Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Iron aluminium sulphate hydrate
Chemical Formula: Fe2+Al2(SO4)4.22H2O
Method(s) of Verification: material from Parys Mountain and Trwyn Cae Iago verified by a combination of XRD and EDS analyses. Material from Gwyfynydd Mine is mid-series between halotrichite and pickeringite. See Cotterell (2009) for a comprehensive account.

Chemical Group:

  • Sulphates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
Loose aggregates of fibrous halotrichite from Parys Mountain, Amlwch, Anglesey. The box is 75 mm long. National Museum of Wales specimen, no. 27.111.GR.276, ex G.J. Williams collection. © National Museum of Wales.
Introduction: a secondary mineral that typically develops as a result of alteration of pyrite, and is found on mine walls, along with other sulphate minerals. Halotrichite forms a series with pickeringite.
Occurrence in Wales: halotrichite was once thought to be quite widespread in Wales, being reported on occasion in sizable quantities, most notably from collieries in the South Wales coalfield, but also as post-mining efflorescence at Parys Mountain and Gwynfynydd Mine (Bevins, 1994). Identification had been made on the basis of XRD of specimens in the National Museum of Wales' collections. However, a review of all such occurrences (Cotterell, 2009) has shown that many are in fact the Mg-analogue, pickeringite. A full characterisation of Welsh halotrichite group minerals has revealed that samples from Parys Mountain and Trywn Cae Iago are near end-member halotrichite and material from Gwynfynydd Mine is mid-series. All specimens from the Coal Measures in South Wales are pickeringite.

Key Localities:

  • Gwynfynydd Mine, Ganllwyd, Dolgellau, Gwynedd: a pinky-cream coloured nodular aggregate of fibres (National Museum of Wales specimen, NMW 83.41G.M.9093) of mid-series composition, was collected in 1961.
  • Parys Mountain, Anglesey: halotrichite of near end-member composition occurs as post-mining efflorescence in sheltered recesses in the opencast workings. National Museum of Wales specimen no. NMW 27.111.GR.276 (ex G.J. Williams collection) is a small box containing loose aggregates of silky white fibrous halotrichite associated with minor siderotil and melanterite. The lack of any matrix suggests that it formed post-mining as efflorescence.
  • Trwyn Cae Iago, near Porthmadog: halotrichite occurs as an encrustation of minute fibres (National Museum of Wales specimen, NMW 79.10G.M.2).


  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Bevins, R.E. & Mason, J.S., 2000. Welsh Metallophyte and Metallogenic Evaluation Project. Results of a minesite survey of Glamorgan and Gwent compiled by the National Museums and Galleries of Wales. CCW Contract Science Report No. 386.
  3. Cotterell, T.F., 2009. A review of halotrichite group minerals in Wales. UK Journal of Mines and Minerals, 30, 43-47.