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Cymraeg

Witherite

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.

Chemical Group:

Geological Context:

Unusual elongated prismatic witherite crystals on Precambrian sediments from Dolyhir Quarry. Photo D.I. Green © D.I. Green.
Pyramidal witherite crystals from Pennant Mine, St. Asaph, Clwyd. National Museum of Wales Collection (NMW 83.41G.M.5437), ex R.J. King Collection. Photo D.I. Green, © National Museum of Wales.
Hexagonal witherite crystal aggregate on matrix. Llantrisant, Mid Glamorgan. I.E. Jones Collection. Photo M.P. Cooper, © National Museum of Wales.
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.

Key Localities:

References:

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. Davies, W., 1810. General view of the agriculture and domestic economy of North Wales. London. 510 pp.
  7. Greg, R.P. & Lettsom, W.G., 1858. Manual of the Mineralogy of Great Britain and Ireland. John van Voorst, London, 483pp.
  8. 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.
  9. 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.
  10. Phillips, W., 1823. An elementary introduction to the knowledge of Mineralogy. 3rd edition. London. 406pp.
  11. 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.