Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Aluminium oxide
Chemical Formula: Al2O3
Method(s) of Verification: All occurrences cited - verified by polarizing microscope.
- Oxides & Hydroxides
- Sedimentary: allogenic (detrital)
Introduction: the mineral corundum, which includes the varieties ruby and sapphire, can be formed by both igneous and metamorphic processes. In igneous rocks it is typically found in silica-poor compositions, such as pegmatites or other rocks associated with nepheline syenites (Deer et al., 1992). It may also be found in aluminium-rich nodules in igneous rocks. Metamorphic corundum forms in silica-poor rocks in contact metamorphic zones and in thermally or contact metamorphosed bauxite deposits (Deer et al., 1992).
Occurrence in Wales: corundum has been recorded from altered rhyolitic lavas and rhyolitic tuffs from a borehole near Treffgarne, Pembrokeshire (Brown et al., 1987). Previously Heard (1922) identified prismatic-blue corundum crystals in sandstones and shales of the Pennant Series (Upper Coal Measures) of Carboniferous age exposed to the east of the River Taff, in South Wales.
There are no key localities for this specimen.
- 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.
- Deer, W.A., Howie, R.A. & Zussman, J., 1992. An Introduction to Rock-Forming Minerals. Longman Scientific & Technical, 696pp.
- Heard, A., 1922. The petrology of the Pennant Series, east of the River Taff. Geological Magazine, 59, 83-94.