Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Aluminium sulphate hydrate
Chemical Formula: Al2(SO4)3.17H2O
Method(s) of Verification: three specimens labelled as alunogen in the National Museum of Wales' collection have been discredited by XRD (numbers NMW X-674, X-676 & X-704). The first verified alunogen in Wales, from Trwyn Cae Iago, was identified by XRD (no. NMW X-1937).

Chemical Group:

  • Sulphates
Introduction: alunogen occurs as an alteration product of pyritic rocks, often forming in the post-mining environment, for example on the walls of old coal mines.
Occurrence in Wales: Bevins (1994) recorded that the three specimens in the National Museum of Wales Mineral Collection with original labels stating that they are alunogen are on the basis of X-ray diffraction in fact halotrichite. Cotterell (2009) has since demonstrated by a combination of XRD and EDS data that the specimens from Cymmer and Deep Navigation collieries in south Wales are actually pickeringite, while the specimen from Parys Mountain on Anglesey is halotrichite. Perhaps more importantly, Cotterell (2009) identified minor alunogen intergrown with halotrichite from Trwyn Cae Iago, near Porthmadog.

Key Localities:

  • Trwyn Cae Iago, near Porthmadog: minor alunogen was detected by XRD intergrown with halotrichite on National Museum of Wales specimen, no. NMW 79.10G.M.1 (Cotterell, 2009).


  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Cotterell, T.F., 2009. A review of halotrichite group minerals in Wales. UK Journal of Mines and Minerals, 30, 43-47.