Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Hydrated iron sulphate
Chemical Formula: FeSO4.5H2O
Method(s) of Verification: Parys Mountain - XRD (National Museum of Wales, NMW X-857)

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
Pale blue siderotil and melanterite with silky white fibrous halotrichite from Parys Mountain, Anglesey. The box is 75 mm long. National Museum of Wales specimen no. NMW 27.111.GR.276, ex G.J. Williams collection. © National Museum of Wales.
Introduction: siderotil is a rare supergene mineral formed by the oxidation of iron sulphides. It tends to occur in association with other supergene sulphates such as jarosite, halotrichite and copiapite.
Occurrence in Wales: only one record of siderotil is known from the UK and this is from the Parys Mountain Copper Mine on Anglesey (Jenkins et al., 2000). However the work done in identifying the post-mining products of pyrite decay at this site has been relatively thorough compared to many similar Welsh and UK localities, where assemblages of unidentified iron salts have yet to be researched.

Key Localities:

  • Parys Mountain, Anglesey: as a bluish, earthy phase associated with halotrichite and formed by post-mining sulphide decay (Jenkins et al., 2000; Cotterell, 2009).


  1. Cotterell, T.F., 2009. A review of halotrichite group minerals in Wales. UK Journal of Mines and Minerals, 30, 43-47.
  2. 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.