Hydronium jarosite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence - 3rd UK recording
Distribution: Rare
Chemical Composition: Hydronium iron sulphate hydroxide
Chemical Formula: (H3O)Fe3+3(SO4)2(OH)6
Method(s) of Verification: Brynycastell Mine - XRD (NMW X-1448 & X-1454) and EMPA at the National Museum of Wales.

Chemical Group:

  • Sulphates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
  • Supergene: in situ natural oxidation & weathering deposits
Hydronium jarosite pseudomorphs after prismatic gypsum crystals from Brynycastell Mine. Field of view 9 mm across. National Museum of Wales Collection (NMW 2006.11G.M.6). © National Museum of Wales.
Introduction: hydronium jarosite, formerly called carphosiderite, is a member of the alunite group of minerals resulting typically from the oxidation of sulphide-rich orebodies, in particular those containing pyrite.
Occurrence in Wales: ‘carphosiderite’ is reported to occur abundantly in underground mine workings at Parys Mountain, Anglesey (Bevins, 1994). Jenkins et al. (2000) described earthy yellow jarosite as the characteristic supergene mineral both above and below ground at Parys Mountain, but suggest that the hydronium variety is possibly included although has yet to be confirmed by quantitative chemical analysis. Recent analysis of orange-yellow crusts, some of which pseudomorph small selenite crystals, at Brynycastell Mine, near Dolgellau has shown hydronium jarosite to be present (Cotterell, 2006).

Key Localities:

  • Brynycastell Mine, Cross Foxes, Dolgellau, Gwynedd: straw-yellow post-mining efflorescence developed within sheltered parts of opencast workings are shown by Cotterell (2006) to be hydronium jarosite with gypsum.
  • Parys Mountain, Anglesey: described by Jenkins et al. (2000) as possibly occurring with jarosite, but it is noted that this has not been confirmed.


  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Cotterell, T.F., 2006. Hydronium jarosite from Brynycastell Mine, Cross Foxes, Dolgellau, Gwynedd, Wales. Journal of the Russell Society, 9, 65-67.
  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.