Mineral Database

Mineral Database
Mineralogy of Wales

Pickeringite

Crystal System: Monoclinic
Formula: MgAl2(SO4)4•22H2O
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Magnesium aluminium sulphate hydrate
Method(s) of Verification: specimens from the Coal Measures in South Wales are confirmed as near end-member pickeringite by both XRD and EDS (see, Cotterell, 2009 for full details); material from Gwynfynydd mine is mid-series in composition.

Silky white fibres (up to 25 mm long) of pickeringite from the Five-Feet Seam, Cymmer Colliery. National Museum of Wales Collection (NMW 28.584.GR.1). Photo M.P. Cooper, © National Museum of Wales.

Chemical Group:
  • Sulphates
Geological Context:
  • Supergene : post-mining oxidation & weathering deposits
Introduction: pickeringite is a supergene mineral that forms commonly on the walls of mines as an alteration product of pyrite. It forms a series with halotrichite (FeAl2(SO4)4•22H2O.
Occurrence in Wales: two occurrences of pickeringite in Wales were recorded by Bevins (1994), both forming post-mining deposits related to oxidation of sulphides. A review of halotrichite group minerals in Wales (Cotterell, 2009) has shown that material from Gwynfynydd mine is actually mid-series in composition and material from Parys Mountain is near end-member halotrichite. However, all of the specimens of coarse silky-white fibrous halotrichite from the South Wales Coalfield have been shown to be near end-member pickeringite in composition. Therefore pickeringite is now classified as a valid Welsh mineral species.
Key Localities:
  • Gwynfynydd Mine, Ganllwyd, Dolgellau, Gwynedd: a fibrous aggregate collected in 1961 (National Museum of Wales specimen, no. NMW 83.41G.M.9093) is mid-series in composition - two EDS analyses are Mg dominant (pickeringite) and one Fe dominant (halotrichite).
  • South Wales Coalfield: A comprehensive review of halotrichite group minerals in Wales (Cotterell, 2009) has revealled that all specimens from the Coal Measures of South Wales are near end-member pickeringite in composition. Localities include, Deep Navigation Pit, Treharris and Cymmer Colliery, Porth.
References:
  • Bevins, R.E., 1994 A Mineralogy of Wales  National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  • Cotterell, T.F., 2009 A review of halotrichite group minerals in Wales.  UK Journal of Mines and Minerals, 30, 43-47.