬Ám) inclusions of millerite in galena from a Mississippi Valley-type lead-vein occupying a fault-plane at this site. Associated minerals were pyrite, marcasite, chalcopyrite and rare sphalerite.
  • Gwaithgoch Mine, Pontrhydygroes, Ceredigion: recently discovered (T.F. Cotterell, unpublished data) as minute golden needles on samples collected during Minescan fieldwork in 1996. The samples consist of late (A2) ullmannite-chalcopyrite bearing quartz veinstone and the millerite occurs in close association with these sulphides.
  • Henfwlch Mine, Ceulanymaesmawr, Ceredigion: present in association with chalcopyrite, siegenite, tucekite and galena in quartz and ferroan dolomite, millerite from this locality is more readily observed in polished sections. Noted by Mason (1998).
  • Hyddgen Mine, Uwchygarreg, Powys: noted by Mason (1998), millerite occurs rarely with chalcopyrite, galena and tucekite. It is generally so fine-grained that ore petrology is best suited to its study.
  • South Wales Coalfield: millerite-bearing ironstone nodules are present throughout the coalfield. The nodules contain septarian geodes lined with siderite crystals upon which millerite and a variety of other sulphides (galena, siegenite, sphalerite, chalcopyrite and pyrite) have crystallized (Firth, 1971). Quartz, barite and various mineral waxes are also present (Firth & Eglinton, 1971). Millerite is most frequently observed in ironstones associated with the bituminous coals but has also been found in the higher-temperature anthracite zone in the northwestern part of the coalfield. Notable specimens have been recovered in the last two decades from tips at, among others, Coed Ely Colliery, Llantrisant; International Colliery, Blaengarw; Gelli and Ferndale collieries, Rhondda; Deep Navigation Colliery, Treharris and Wyndham Colliery, Ogmore Vale. The demise in the deep mining industry coupled with the increase in opencast sites has led to further millerite finds in recent years from sites including Park Slip West near Bridgend and Nant Helen near Ystradgynlais; at the latter site millerite notably occurs, with chalcopyrite, quartz and ankerite, in a different setting - on mineralized joints cutting thick sandstone beds (Bevins & Mason, 2000).

References:

  1. Bevins, R.E. & Mason, J.S., 2000. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Glamorgan and Gwent. National Museums & Galleries of Wales, Cardiff
  2. Firth, J.N.M., 1971. The Mineralogy of the South Wales Coalfield. Unpublished Ph.D. thesis, University of Bristol.
  3. Firth, J.N.M. & Eglinton, G., 1971. Hatchettine from the South Wales Coalfield. Advances in Organic Geochemistry, 613-628.
  4. Haidinger, W., 1845. Handbuch der bestimmenden Mineralogie. Vienna.
  5. 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.
  6. Jones, J.A. & Moreton, N.J.M., 1977. The Mines and Minerals of Mid-Wales 40pp.
  7. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  8. 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.
  9. 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.
  10. Miller, W.H., 1842. On the specific gravity of sulphuret of nickel. The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science, 20, 378-379.
  11. 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.