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Cymraeg

Pyromorphite

Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Lead chloro-phosphate
Chemical Formula: Pb5(PO4)3Cl
Method(s) of Verification: Many localities confirmed by XRD including: Llangunnor (NMW X-816); Ysbyty Ystwyth (NHM, x11054); Dylife (NHM, x13482); Eaglebrook (NHM, 6347F, 6349F & 8500F); Esgair Hir (NHM, 6801F & 6870F); Dyfngwm (NHM, 6874F); Frongoch (NHM, 8582F).

Chemical Group:

Geological Context:

Prismatic pyromorphite crystals on quartz from Bwlch-glas Mine, in the Central Wales Orefield. Specimen 8 cm x 10 cm. B.R. Moore Collection. Photo M.P. Cooper, © National Museum of Wales.
Pale mauve to brown prismatic pyromorphite crystals up to 3 mm long from Frongoch Mine, in the Central Wales Orefield. J.S. Mason Collection (no. JMFG505). Photo M.P. Cooper, © National Museum of Wales.
Introduction: pyromorphite is a secondary mineral which forms in the oxidized zone of lead-bearing mineral deposits.
Occurrence in Wales: pyromorphite is widespread across large parts of Wales, particularly Central Wales, where it is invariably associated with the old lead mines. The earliest reference to pyromorphite in Wales is by the naturalist Thomas Pennant (1726-1798) when, in 1778 he described a ‘green lead ore’ as being found in small amount some years previous in the Silver Rake on Halkin Mountain. Pennant had himself seven samples of this material, although by 1913 only one had remained with its label attached (Campbell Smith, 1913). Sowerby in 1804, mentioned phosphate of lead from Wales. Although no further details are given, this is likely to refer to Pennant’s locality as Sowerby had links with Thomas Pennant. Green lead ore, or pyromorphite was first noted in Central Wales by Smyth (1848), occasionally coating stones dumped on the surface of mines as microscopic crystals. Smyth postulated that the pyromorphite formed through the reaction of galena to decomposing organic matter resulting in a loss of sulphur to gain phosphoric acid. In 1858, Greg & Lettsom described hair-brown pyromorphite from near Devil’s Bridge in North Wales. Devil’s Bridge is actually in Central Wales, but the mine that they refer to is largely regarded to be Frongoch, a large (by Mid-Wales standards) lead-zinc mine, situated only a couple of mines from Devil’s Bridge. In 1922, O.T. Jones described pyromorphite from a number of other mines and mineral lodes in Mid-Wales, but with very little detail. During the 1970s fine specimens of crystallized pyromorphite appeared on the market labelled as ‘from near Plynlimon’. The actual locality was Bwlch-glas Mine, situated some six miles from Plynlimon in Central Wales. Specimens collected during this time are some of the finest Welsh pyromorphites particularly in terms of size and aesthetics and are now considered British classics. In South Wales pyromorphite is comparatively rare. It is known from a number of mines in Carmarthenshire and a single specimen from Mid Glamorgan is in the mineral collection of National Museum of Wales, but the fact that this is the only example has brought its authenticity into question.

Key Localities:

References:

  1. Braithwaite, R.S.W., 1982b. Pyromorphite, wulfenite and other minerals from Bwlch-Glas mine, Central Wales. Mineralogical Record, 13, 151-153.
  2. Campbell Smith, W., 1913. The mineral collection of Thomas Pennant (1726-1798). Mineralogical Magazine, 16, 331-342.
  3. Greg, R.P. & Lettsom, W.G., 1858. Manual of the Mineralogy of Great Britain and Ireland. John van Voorst, London, 483pp.
  4. Hall, G.W., 1971. Metal Mines of Southern Wales. Privately Published.
  5. Jones, A.D., 1983. Nant-y-Cagl. Mineral Realm, 3, 42-76.
  6. Jones, A.D., 1987. The Minerals of Llechwedd Helyg. UK Journal of Mines and Minerals. 3, 25-27.
  7. Jones, J.A. & Moreton, N.J.M., 1977. The Mines and Minerals of Mid-Wales 40pp.
  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. Mason, J.S., 2004. The development of supergene lead mineralization in Central Wales. UK Journal of Mines and Minerals, 24, 35-46.
  10. Pennant, T., 1778. Tour in Wales.
  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.
  12. Sowerby, J., 1817. British Mineralogy: or coloured figures intended to elucidate the mineralogy of Great Britain. Vol. V. London.
  13. Traill, T.S., 1821. Observation on the mineralogy of Halkin Mountain, in Flintshire; with particular account of the recently discovered Buhrstone and porcelain-clay of that place. Edinburgh Philosophical Journal, 4, 246-261.