Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Hydrated cobalt arsenate
Chemical Formula: Co3(AsO4)2.8H2O
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.

Chemical Group:

  • Arsenates

Geological Context:

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


  1. 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.
  2. 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.
  3. 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.
  4. Jenkins, D.A. & Johnson, D.B., 1993. Abandoned metal mines: a unique mineralogical and microbiological resource. Journal of the Russell Society, 5, 40-44.
  5. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  6. 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.
  7. 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.