Serpierite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Calcium copper zinc sulphate hydroxide hydrate
Chemical Formula: Ca(Cu,Zn)4(SO4)2(OH)6.3H2O
Method(s) of Verification: XRD from these mines: Brynarian - (NMW X-1038); Dylife - (NMW X-39 & NHM 4130F); Eaglebrook - (NMW X-772, 1037 & 1125); Frongoch – (NHM); Henfwlch - (NMW X-1344); Nant-y-creiau - (NMW X-1270); Penrhiw - (NMW X-883); Ystrad Einion – (NMW X-1044)

Chemical Group:

  • Sulphates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
Serpierite rosettes to 5 mm from Ystrad Einion Mine, in the Central Wales Orefield. S.A. Rust Collection (no. 1973). Photo M.P. Cooper, © National Museum of Wales.
Radiating aggregates of serpierite crystals (up to 1 mm long) from Ystrad Einion Mine, Central Wales. J.S.Mason Collection (no. JMYE006). Photo M.P. Cooper, ©, National Museum of Wales.
Feathery sprays of pale blue serpierite from Eaglebrook Mine in the Central Wales Orefield. Specimen and photo S.A. Rust. © S.A. Rust.
Introduction: serpierite is an attractive secondary species which, forms in post-mining supergene environments associated with primary zinc-copper mineralization.
Occurrence in Wales: Jones & Moreton (1977) produced the first written account of serpierite from Wales, describing sky-blue tufted aggregates from Dylife Mine in the Central Wales Orefield. Specimens had, in fact, been collected from Dylife from the late 1960’s (National Museum of Wales Mineral Collection numbers NMW 68.378.GR.27 & NMW 68.576.GR.17) by D. Hardman and R.S.W. Braithwaite. Many more occurrences have come to light from the Central Wales Orefield including, stunning freestanding crystal sprays from underground localities such as Ystrad Einion Mine (Mason & Rust, 1997) and Henfwlch Mine (T.F. Cotterell, unpublished data). The best Ystrad Einion specimens are considered to be some of the finest ‘natural’ serpierite collected in the British Isles (Mason & Rust, 1997). Most occurrences, however, are derived from mine dumps. The discovery of flat-lying radiating sprays of crystals from Llanegan Mine on the Llŷn in Gwynedd by Dr. D. Roe represents, the only occurrence of serpierite outside of the Central Wales Orefield.

Key Localities:

  • Dylife Mine, Penegoes, Powys: radial aggregates of sky-blue lath-like crystals occur elongated and flattened on fractures in calcite and within grey mudstone cut by quartz-calcite veins, containing spots of chalcopyrite. Jones & Moreton (1977) also report botryoidal crusts of serpierite with satiny surfaces, associated with calcite or more rarely linarite. Most specimens are believed to have been collected from the large dumps opposite the Star Inn.
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: pale blue microscopic, radiating blade-like crystals are noted by Braithwaite (1982). Jones (1983) provides a detailed account of serpierite from Nant-y-cagle (Eaglebrook). Visual identification is awkward due to the presence of devilline and aurichalcite, two minerals very similar in appearance to serpierite.
  • Esgairhir Mine, Tal-y-bont, Ceredigion: only a very small number of specimens have been collected, typically displaying masses of light blue, intergrown, lath-like crystals in cavities to 2 mm|3 (Rust, 1985; Rust & Mason, 1988).
  • Frongoch Mine, Devil's Bridge, Ceredigion: Green et al. (1996) describe broad blue-green plates to 0.5 mm and wispy sky-blue lath-like crystals to 0.7 mm in the small number of specimens that have been found.
  • Henfwlch Mine, Ceulanymaesmawr, Ceredigion: fine examples of post-mining serpierite have been collected from underground workings (T.F. Cotterell, unpublished data). Serpierite is widely developed within backfill consisting, of low-grade sulphide vein material. Sprays of sky blue to pale turquoise blue feathery acicular crystals liberally coat fragments of weathered veinstone. Undamaged freestanding sprays are restricted to cavities due to the fragile nature of serpierite. This occurrence shows striking similarities to that described by Mason & Rust (1997) at Ystrad Einion Mine.
  • Llanegan Mine, Llŷn, Gwynedd: flat-lying pale blue-green sprays have been collected by Dr. D. Roe.
  • Nant-y-creiau Mine, Devil’s Bridge, Ceredigion: matted aggregates of light blue feathery microcrystals and pale turquoise-blue bladed microcrystals occur within a block of corroded sulphide-dominated veinstone discovered by T.F. Cotterell in a dried-up river bed, cutting through the minesite. Primary sulphides include galena, sphalerite, chalcopyrite and pyrite, while extensive alteration has resulted in a wide range of secondary species including, serpierite, brochantite, linarite, anglesite, cerussite, sulphur, a leadhillite group mineral, and a langite group phase.
  • Penrhiw Mine, Ystumtuen, Ceredigion: serpierite forms a minor component of the dump-formed secondary assemblage, described in detail by Mason & Green (1995). Typical tangled pale-blue fibres occur in minute quantities both as coatings on dolomite and overgrowths on associated copper and zinc secondary species including ramsbeckite, schulenbergite and posnjakite. Serpierite always occurs late in the secondary assemblage.
  • Ystrad Einion Mine, Furnace, Ceredigion: the finest Welsh examples of serpierite have been collected from a short backfilled passage off from one of the main adits. Typically, specimens display groups of pale blue lath-like crystals to 5 mm on a matrix of corroded creamy dolomite, in association with hemimorphite, brochantite, malachite, ktenasite and schulenbergite (Mason & Rust, 1997).

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Braithwaite, R.S.W., 1982a. Wroewolfeite in Britain. Mineralogical Record, 13, 167-174.
  3. Camm, G.S., 1993. The Minerals and Mines of the Principality of Wales. The Natural History Museum, London. 231pp.
  4. 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.
  5. Jones, A.D., 1983. Nant-y-Cagl. Mineral Realm, 3, 42-76.
  6. Jones, J.A. & Moreton, N.J.M., 1977. The Mines and Minerals of Mid-Wales 40pp.
  7. Mason, J.S. & Green, D.I., 1995. Supergene minerals including exceptional ramsbeckite from Penrhiw Mine, Ystumtuen, Dyfed. UK Journal of Mines & Minerals, 15, 21-27.
  8. Mason, J.S. & Rust, S.A., 1997. The Mineralogy of Ystrad Einion Mine, Dyfed, Wales. U.K. Journal of Mines and Minerals, 18, 33-36.
  9. Rust, S., 1985. British Micro Localities. No. 12: Esgair Hir Mine (SN 734913), Ceulanymaesmawr, Dyfed, Wales British Micromount Society Newsletter. 13. 13-14.
  10. Rust, S.A. & Mason, J.S., 1994. An unusual occurrence of arsenate minerals at Gwaith-yr-Afon mine, Dyfed, Wales. Journal of the Russell Society, 5(2), 109-113.
  11. Rust, S.A. & Mason, J.S., 1988. The minerals of Esgair-Hir mine, Dyfed, Wales. UK Journal of Mines & Minerals, 5, 35-43.