Smithsonite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Zinc carbonate
Chemical Formula: ZnCO3
Method(s) of Verification: Cwmystwyth Mine - XRD (NMW X-1291); Dolyhir Quarry - EMPA (Mike Rothwell); Llantrisant -XRD (NMW X-824); Machen Quarry - XRD (NMW X-960); Ochr-Chwith - XRD (NHM, x14629); Rhosesmor Mine - XRD (NMW, REB 37); Rhyd-y-Gwern Mine – XRD (NHM, x15836).

Chemical Group:

  • Carbonates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
  • Supergene: in situ natural oxidation & weathering deposits
Introduction: smithsonite is a secondary zinc carbonate typically found in the oxidized zone of zinc-bearing ore bodies or as a replacement of adjacent calcareous rocks. Smithsonite is frequently confused with the similar looking zinc silicate, hemimorphite, both of which were at one time known by the name calamine. When well crystallized identification is straightforward to the trained eye, however, both can form botryoidal masses of similar colours. Associated species include hemimorphite, cerussite, malachite, azurite, anglesite, pyromorphite, aurichalcite, and hydrozincite.
Occurrence in Wales: smithsonite is one of the earliest minerals to have been reported in Wales. Smithsonite has long been known to occur in Flintshire. Thomas Pennant (1726-1798) kept a catalogue of mineral specimens collected from the mines of Flintshire within which forty-five specimens are described (Campbell Smith, 1913). Indeed the first analysis of smithsonite appears to have been on material from “Holywell, Anglia” (Bergmann, 1780), clearly the well-known locality in Flintshire (Clark, 1993). Flintshire, and in particular the Halkyn District remain the most important region in Wales for smithsonite, although a number of other minor discoveries have been made across Wales. In Central Wales smithsonite occurs at a limited number of localities, although it has been reported from many more. Many of the sites described by Jones & Moreton (1977) have been shown to be hemimorphite (T.F. Cotterell, unpublished data). Smithsonite has however, been confirmed from Cwmystwyth Mine, but only as thin coatings on sphalerite, and never as crystals (T.F. Cotterell, unpublished data). In South Wales smithsonite is known from oxidized veins cutting the southern outcrop of Carboniferous Limestone. Notable localities include Machen Quarry and the Clive mines both near Caerphilly, but specimens are always small.

Key Localities:

  • Cefn-yr-Ogof, Llanddulas, Denbighshire: traces of porous smithsonite are reported from this locality (Bevins & Mason, 1999).
  • Central Wales Orefield: smithsonite has been reported from a number of localities including Esgairfraith, Copper Hill and Brynyrafr (Jones & Moreton, 1977), although their description is more akin to hemimorphite, that is widespread within Mid-Wales. Specimens from these localities within the mineral collection of the National Museum of Wales all appear to be hemimorphite. A second account of smithsonite by Jones & Moreton (1977) may prove to be correct as they note lustrous bright yellow botryoidal crusts found at Eaglebrook Mine. Jones (1983) expands upon this description, suggesting a range of colour from light yellow through light yellow-green to almost khaki, usually forming botryoidal crusts on calcite or dolomite and often with an overgrowth of 'limonite'. Smithsonite is likely to be more common, the dull, relatively unattractive appearance allowing it to be easily overlooked (Jones, 1983). Recent investigations into the identification of vivid canary yellow crusts on sphalerite collected from the Graig opencut at Cwmystwyth Mine has shown them to be smithsonite and not greenockite as many believe (T.F. Cotterell, unpublished data).
  • Clive mines, Draethen, South Wales: similar geological conditions to the nearby Machen Quarry. At Clive, smithsonite is more common, but not as well-formed, typically occurring as inconspicuous, minute, crystalline aggregates within cavities in highly altered veinstone.
  • Dolyhir Quarry, Old Radnor, Powys: extremely rare. Small brownish lenticular crystals are confirmed as smithsonite by Mike Rothwell (D.I. Green pers. comm.).
  • Halkyn District, Flintshire: smithsonite appears to have been common in the oxidized, near surface parts of the veins running through this district. Early references to smithsonite from Flintshire focus on Holywell (Bergmann, 1780; Phillips, 1823; Greg & Lettsom, 1858; Hall, 1868), where it occurs in obtuse rhomboids and as pseudomorphs. The mineral collection of Thomas Pennant (1726-1798) contained forty-five specimens of smithsonite from Flintshire including, examples from Pen-y-Bryn, Quoitia Mawr, Butler’s Fields, Pant-y-Godidw, Driniog, Bryn-driniog, Llin eer and Ty Maen all near Holywell (Campbell Smith, 1913). Pseudomorphs and botryoidal masses are also recorded from Milwr Mine (Mountain, 1924), while traces of smithsonite are reported from the Cambria and Gladstone mines near Holywell (Bevins & Mason, 1999). Pennant also recorded smithsonite from Halkyn Mountain noting, calamine from Pentre Halkin, while also listing eight specimens from Moel-y-Crio: seven specimens of calamine (smithsonite) pseudomorphs after calcite, including a large hollow crystal showing a twinned scalenohedron and a single epimorph of green calamine on cubes of purple fluor (Campbell Smith, 1913). Large masses of dense botryoidal creamy grey smithsonite are known from West Halkyn mine (e.g.NMW 83.41G.M.5502). Attractive crystallized specimens have been collected at Rhosesmor Mine at the southern end of Halkyn Mountain. Fine large aggregates of fragile sugary khaki-green crystals were collected by R. Starkey in 1997 from Powell’s Lode Cavern while, a specimen in the G.J. Williams collection, National Museum of Wales (NMW 27.111.GR.307) displays small, but attractive, green prismatic microcrystals directly overgrowing orange sphalerite. A fine specimen of botryoidal yellow smithsonite from Halkyn Mountain was in the Richard Barstow mineral collection and now forms part of the National Museum of Wales collection (Specimen no. NMW 85.70G.M.35). Smithsonite is also reported at Bryngwiog, Dog Pit, Halkyn East, Halkyn Mountains, Union Vein and Wagstaff workings all on Halkyn Mountain (Bevins & Mason, 1999). To the north and west of Halkyn Mountain lie Trelogan, Talacre and Talargoch mines. Thomas Pennant described specimens of smithsonite in his collection, from all three of these mines (Campbell Smith, 1913). At Trelogan, smithsonite (calamine) pseudomorphs after calcite are recorded, while at Talacre, black calamine (smithsonite) is noted. At Talargoch, near Dyserth, fine, canary-yellow, botryoidal specimens are described as, ‘from a Drift the side of the Hill with a stream running through it’ (Campbell Smith, 1913). Finally, smithsonite is reported from a number of sites to the west of Mold in the southern end of the Halkyn District. Bevins & Mason (1999) note ‘calamine’ from the Cat Hole and Pant-y-Buarth mines. Recently, small (<1 mm), but perfectly formed colourless, rhombohedral crystals lining boxwork cavities in limestone, where sphalerite has dissolved away, were collected at Cefn Mawr Quarry (National Museum of Wales specimens).
  • Halkyn-Minera district, NE Wales: smithsonite is the major secondary mineral within the limestone-hosted Mississipi Valley Type, Halkyn-Minera orefield of NE Wales.See individual entries for the Halkyn and Minera Districts.
  • Llangynog Orefield, NW Powys: smithsonite is reported in small amounts (Williams, 1985). At Ochr Craig mine frosted crusts of aggregated rounded crystals coat weathered ferroan dolomite with bright blue aurichalcite in association (National Museum of Wales specimen NMW 87.43G.M.58).
  • Llantrisant area, South Wales: a small fragment of sphalerite-bearing limestone encrusted, with cream botryoidal smithsonite is in the Mineral Collection of the National Museum of Wales (NMW 48.264.GR.448). The specific location where this was collected is not known.
  • Llanymynech, Welshpool, Powys: Greg & Lettsom (1858) list calamine (smithsonite) from Llanymynech Rocks, Denbighshire. Llanymynech is actually in Powys, but the old county boundary lies only four miles away. No further details are known. However, Arthur Russell collected smithsonite from Moelydd Hill mine near Llanyblodwell, barely three miles to the north-west and just over the border into England (National Museum of Wales specimen no. NMW 27.111.GR.306).
  • Machen Quarry, Caerphilly, South Wales: smithsonite is generally very rare, particularly when compared with the abundant hemimorphite at this locality. Small, brownish, rounded seed-like crystals (up to 2-3 mm) have been collected on occasion (Plant & Jones, 1995), while creamy botryoidal crusts found coating galena veinstone in 1982 are smithsonite, but the majority of specimens thought to be smithsonite are actually the zinc silicate hemimorphite.
  • Minera District, Flintshire: smithsonite is not as common as in the Halkyn District, but is still an important secondary phase. Bevins (1994) reports smithsonite from Minera Mine and Bevins & Mason (1999) record smithsonite replacing sphalerite at Minera South. Minor quantities of smithsonite are also recorded replacing sphalerite at Park and Pool Park mines and microcrystalline smithsonite occurs at Cefn-y-Gist (Bevins & Mason, 1999).
  • Ochrwyth Quarry, Risca, Gwent: associated with barite in material from ‘Ochr-Chwith’ analysed by the Natural History Museum.
  • Rhyd y Gwern pits, Machen, South Wales: smithsonite has been confirmed, associated with dolomite/ankerite in material analysed by the Natural History Museum.

References:

  1. Bergmann, . Opuscula 2, 324.
  2. Bergmann, . J. Phys. 16, 17.
  3. 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.
  4. Bevins, R.E. & Mason, J.S., 2000. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Glamorgan and Gwent. National Museums & Galleries of Wales, Cardiff
  5. Bevins, R.E. & Mason, J.S., 1999. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Clwyd. National Museums & Galleries of Wales, Cardiff.
  6. Campbell Smith, W., 1913. The mineral collection of Thomas Pennant (1726-1798). Mineralogical Magazine, 16, 331-342.
  7. Greg, R.P. & Lettsom, W.G., 1858. Manual of the Mineralogy of Great Britain and Ireland. John van Voorst, London, 483pp.
  8. Hall, T.M., 1868. The Mineralogists' Directory. London, 168pp.
  9. Mountain, E.D., 1924. Calcite crystals from Holywell, North Wales. Mineralogical Magazine, 20, 212-216.
  10. Phillips, W., 1823. An elementary introduction to the knowledge of Mineralogy. 3rd edition. London. 406pp.
  11. Plant, S.P. & Jones, I.E., 1995. Minerals of Machen Quarry, Mid Glamorgan, Wales. Journal of the Russell Society, 6(1), 31-36.
  12. Williams, R.A., 1985. The Old Mines of the Llangynog District. British Mining, No. 26. Northern Mine Research Society, 128pp.