Mineral Database (Saesneg yn unig)

Mineral Database (Saesneg yn unig)
Mineralogy of Wales

Alunite

Crystal System: Trigonal
Formula: KAl3(SO4)2(OH)6
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Potassium aluminium sulphate hydroxide
Method(s) of Verification: Treffgarne - polarizing microscope & XRD (British Geological Survey).
Chemical Group:
  • Sulphates
Introduction: alunite tends to form in the near-surface epithermal environment in acidic conditions, for example within fumarolic systems.
Occurrence in Wales: classic volcanogenic epithermal mineralization is unusual in Wales. However, Brown et al. (1987) identified alunite in altered acid volcanic rocks belonging to the Roch Rhyolite Group near Treffgarne in Pembrokeshire. The find was made during a British Geological Survey exploration programme for gold.
Key Localities:
  • Treffgarne, Pembrokeshire: identified from BGS drillcore (Brown et al., 1987). Alunite forms intergrowths with quartz and white mica, also intergrowths with corundum and, veinlets with pyrophyllite. It was only found in one out of the three boreholes drilled. The occurrences are within a major zone of hydrothermal alteration with high levels of Al, Ba, Sr, Fe and S. Pyritization and sericitization are the most obvious effects; enhanced (but only of academic interest) gold levels were recorded in some samples.
References:
  • Brown, M.J. and others, 1987 Volcanogenic mineralisation in the Treffgarne area, south-west Dyfed, Wales.  Mineral Reconnaissance Programme Report, British Geological Survey, No. 86.