Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Iron oxide
Chemical Formula: alpha-Fe2O3
Method(s) of Verification: all localities cited - identified by physical properties.
- Oxides & Hydroxides
- Hydrothermal: limestone hosted hematite deposits
- Hydrothermal: alpine type veins
- Hydrothermal: mesothermal polymetallic veins
Hexagonal platy hematite crystal (10 mm across) from Penrhyn Quarry, Bethesda, Gwynedd. National Museum of Wales Collection (NMW 85.70G.M.28) ex R.W. Barstow Collection. © National Museum of Wales.
Specular hematite from Taff's Well Quarry, nr. Cardiff. I.E. Jones Collection. © National Museum of Wales.
Introduction: hematite is an extremely common mineral, occurring in a wide range of settings from hydrothermal mineral veins (from which it is often mined as an iron ore) to red sandstones, to which it gives the distinctive colour both to the rocks and to the soils overlying them. In terms of ore deposits, it tends to occur with other oxide minerals rather than sulphides in many cases, but in certain associations, such as skarns, it occurs with a wide range of sulphides and silicates. Alpine-fissure type veins may contain platy hematite crystals, as in the famed 'Iron Roses' from the Alps. Hematite is generally easy to identify due to its blood-red streak - the name is a reference to the fact that when powdered it is blood-coloured.
Occurrence in Wales: hematite is widespread in Wales, but notable occurrences are few. Most records detail hematite as inclusions within other minerals or rocks, giving them a characterisitic red colour. Localities noted for good crystallized specimens include the iron mines of South Wales and Penrhyn Slate Quarry, near Bethesda in North Wales.
- Central Wales Orefield: supergene hematite occurs at several localities as thin bright red coatings on leached brecciated mudstone from the oxidation zones of the lead-zinc (Pb-Zn) lodes. It is particularly conspicuous at Frongoch Mine.
- Cwm Llan, Snowdon, Gwynedd: hematite occurs as sheaves of platy crystals associated with quartz and magnetite in veins and infilled vesicles in pillowed basalts outcropping near the head of the cwm. It is also an important component of the magnetite-dominated breccia exposed in the closely ajacent Shadow Gully (Colman & Appleby, 1990).
- Dolgellau Gold-belt, Gwynedd: earthy red hematite is a minor component of a late-stage, crosscutting open fissure-type assemblage, occurring with calcite, marcasite, goethite and quartz, for example at Gwynfynydd and Prince Edward mines (Mason et al., 2002). Minor hematite is also present as small, platy crystals on joints in a contact-metamorphosed highly altered carbonate-rich intrusive sill exposed in the bed of Afon Mawddach in Coed-y-Brenin, Gwynedd. Associated minerals in this skarn-like assemblage are magnetite, andradite, chalcopyrite and epidote. The locality is generally inaccessible as it is usually deep underwater. (M.J. Liezers & J.S. Mason, unpublished data).
- Harlech area, Gwynedd: the extremely fine-grained banded red and yellow manganese ores occurring within the Lower Cambrian Hafotty Formation of the Harlech area, contain hematite within the red bands, along with a number of other iron and manganese-bearing minerals (Bennett, 1987b).
- North-east Wales: hematite of a generally earthy nature was mined at a number of localities, principally adjacent to the Vale of Clwyd, such as Cwm and other iron mines near Dyserth and the Bodfari Iron Mine to the NE of Denbigh (Le Neve Foster, 1882; Strahan, 1885; Warren et al., 1984).
- Penrhyn Quarry, Bethesda, Gwynedd: the finest Welsh specimens were recovered in the 1970s, consisting of lustrous pseudohexagonal metallic grey plates in quartz associated with chlorite. These occurred in alpine fissure-type veins in and around boudin neck-zones in basalt dykes cutting Cambrian slate. Crystals to 30 mm were present on the best specimens, several of which were acquired by the National Museum of Wales from the collection of the late Richard Barstow in the 1980s.
- South Wales: an important iron-mining area is hosted by the Carboniferous Limestone to the south of the 'south crop' of the South Wales Coalfield. The distinctive style of mineralization involved extensive dolomitization and volume-loss of the limestone, with the development, along fracture-zones, of systems of richly mineralized solution-cavities. Hematite accompanies goethite, quartz, dolomite, calcite and baryte in a sulphide-free assemblage which provided large tonnages of ore, particularly at the Llanharry and Mwyndy mines near Llantrisant and the Garth Mine near Pentyrch (Vivian, 1885; Rankin & Criddle, 1985). Hematization of the Carboniferous Limestone is widespread throughout its outcrop from the Gower Peninsula in the west to the Machen area in the east: this is at least in part influenced by the proximity of the unconformably overlying red-beds of the Triassic Keuper Marl, in which disseminated hematite is a major component.
- Bennett, M.A., 1987b. Genesis and diagenesis of the Cambrian manganese deposits, Harlech, North Wales. Geological Journal, 22, 7-18.
- Colman, T.B. & Appleby, A.-K., 1991. Volcanogenic quartz-magnetite-hematite veins, Snowdon, North Wales. Mineralogical Magazine, 55, 257-262.
- Foster, C. Le Neve, 1882. On the occurrence of cobalt ores in Flintshire. Transactions of the Royal Geological Society of Cornwall, 10, 107-112.
- 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.
- Pulfrey, W., 1933. The iron-ore oolites and pisolites of North Wales. Quarterly Journal of the Geological Society of London, 89, 401-430.
- Rankin, A.H. & Criddle, A.J., 1985. Mineralizing fluids and metastable low-temperature inclusion brines at Llanharry iron deposit, South Wales. Transactions of the Institution of Mining and Metallurgy (Section B: Applied earth science), 94, B126-B132.
- Raybould, J.G., 1974. Ore textures, paragenesis and zoning in the lead-zinc veins of mid-Wales. Transactions of the Institution of Mining and Metallurgy (Section B: Applied earth science), 83, B112-B119.
- Strahan, A., 1885. The geology of the coasts adjoining Rhyl, Abergele and Colwyn. Memoirs of the Geological Survey of England and Wales. Explanation of Quarter-Sheet 79 N.W.
- Vivian, S., 1885. The hematite deposits of the southern outcrop of the Carboniferous Limestone of South Wales. Transactions of the South Wales Institute of Engineers, 14, 164-175.
- Williams, D., 1930. The geology of the country between Nant Peris and Nant Ffrancon (Snowdonia). Quarterly Journal of the Geological Society of London, 86, 191-232.