Hemimorphite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Zinc silicate hydroxide hydrate
Chemical Formula: Zn4Si2O7(OH)2.H2O
Method(s) of Verification: Brittanic Merthyr: XRD (National Museum of Wales, NMW X-1036); Cwmystwyth Mine: XRD (NMW X-1279); Dylife Mine: XRD (NHM, 3274F); Frongoch Mine - XRD (Manchester Museum); Henfwlch Mine - XRD (NMW X-1292); Machen Quarry - XRD (NMW X-808, X-1035 & X-1190).

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Hemimorphite crystal aggregates (each up to 1 mm across) on quartz. Collected 1980 from Level Fawr, Cwmystwyth Mine. National Museum of Wales specimen (NMW 87.43G.M.38), ex A. Dean Collection, © National Museum of Wales.
Well-formed, clear, prismatic hemimorphite crystals in cavity in baryte vein. Machen Quarry. National Museum of Wales specimen (NMW 95.28G.M2), © National Museum of Wales.
Introduction: hemimorphite is a secondary mineral species found typically in the oxidized zone of zinc-bearing mineral deposits. During the eighteenth century this mineral was grouped with other secondary zinc species including smithsonite and hydrozincite under the name calamine. This has caused considerable problems when reviewing old literature particularly as the term was still being used well into the nineteenth century.
Occurrence in Wales: hemimorphite was, during the eighteenth century grouped with other secondary zinc species including smithsonite and hydrozincite under the name calamine. This causes considerable problems when reviewing old literature particularly as the term was still being used well into the nineteenth century. In 1848, W.W. Smyth described the presence of hemimorphite in Central Wales, ‘Calamine, the siliceous species, has occurred in considerable quantity at Nant y Creiau mine, below the adit level, but has been thrown away as a worthless “spar”: elsewhere it may be seen investing blende with a thin crust’. Only by the use of the term ‘siliceous species’ can the true identity of this ‘calamine’ be made – smithsonite and hydrozincite both being carbonates. Prior to this, Sowerby (1817) illustrated, white botryoidal silical oxide of zinc from North Wales. The description is certainly of hemimorphite and although no more accurate locality information is given, the specimen is likely to have been derived from Flintshire. Although hemimorphite was clearly known about, the earliest specimen of hemimorphite in the National Museum of Wales collections dates from 1927. G.J. Williams, the former H.M. Mine Inspector for North Wales had in his collection a sample from Devil’s Bridge, displaying finely crystalline spherical hemimorphite crystal aggregates encrusting quartz crystals overgrowing sphalerite veinstone. No new discoveries were made then, until, in the late 1960s mineral collectors began to take an interest in the minerals of the Central Wales Orefield. A number of new localities were found along with the rediscovery of G.J. Williams’ original site. In South Wales, quarrying of the Carboniferous Limestone during the 1980s and 90s resulted in the discovery of the some of the finest Welsh hemimorphite specimens, particularly at Machen Quarry, near Caerphilly. Hemimorphite occurs here within barite-galena veins cutting dolomitic limestone. Many of the veins had been worked at one time for the lead ore, therefore, it is not suprising that hemimorphite has also been found at a number of disused mine sites within the same area.

Key Localities:

  • Bwlchrhennaid Mine, Goginan, Ceredigion: feathery microcrystals coat pale grey mudstone from the dumps at this mine. Rare orange tabular wulfenite microcrystals form an attractive, but unusual association (Rothwell & Mason, 1992).
  • Coed Cefn Pwll-du, Draethen, South Wales: well-formed bladed crystals (up to 4 mm in length) line cavities within barite cutting dolomitic limestone from an outcrop containing an assemblage very similar in appearance to that from Machen Quarry.
  • Craig Rhiwarth Mine, Llangynog, Powys: specimens collected by W.W. Smyth in 1846 were later described by Arthur Russell as containing hemimorphite (Williams, 1985). Specimen no. 694 in the British Micromount Society Reference Collection is hemimorphite collected by M. Rothwell.
  • Cwmleyshon Quarry, Draethen, South Wales: similar in appearance to material from Coed Cefn Pwll-du and Machen Quarry. At Cwmleyshon cream coloured translucent crystals exceptionally reach 36 mm in length although are typically only 1 or 2 mm long. Some of the largest crystals were collected in 1990 by Dr. F. Ince (e.g. National Museum of Wales specimen, NMW 90.47G.M.1.).
  • Cwmystwyth Mine, Cwmystwyth, Ceredigion: post-mining crusts composed of radiating spherical crystal aggregates (up to 2.5 mm) coat tunnel walls underground within Level Fawr. This occurrence has been known about for some time. The collection of G.J. Williams, the former H.M. Mine Inspector for North Wales, contains a specimen of hemimorphite labelled as Devil’s Bridge, Cardiganshire. This specimen, now in the National Museum of Wales Collection (NMW 27.111.GR.335) is almost certainly from underground workings at Cwmystwyth Mine, where from the late 1970s onwards collectors have continued to find good specimens.
  • Dylife Mine, Penegoes, Powys: this mineral was common, particularly from the large dumps opposite the Star Inn. During the late 1960s, R.S.W. Braithwaite collected sugary opaque white botryoidal hemimorphite, as represented by National Museum of Wales specimen NMW 68.576.GR.16, while Jones & Moreton (1977) describe tufts of colourless transparent tabular crystals on sphalerite.
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: this mineral was originally noted without description by Jones & Moreton (1977). Jones (1983) provides a detailed account of hemimorphite from Eaglebrook where hemimorphite is often overlooked because of superficial weathering. The mineral is actually of fairly frequent occurrence, usually in close association with quartz. Crystals are typically water clear and well-formed, and on occasion reach 3 mm in length, but are prone to heavy brown iron staining as a result of weathering.
  • Esgair Fraith Mine, Tal-y-bont, Ceredigion: creamy coloured hemimorphite, veining siltstones, occurs on the dumps of the Eastern Shaft. Porous masses up to 60 x 40 mm have been found containing fine bladed crystals. Specimens in the National Museum of Wales Collection labelled Esgair Hir Mine are believed to be derived from Esgair Fraith Mine – the eastward extension of the Esgair Hir Lode. Material collected by A. Dean in 1976 originally labelled smithsonite has, since been shown to be hemimorphite and this is also likely to be the case with the smithsonite reported by Jones & Moreton (1977). Crystals from Esgair Fraith are amongst the finest to have been collected in Central Wales.
  • Frongoch Mine, Devil's Bridge, Ceredigion: hemimorphite is rare at this locality, somewhat surprisingly given the abundance of primary sphalerite. It is occasionally found forming characteristic drusy bladed crystals lining cavities to 2 cm x 1 cm and as radiating crystal groups to 3 mm on goethite.
  • Halkyn, Flintshire: small spherical crystal clusters scattered on calcite containing sphalerite inclusions are displayed on National Museum of Wales specimen NMW 68.378.GR.24.
  • Hendy Quarry, Miskin, South Wales: listed by S.P. Plant in 1993 as part of the supergene zinc-copper assemblage (British Micromount Society Newsletter No. 35).
  • Henfwlch Mine, Ceulanymaesmawr, Ceredigion: botryoidal crusts and occasional crystal clusters within ferroan dolomite have been collected from the adit level dumps. Recently, fine specimens of colourless stepped crystals lining cavities in crumbly brown corroded ferroan dolomite have been collected underground (T.F. Cotterell, unpublished data).
  • Holywell, Flintshire: listed without description by Neil (1907).
  • Llangynog Mine, Llangynog, Powys: British Micromount Society Reference Collection specimen no. 680 is hemimorphite collected by M. Rothwell.
  • Llettyhen Mine, Bontgoch, Ceredigion: attractive white to pale blue, thick, botryoidal crusts are found covering areas up to 5 cm across and less commonly as radiating tabular crystals up to 7 mm long.
  • Machen Quarry, Caerphilly, South Wales: hemimorphite is described by Plant & Jones (1995) as relatively common from this locality. Well-formed water clear to honey-coloured, radiating, tabular crystals line cavities up to 5 cm across, in barite veins cutting dolomitic limestone. Crystals are usually less than 1 mm, but occasionally reach 4 mm. Hemimorphite was known from the late 1970s, although the quality of early specimens does not compare with those found in recent years. The best examples show a high degree of lustre and represent the finest hemimorphite from Wales.
  • Minera District, Flintshire: 3-5 mm brown sheaves on quartz and calcite are reported by Bevins (1994).
  • Nant Mine, Llangunnor, Carmarthenshire: a solid greyish-white mass labelled as hemimorphite is present on a specimen in the collections of the National Museum of Wales, but formal identification has not been made.
  • Old Esgairlle Mine, Ponterwyd, Ceredigion: 'dry bone' hemimorphite is found as a pale-grey coating on mudstone. In a similar paragenesis to Bwlchrhennaid Mine, hemimorphite also occurs in association with wulfenite.

There are no key localities for this specimen.

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Green, D.I., Rust, S.A. & Mason, J.S., 1996. Classic British mineral localities: Frongoch Mine, Dyfed. UK Journal of Mines & Minerals, 17, 29-38.
  3. Jones, A.D., 1983. Nant-y-Cagl. Mineral Realm, 3, 42-76.
  4. Jones, J.A. & Moreton, N.J.M., 1977. The Mines and Minerals of Mid-Wales 40pp.
  5. Neil, J.S., 1907. British Minerals and where to find them. Murby’s Science Series. 224pp.
  6. Plant, S.P. & Jones, I.E., 1995. Minerals of Machen Quarry, Mid Glamorgan, Wales. Journal of the Russell Society, 6(1), 31-36.
  7. Rothwell, M. & Mason, J.S., 1992. Wulfenite in the British Isles. UK Journal of Mines & Minerals, 11, 30-41.
  8. 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.
  9. Sowerby, J., 1817. British Mineralogy: or coloured figures intended to elucidate the mineralogy of Great Britain. Vol. V. London.
  10. Williams, R.A., 1985. The Old Mines of the Llangynog District. British Mining, No. 26. Northern Mine Research Society, 128pp.

There are no references for this specimen.