Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Potassium aluminium sulphate hydroxide
Chemical Formula: KAl3(SO4)2(OH)6
Method(s) of Verification: Treffgarne - polarizing microscope & XRD (British Geological Survey).
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.
- 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.
- 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.