Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Potassium iron sulphate hydroxide
Chemical Formula: KFe33+(SO4)2(OH)6
Method(s) of Verification: Parys Mountain - XRD (Dr. D. Jenkins, ex UCNW Bangor); Dolyhir Quarry - XRD (Dr. D.I. Green, Manchester Museum); Penarth - XRD (National Museum of Wales, NMW X-1595).

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
  • Supergene: in situ natural oxidation & weathering deposits
Powdery, pale yellow jarosite crust on Rhaetic shale from Penarth, nr. Cardiff, South Wales. National Museum of Wales specimen. Photo T.F. Cotterell, © National Museum of Wales.
SEM image of jarosite from Dolyhir Quarry, Old Radnor, Powys. Scale bar 10 microns (0.01 mm). Image D.I. Green, © D.I. Green.
Introduction: jarosite is a secondary sulphate that forms under acidic conditions in the oxidized zone of sulphide and in particular pyrite-bearing mineral deposits.
Occurrence in Wales: there are very few records of jarosite from Wales, although it is probably much more common. Jarosite was, identified as a major component of post-mining efflorescence developed on the floor of the Cae Coch Pyrite Mine (Johnson et al., 1979) and has subsequently been identified in a similar environment at Parys Mountain on Anglesey (Jenkins et al., 2000). Jarosite has also recently been confirmed at Dolyhir Quarry forming microcrystalline coatings with brochantite in thin fractures (D.I. Green, unpublished data) and as yellow crusts on friable shale from coastal cliffs near Penarth in South Wales (T.F. Cotterell, unpublished data).

Key Localities:

  • Cae Coch Mine, Trefriw, Gwynedd: yellow jarosite occurs with fibroferrite and rare copiapite on the floor of the mine (Johnson et al., 1979).
  • Dolyhir Quarry, Old Radnor, Powys: a recent find, forming microcrystalline crusts in fractures associated with brochantite (D.I. Green, unpublished data).
  • Gwaith-yr-Afon Mine, Goginan, Ceredigion: Rust & Mason (1994) tentatively identified thin yellow-brown to golden-yellow drusy crusts of platy crystals on corroded chalcopyrite as jarosite.
  • Parys Mountain, Anglesey: post-mining mineralogy at Parys Mountain is dominated by sulphate minerals of which the characteristic mineral both above and below ground is earthy yellow jarosite (Jenkins et al., 2000).
  • Penarth and Lavernock, South Wales: yellow crusts of jarosite occur on shale exposed in sea cliffs a short distance south of Penarth (T.F. Cotterell/A. Dean, unpublished data).


  1. Jenkins, D. A., Johnson, D. B. & Freeman, C., 2000. Mynydd Parys Cu-Pb-Zn mines: mineralogy, microbiology and acid mine drainage. pp. 161-179. In: Environmental Mineralogy: Microbial Interactions, Anthropogenic Influences, Contaminated Land and Waste Management (Cotter-Howells, J. D., Campbell, L. S., Valasami-Jones, E. & Batchelder, M., eds.). The Mineralogical Society of Great Britain & Ireland, London.
  2. Johnson, D.B., Kelso, W.I., & Jenkins, D.A., 1979. Bacterial streamer growth in a disused pyrite mine. Environmental Pollution, 18, 107-118.
  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.