Mineral Database

Mineral Database
Mineralogy of Wales

Erythrite

Crystal System: Monoclinic
Formula: Co3(AsO4)2.8H2O
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Hydrated cobalt arsenate
Method(s) of Verification: Great Orme Mines - XRD (Jenkins & Johnson, 1993); Clogau Mine - XRD at the National Museum of Wales (NMW X-1653); all other cited occurrences - visual, based on the highly distinctive colour.

Sparkling pink erythrite microcrystals from Clogau Mine. Specimen J.S. Mason (JM2529). Photo D.I. Green, © National Museum of Wales.

Erythrite encrusting quartz from Vigra Mine, in the Dolgellau Gold-belt. Vertical field of view 7.5 cm. National Museum of Wales Collection (NMW 83.41G.M.8092), ex R.J. King Collection. Photo M.P. Cooper, © National Museum of Wales.

Chemical Group:
  • Arsenates
Geological Context:
  • Supergene : post-mining oxidation & weathering deposits
  • Supergene : in situ natural oxidation & weathering deposits
Introduction: erythrite is a supergene mineral that forms during weathering of primary Co- and As-bearing sulphide assemblages. It is of frequent occurrence in the 'post-mining' supergene environment: that is, it often occurs, where the precursor minerals are available, as crusts or coatings around fragments of rock in mine-tips or on mine walls underground. Known for many years to miners as 'cobalt bloom', erythrite is a very conspicuous mineral that is easily observed even when present in minuscule quantities, due to its vivid pink colour. There are a number of similar-coloured cobalt minerals in existence but in general erythrite is the most abundant of them.
Occurrence in Wales: erythrite was known from the Dolgellau Gold-belt (its chief Welsh occurrences being in mines in this area) nearly 80 years ago (Dewey & Eastwood, 1925). It has subsequently been noted by Gilbey (1968) and by Saich & Rust (1987). All occurrences are intimately associated with cobaltite, usually in its typical (for the Gold-belt) paragenetic position as rich disseminations in sheared mudstone and quartz. The erythrite is of post-mining origin, forming pink encrustations on masses of such veinstone. More recently, scattered occurrences have been noted across North and Central Wales (see: key locality entries).
Key Localities:
  • Bontddu, Dolgellau, Gwynedd: Saich & Rust (1987) described pink to deep reddish-pink drusy crusts and spheres of erythrite to 0.6 mm from an old trial level, at SH 657 194, to the NW of Bontddu.
  • Central Wales Orefield: erythrite as thin, filmy pale pink coatings and occasional spheroids is occasionally found on mine tips where the early (A1) polymetallic mineralization was worked: the diverse primary ore assemblage locally includes minor sulpharsenides (Mason, 1994; Rust & Mason, 1994). The Darren Lodes are the chief source for these primary phases and Darren, Cerrigyrwyn and Gwaithyrafon mines are the main localities, but in specimen terms the material is unimpressive.
  • Clogau Mine, Bontddu, Gwynedd: localized occurrences of lustrous, microcrystalline erythrite on cobaltite-bearing quartz have recently been noted underground at Clogau Mine (J.S. Mason, unpublished data).
  • Drws-y-Coed Mine, Nantlle, Gwynedd: traces of post-mining erythrite were found on the tips in 1997 (Bevins & Mason, 1998). Arsenopyrite is abundant at this site although the source of the cobalt remains to be discovered.
  • Great Orme Copper Mines, Llandudno, Gwynedd: the presence of erythrite, as 10-20 µm diameter spheres in copper-ore from the Roman Shaft has been recorded by Jenkins & Johnson (1993). The occurrence is very minor.
  • Nant-y-mwyn Mine, Rhandirmwyn, Carmarthenshire: very rare at this locality as microscopic areas of pink crystals in quartz cavities (Steve Rust Collection) and as earthy pink masses in a part-weathered galena/pyrite matrix (National Museum of Wales Collection, ex J.S. Mason, collected 1991). It is anticipated that small amounts of Co-bearing sulpharsenides occur with the pyrite.
  • Panorama Mine, Barmouth, Gwynedd: as encrustations on cobaltite-bearing veinstone, as described by Gilbey (1968).
  • Vigra Mine, Dolgellau gold-belt, Gwynedd: erythrite is conspicuous as vivid pink, poorly microcrystalline spots and encrustations on cobaltite-bearing veinstone in the tips at this mine. Specimens are best described as colourful rather than well-crystallized.
References:
  • Bevins, R.E. & Mason, J.S., 1998 Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Gwynedd.  National Museums of Wales, Cardiff.
  • Dewey, H. & Eastwood, T., 1925 Copper ores of the Midlands, Wales, the Lake District and the Isle of Man.  Memoirs of the Geological Survey. Special Report on the Mineral Resources of Great Britain, 30.
  • Gilbey, J.W., 1968 The mineralogy, paragenesis and structure of the ores of the Dolgellau Gold Belt, Merionethshire, and associated wall rock alteration.  Unpublished Ph.D thesis, University of London, UK.
  • Jenkins, D.A. & Johnson, D.B., 1993 Abandoned metal mines: a unique mineralogical and microbiological resource.  Journal of the Russell Society, 5, 40-44.
  • Mason, J.S., 1994 A Regional Paragenesis for the Central Wales Orefield.  Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  • 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.
  • Saich, D.A. & Rust, S.A., 1987 Micro-minerals from a trial level in Wales.  U.K. Journal of Mines and Minerals, No. 3, 3-4.