Scorodite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Hydrated iron arsenate
Chemical Formula: FeAs04.2H20
Method(s) of Verification: Tan-y-garth - XRD (National Museum of Wales, NMW X-103); all other occurrences reported are based on visual identifications

Chemical Group:

  • Arsenates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
Scorodite forming a microcrystalline crust. Bethesda. National Museum of Wales Collection (NMW 78.85G.M.20). Photo D.I. Green. © National Museum of Wales.
Introduction: scorodite is a common supergene mineral in areas where primary arsenopyrite-bearing mineralization is present. It may be the only supergene species present, or in more evolved oxidized deposits it may occur accompanied by pharmacosiderite and a range of base-metal arsenates (depending upon the geochemistry of the primary sulphide mineralization). It occurs typically associated with part-weathered, crumbly and cracked arsenopyrite, as pale greenish-yellow discernably crystalline crusts. The association with this matrix is often diagnostic in identifying the material as scorodite.
Occurrence in Wales: as arsenopyrite is widespread and locally common in Wales, it is perhaps unsurprising that quantities of scorodite have been found at a number of localities. In all of these, it is thought to be post-mining in origin. First reported by Gilbey (1968) from the Dolgellau Gold-belt, it has since been noted from three other Welsh mining districts, namely the Snowdon Caldera, the Central Wales Orefield and the Dolaucothi gold mine. None of the known localities have produced scorodite in specimens of anything more than academic quality.

Key Localities:

  • Dolaucothi Gold Mine, Pumpsaint, Carmarthenshire: scorodite occurs rarely as thin greenish-grey coatings on part-weathered arsenopyrite (J.S. Mason, unpublished data).
  • Dolgellau Gold-belt, Gwynedd: first recorded by Gilbey (1968) from numerous mines in the area, scorodite is widespread in trace to minor amounts, and shadows arsenopyrite in its distribution. It has been found at Clogau, Vigra, Cefn Coch, Tyddyn Gwladys and Prince Edward mines (Gilbey, 1968; Bevins & Mason, 1998) and in pyrite-arsenopyrite bearing boulders in the bed of the Afon Wen (J.S. Mason, unpublished data). It is perhaps best developed in the Foel Ispri mines, near Llanelltyd, where sub-millimetre thickness greenish-yellow crusts fill small cavities in pyrite-arsenopyrite rich veinstone.
  • Gwaith-yr-Afon Mine, Goginan, Ceredigion: greyish to pale yellow sparsely scattered crystal aggregates (up to 0.75 mm) associated with limonite and pharmacosiderite on quartz, have been identified visually as scorodite (Mason & Rust, 1995).
  • Snowdon, Gwynedd: as in the Dolgellau Gold-belt, scorodite shadows arsenopyrite in its occurrence and is thus present at mines around the caldera margin, such as Cwm Bychan, near Beddgelert; Llanberis; Moel Fleiddiau, near Blaenau Ffestiniog and Ceunant and Tan-y-garth mines near Bethesda: at the latter site it is particularly common (Bevins & Mason, 1998). Typically it forms thin microcrystalline crusts in association with iron oxides on decomposing arsenopyrite.

References:

  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. 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.
  3. 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.