Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Iron sulphate hydroxide hydrate
Chemical Formula: Fe3+(SO4)(OH).5H2O
Method(s) of Verification: Cae Coch Mine - XRD & wet chemical analysis; Parys Mountain - XRD.

Chemical Group:

  • Sulphates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
Scanning electron micrograph of matted hair-like fibroferrite on gypsum from Cae Coch Mine. National Museum of Wales specimen (NMW 2002.9G.M.4). © National Museum of Wales.
Introduction: fibroferrite forms in highly acidic conditions, particularly in the altered portions of sulphide ore bodies and on the walls of underground mine workings in the post-mining supergene environment.
Occurrence in Wales: fibroferrite is known from two localities in North Wales, where it has formed in underground workings as a product of the post-mining oxidation of pyrite (Johnson & Jenkins, 1979; Jenkins et al., 2000). Given the number of localities where similar environments exist throughout North and Central Wales, but where the post-mining mineralogy remains to be investigated, it is anticipated that further occurrences may well come to light.

Key Localities:

  • Cae Coch Mine, Trefriw, Gwynedd: fibroferrite forms copious mounds with jarosite and, more rarely, copiapite on the floor of the mine (Johnson et al., 1979).
  • Parys Mountain, Anglesey: fibroferrite is present in minor amounts, with other iron sulphates, in overhangs and recesses and other sites protected from the elements within the two major opencasts (Jenkins et al., 2000).


  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.