Marcasite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Iron sulphide
Chemical Formula: FeS2
Method(s) of Verification: Vaynor Quarry - XRD (National Museum of Wales, NMW X-843); all other occurrences identified by visual means or reflected light microscopy.

Chemical Group:

  • Sulphides

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
  • Hydrothermal: Mississippi Valley Type veins
  • Hydrothermal: mesothermal polymetallic veins
  • Hydrothermal: epithermal polymetallic veins & pipes
  • Hydrothermal: volcanogenic massive sulphides
  • Sedimentary
  • Hydrothermal: sedimentary exhalative deposits
  • Hydrothermal: copper-dolomite
Marcasite, associated with banded sphalerite and calcite in a cut and polished slab (25 cm across) from West Cwmheisian Mine. © J.S. Mason.
Supergene marcasite in a polished section from Panorama Mine. The banded texture is characteristic; the darker grey bands are iron oxides. Field of view ca. 0.5 mm across. © J.S. Mason.
Cog-wheel marcasite crystals up to 10 mm across from Gwynfynydd Mine, in the Dolgellau Gold-belt. National Museum of Wales Collection (NMW 86.95G.M.2) ex R.W. Barstow Collection. © National Museum of Wales.
Introduction: marcasite occurs in a variety of settings but is most commonly seen as a primary mineral in low-temperature hydrothermal mineral veins, where it is associated with pyrite, sphalerite, wurtzite, galena and other sulphides. Supergene marcasite is also widespread and is commonly formed during the weathering of pyrrhotite. It also occurs in sedimentary rocks, where it is most commonly found forming, or occurring within, concretionary nodules. Marcasite is easy to identify when well-crystallized, forming prismatic crystals, often twinned into cogwheel-like forms. Massive marcasite is more of a yellow-white colour compared to pyrite but it rapidly tarnishes due to the onset of decay, after which its noxious, sulphurous smell is distinctive. The instability of marcasite when exposed to moist air is a well-known problem: the initial products include sulphuric acid which goes on to attack other sulphides and wallrock minerals, thus liberating other metals which can lead to a significant pollution problem in waters draining from mineworkings. The same process can affect inappropriately-stored marcasite specimens, the conservation of which is a well-known problem to museum curators.
Occurrence in Wales: marcasite is a widespread mineral in Wales, occurring in considerable amounts in some mining districts, notably in the Central Wales and Llanrwst Orefields, where its abundance and ready decomposition into acidic iron-rich solutions causes problems in some areas due to polluted minewater. Although some fine specimens have been obtained, the mineral's instability has made the preservation of many of these impossible. Locality data below focuses on the mineral's distribution as this is in most cases far more relevant than the specimens recovered.

Key Localities:

  • Central Wales Orefield: marcasite is widespread as a late-stage mineral in the polyphase lodes of this area and was in fact worked in a limited manner as 'pyrites', for sulphuric acid manufacture (Jones, 1922). As a late-stage mineral it often net-veins earlier mineralization (Mason, 1994, 1997): this contamination affected some lead-zinc ores so badly as to render them unsaleable. Well-formed crystals have been collected at the Brynyrafr, Plynlimon, Bacheiddon, Esgairfraith and Eaglebrook mines but these decompose rapidly unless kept in a controlled environment. The greatest concentration of marcasite, however, is along the Castell Lode in the Ystumtuen area: here it occurs in such abundance as to cause serious environmental problems due to acid mine drainage.
  • Dolgellau Gold-belt, Gwynedd: a widely-distributed late-stage calcite-marcasite (plus sphalerite, galena, quartz, goethite, pyrite, hematite) assemblage is present in this area and is frequently observed crosscutting earlier veins such as the Gold-belt lodes (Mason et al., 2002). Some fine cogwheel-twinned marcasite crystals have been recovered from the assemblage, notably at Gwynfynydd Mine (National Museum of Wales specimens, e.g. NMW 86.95G.M.2, ex R.W. Barstow Collection). Further fine marcasite crystals in the National Museum of Wales Collection are from the railway tunnel between Blaenau Ffestiniog and Roman Bridge. Secondary marcasite, as an alteration product of pyrrhotite in the Gold-belt lodes, is also widespread (Gilbey, 1968; Mason et al., 2002).
  • Great Orme Copper Mines, Llandudno, Gwynedd: marcasite is one of the few sulphide associates of chalcopyrite in the copper-dolomite association worked at this locality. It forms generally small (20-30 ┬Ám) bladed crystals in or close to chalcopyrite (Ixer & Davies, 1996).
  • Llanrwst Orefield, Gwynedd: marcasite is a widespread relatively late-stage mineral from this lead-mining district (Haggerty, 1995). Some excellent specimens, forming cockscomb crystal groupings associated with calcite and minor chalcopyrite, have been recovered from the area, the best examples in the National Museum of Wales Collection being those from the Trecastell and Parc mines.
  • South Wales: in the lead-zinc orefield of South Wales minor marcasite is a widespread late-stage mineral in this zone of Mississippi Valley Type (MVT) mineralization. Occurring as crosscutting veinlets traversing galena and baryte, it has been noted across the district from Machen in the east to the Cowbridge area in the west. In the northern outcrop of the Carboniferous Limestone, interesting micro-specimens have been obtained at Vaynor Quarry near Merthyr Tydfil in which elongated blades of marcasite occur with pyrite cubes at either end, resembling dumb-bells in shape.
  • South Wales Coalfield: marcasite is a relatively minor sulphide in the millerite-bearing septarian nodule assemblage. Specimens are, however, known from the following coalmines: Coed Ely, Wylie, Parc, Big Pit and Llanharan West Opencast. The best known specimen from the coalfield (which by far outstrips all others from the area) is a 10 x 5 cm plate of twinned cockscomb crystals to 25 mm, on gritstone matrix, from Big Pit (Natural History Museum Collection).

References:

  1. Gilbey, J.W., 1968. The mineralogy, paragenesis and structure of the ores of the Dolgellau Gold Belt, Merionethshire, and associated wall rock alteration. Unpublished Ph.D thesis, University of London, UK.
  2. Haggerty, R., 1995. The Mineralization of the Llanwrst Orefield, North Wales. UK Journal of Mines and Minerals, 15, 5-10.
  3. 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.
  4. Jones, O.T., 1922. Lead and zinc. The mining district of North Cardiganshire and West Montgomeryshire. Memoirs of the Geological Survey. Special Report of the Mineral Resources of Great Britain, 20.
  5. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  6. 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.
  7. Mason, J.S., Bevins, R.E. & Alderton, D.H.M., 2002. Ore Mineralogy of the mesothermal gold lodes of the Dolgellau Gold Belt, North Wales. Transactions of the Institution of Mining and Metallurgy (Section B, Applied earth science), 111, B203-B214.