Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Barium carbonate
Chemical Formula: BaCO3
Method(s) of Verification: Dolyhir Quarry - XRD at the National Museum of Wales; all other occurrences have been identified by visual methods.
- Hydrothermal: Mississippi Valley Type veins
- Hydrothermal: epithermal polymetallic veins & pipes
Introduction: witherite occurs typically, along with baryte and other Ba-bearing minerals, in low-temperature hydrothermal mineral veins, frequently of Mississippi Valley type (MVT) affinity. Other commonly associated mnerals include calcite, sphalerite and galena. Witherite is identified by the fact that it is very dense for a nonmetallic mineral (a property shared with baryte) and its effervescence in dilute hydrochloric acid, demonstrating that it is a carbonate. The association with low-temperature vein mineralization is also diagnostic.
Occurrence in Wales: the earliest account of witherite in Wales appears to be that made by Davies (1810) in which he described, 'barytes united with carbonic acid, the terra ponderosa aërata, at Pennant, between St Asaph and Holywell'. Phillips (1823) referred directly to witherite from 'Flintshire, near St Asaph'. This rather vague locality information, repeated by Greg & Lettsom (1858), almost certainly corresponds with Pennant Mine where witherite was actively mined during the early 20th century (Carruthers et al., 1915). To the south, in the Central Wales Orefield, Smyth (1848) described the occurrence of witherite from Penyclun Mine, 'exhibiting in its druses well-formed crystals, formed by the combination of the rhomboidal prism with the double six-sided pyramid'. An original specimen of Penyclun witherite, from Smyth's collection, is held by the National Museum of Wales. In the early twentieth century, Carruthers et al. (1915) undertook a detailed review of barium mineralisation in Wales and they also noted, in addition to the above occurrences, the presence of witherite at several mines in the Llangynog Orefield and at further localities in Central Wales. Further minor occurrences have subsequently been identified by mineralogists at localities in South Wales, particularly in the Llantrisant area, and from Dolyhir Quarry in the Borderlands, where some fine specimens have been obtained in recent years.
- Bryntail Mine, Llanidloes, Powys: noted by Jones (1922). Baryte is the chief Ba-bearing mineral at this mine, unlike the nearby Penyclun Mine where the reverse is the case. However, massive witherite is present on the dumps just east of the farmhouse.
- Cwm Orog Mine, Llangynog, Powys: witherite was recorded at this and neighbouring mines by Carruthers et al. (1915). Some fine specimens are known from Cwm Orog, which is significantly richer in Ba minerals than the neighbouring localities (e.g. NMW 87.21G.M.24). Also recorded from Craig Rhiwarth, Craig-y-Mwyn and Llangynog mines.
- Dolyhir Quarry, Old Radnor, Powys: well-crystallized witherite was obtained in 2002, when hand specimens showing elongated prismatic crystals up to 10 mm in length with shallow pyramidal terminations were found in veins hosted by Precambrian metasediments. Further specimens have been found since. The veins in which the witherite occurs are sulphide-poor, being dominated by baryte and calcite with subordinate barytocalcite and harmotome.
- Gorn Mine, Llanidloes, Powys: recorded by Carruthers et al. (1915), and noted by Jones (1922) and Bevins & Mason (1997), witherite occurs as compact, radiating nodular masses surrounded by clay, which may reach several centimetres in size. The occurrence, which is texturally quite different to those at Penyclun and the other mines on the Van Lode (below), was reported by Jones (1922) to be spatially separate from that of the Pb-Zn ores at the mine.
- Halkyn Mountain, Clwyd: a single ex R.J. King specimen in the National Museum of Wales Collection (NMW 83.41G.M5435) is labelled as 'witherite with calcite, Halkyn mines'. No other specimens are known from this area.
- Llantrisant area, South Wales: occurs in cavities in the Triassic Dolomitic Conglomerate along the Llantrisant Bypass road-cuttings, and also at old, small neighbouring lead-mines such as Cefn-Parc and Rhiwsaeson (Bowler & Kingston, 1971; Alabaster, 1990).
- Pennant Mine, St Asaph, Clwyd: 'in pockets in association with baryte in lead- and zinc-bearing veins' (Carruthers et al., 1915).
- Penyclun Mine, Llanidloes, Powys: noted by Smyth (1848) and by Carruthers et al. (1915), witherite occurs in some abundance at Penyclun, where large, vuggy blocks of it used to be common on the tips. Fine specimens, with colourless to white platy translucent to transparent aggregates of witherite crystals to 20 mm, have been obtained from this site (e.g. Morgan & Starkey, 1991). Associated minerals are baryte, sphalerite, galena and calcite.
- Alabaster, C., 1990. Alstonite and barytocalcite from Llantrisant, South Wales and barytocalcite from Holwell, Mendip Hills, England. Journal of the Russell Society, 3, 1-6.
- Bevins, R.E. & Mason, J.S., 1997. Welsh metallophyte and metallogenic evaluation project: Results of a minesite survey of Dyfed and Powys. CCW Contract Science Report No. 156. National Museums & Galleries of Wales.
- Bowler, C.M.L. & Kingston, G.A., 1971. Mineralisation in the Triassic rocks of the Llantrisant area, Glamorgan. 1970 Gregynog Mineral Exploitation Colloquium Report. University College Cardiff, 30-34.
- Carruthers, R.G., Eastwood, T., Wilson, G.V., Pocock, R.W. & Wray, D.A., 1915. Barytes and witherite. Memoirs of the Geological Survey. Special Reports on the Mineral Resources of Great Britain, 2.
- Cotterell, T.F., Green, D.I., Hubbard, N., Mason, J.S., Starkey, R.E. and Tindle, A.G., 2011. The Mineralogy of Dolyhir Quarry, Old Radnor, Powys, Wales. UK Journal of Mines and Minerals, 32, 5-61.
- Davies, W., 1810. General view of the agriculture and domestic economy of North Wales. London. 510 pp.
- Greg, R.P. & Lettsom, W.G., 1858. Manual of the Mineralogy of Great Britain and Ireland. John van Voorst, London, 483pp.
- 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.
- Morgan, D. & Starkey, R., 1991. Harmotome from Pen-y-Clun Mine, Llanidloes, Dyfed, Wales, U.K. Journal of Mines and Minerals, No. 10, 4-6.
- Phillips, W., 1823. An elementary introduction to the knowledge of Mineralogy. 3rd edition. London. 406pp.
- Smyth, W.W., 1848. On the mining district of Cardiganshire and Montgomeryshire. Memoirs of the Geological Survey of Great Britain, 2, part 2, 655-684.