Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Iron oxide hydroxide hydrate
Chemical Formula: Fe5O7OH.H2O
Method(s) of Verification: unknown.

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
Introduction: ferrihydrite is a secondary mineral widespread in the soluble fraction of soils and weathered rock and found in acid mine effluent.
Occurrence in Wales: in Wales, ferrihydrite has only been recorded associated with acid mine drainage, in particular that related to Parys Mountain, on Anglesey. Ferrihydrite is noted forming particulates in the Afon Goch draining the Parys Mountain mining area. Conditions in the stream are ideal for the precipitation of ferrihydrite with highly acidic iron-rich water.

Key Localities:

  • Afon Goch, Anglesey: the Afon Goch drains the Parys Mountain mine workings and is consequently highly contaminated by acid mine drainage. Ferrihydrite is precipitated as particles where, near-neutral tributary waters enter the stream, allowing the stream to act as a sink for the contaminants (Boult & Curtis, 1994; Boult et al., 1994).


  1. Boult, S. & Curtis, C.D., 1994. The predictability of metal flux in a stream heavily polluted by acid mine-drainage. pp. 227-36 in: Trace Substances, Environment and Health (C.R. Cothern, editor). Science Reviews, Northwood, UK.
  2. Boult, S., Collins, D.N., White, K.N. and Curtis, C.D., 1994. Metal transport in a stream polluted by acid mine drainage – The Afon Goch, Anglesey, UK. Environmental Pollution, 84(3), 279-284.
  3. 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.