Amgueddfa Cymru - National Museum Wales

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:

  • Phosphates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
  • Supergene: in situ natural oxidation & weathering deposits
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:

  • Aberdaunant Mine, Llanidloes, Powys: Jones (1922) records that on the dumps from the workings, ‘fragments of grit are abundantly coated with pyromorphite’. G.J. Williams, former Mine Inspector for North Wales had, in his collection a specimen of pyromorphite collected in 1909. This specimen, now forming part of the National Museum of Wales Mineral Collection (specimen no. NMW 27.111.GR.309) matches the description made by Jones (1922).
  • Anglesey: a number of old specimens in the National Museum of Wales Mineral Collection are reputedly from Anglesey, but the validity of these samples is in question as no contemporary examples have been recorded. The samples in question bear a marked resemblance to coarsely crystalline pyromorphite from Roughton Gill Mine in the Caldbeck Fells, Cumbria, England.
  • Bwlch-glas Mine, Tal-y-bont, Ceredigion: arguably the most well-known of all pyromorphite occurrences in Wales. Known from the early 1900s (National Museum of Wales Collection, no. NMW 27.111.GR.313, collected by Arthur Russell in 1917) the finest specimens were discovered during the 1970s following the discovery of wulfenite with pyromorphite on surface dumps, by P. Thomson and R.S.W. Braithwaite. Underground exploration revealed a gossan zone rich in crystalline pyromorphite. The richest specimens came from a breccia-filled pipe, where beautiful bright green seed-like pyromorphite crystals coat quartz crystals (Braithwaite, 1982). National Museum of Wales specimen no. NMW 99.9G.M.1 is an exceptionally large slab (40 x 33 cm) of quartz densely carpeted by dark green seed-like crystals to 7 mm in length and is probably the largest specimen of pyromorphite ever found in Wales.
  • Copa Hill, Cwmystwyth mines, Ceredigion: pyromorphite was collected from Copa Hill by Arthur Russell in 1914 and can still be found as typical bright green to khaki-green crystalline coatings often with a greasy lustre in ancient dumps high up on Copa Hill.
  • Cystanog Mine, Carmarthen, Carmarthenshire: a quantity of pyromorphite was found near the northern shaft and close to the surface in 1853-4 and a further few tons were extracted by Captain Thomas Harvey shortly after 1860 (Hall, 1971). A number of recently collected specimens in the National Museum of Wales Mineral Collection reveal weathered heavy coatings of greenish pyromorphite on vein quartz.
  • Dylife Mine, Penegoes, Powys: pyromorphite was recorded by Jones (1922) on the dumps of opencast workings on Pen Dylife. Specimens are poor by Central Wales standards, typically showing thin green coatings on quartz veinstone.
  • Esgair-Ddu Mine, Pontrhydfendigaid, Ceredigion: mid to dark green microcrystalline pyromorphite encrusts bleached veinstone from the mine dumps (National Museum of Wales specimens).
  • Frongoch Mine, Devil's Bridge, Ceredigion: Greg & Lettsom (1858) provided the first account of this occurrence, describing, ‘brilliant translucent hair-brown crystals on quartz, much resembling the variety from Bleistadt, in Bohemia, at a mine near Devil’s Bridge, in North Wales’. Brown pyromorphite is rare in the British Isles and the presence of large (exceptionally to 25 mm long) thick prismatic crystals on the extensive dumps at Frongoch suggests that this is the locally to which Greg and Lettsom refer. Colours range from green (extremely rare) to mauve and chocolate brown. Coarse platy cerussite is frequently in association and often on heavily bleached mudstone.
  • Gelly-Fowler Fields, Halkyn Mountain, Flintshire: pyromorphite was reported in a large quantity by Traill (1821).
  • Halkyn, Flintshire: a specimen labelled as ‘Picked up near “Windmill”, Halkyn, Flintshire’ is in the G.J. Williams Mineral Collection in the National Museum of Wales. The specimen displays a near solid aggregate of small somewhat greasy green, hexagonal prismatic crystals. Further specimens (NMW 68.378.GR.9 & 74.27G.M.11), acquired from D.B. Hardman consist of flat-lying sprays of lime-green pyromorphite along fractures in chert.
  • Henfwlch Mine, Ceulanymaesmawr, Ceredigion: Jones & Moreton (1977) listed Henfwlch as a locality for green pyromorphite crystals. Occasional fine examples of lime-green pyromorphite have been collected from the dumps, but many are damaged through weathering. In recent years lemon-yellow and toffee-brown crystals have been collected from a gossan zone underground (T.F. Cotterell, unpublished data).
  • Jamaica Mine, Moel Findeg, Mold, Flintshire: National Museum of Wales Mineral Collection specimen NMW 77.35G.M.40, acquired from R.W. Barstow and originally part of the Joseph Neeld collection, comprises a solid, dense, mass of murky-green pyromorphite labelled as from Jamaica Mine, Halkyn Mountain.
  • Llanerch-yr-aur Mine, Llanbrynmair, Powys: National Museum of Wales Mineral Collection specimen NMW 27.111.GR.310 is a slightly weathered example of deep green acicular pyromorphite encrusting siliceous veinstone.
  • Llangunnor, Carmarthenshire: National Museum of Wales Mineral Collection specimen no. NMW 53.146.GR.12 is a porous aggregate of green pyromorphite forming casts around cavities where an earlier mineral has been weathered away.
  • Llechweddhelyg Mine, Penrhyncoch, Ceredigion: pyromorphite from Llechweddhelyg is generally of superb quality (Jones, 1987). Jones (1987) describes in detail the range of forms present. Some of the finest specimens consist of dark vuggy goethitic gossan containing light olive-green curved ‘barrel-shaped’ crystals up to 4 mm long.
  • Nant Mine, Llangunnor, Carmarthenshire: finely crystalline green pyromorphite encrusts massive barite on National Museum of Wales specimen NMW 53.146.GR.1.
  • Nant-y-mwyn Mine, Rhandirmwyn, Carmarthenshire: poorly formed green coatings are found on veinstone (National Museum of Wales specimens).
  • Penycefn Mine, Bontgoch, Ceredigion: a large amount of cerussite with some pyromorphite was described by Jones (1922) from a cross-cut southward from the east end of the Penycefn workings.
  • Rhiwseiseion, near Llantrisant, South Wales: small green prismatic crystals coating quartz occur on National Museum of Wales specimen NMW 25.408.GR.1. The authenticity of this specimen is however in question as this is the only specimen of pyromorphite known from this part of South Wales.
  • Silver Rake, Halkyn Mountain, Flintshire: Thomas Pennant (1726-1798) described a ‘green lead ore’ as being found in small amount some years previous to 1778 (Campbell Smith, 1913). Seven specimens are recorded in Pennant’s catalogues, described as being ‘of a dark green colour, of a stony appearance and breaking with a smooth surface like that of flint’, sometimes covered with ‘minute sparkling green crystallizations’ or with ‘elegant tubera, some of which are smooth, others crystallized’ (Campbell Smith, 1913). Of the seven specimens only one remained in 1913 with its label still attached, a sample consisting of galena, coated with well-crystallized cerussite and some pale-green botryoidal pyromorphite (Campbell Smith, 1913).
  • Tylwch Mine, Llanidloes, Powys: thick crusts of mid to dark green microcrystalline pyromorphite coat specimens of veinstone breccia collected from the top adit dump (National Museum of Wales Collection, e.g. NMW 98.35G.M.654).
  • Vale of Towy Mine, Llandeilo, Carmarthenshire: specimens of pyromorphite from Vale of Towy Mine came onto the market in the late 1990s. It is thought that they were collected from underground workings.

There are no key localities for this specimen.

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.

There are no references for this specimen.