Spinel

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: magnesium aluminium silicate
Chemical Formula: MgAl2O4
Method(s) of Verification: all cited occurrences - polarizing microscope

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Igneous
Introduction: spinel belongs to the large spinel ‘supergroup’ of minerals and is also the most commonly occurring member. It forms a continuous series with hercynite (Fe2+Al2O4) by replacement of magnesium by ferrous iron and with gahnite (ZnAl2O4) by replacement of magnesium by zinc. A third series is formed by replacement of aluminium by chrome to form magnesiochromite (MgCr2O4). Spinel is found as a minor component in igneous rocks.
Occurrence in Wales: spinel is probably present in most ultramafic and mafic igneous rocks in Wales, although records of its occurrence are few.

Key Localities:

  • Preseli Mountains (Mynydd Preseli), Pembrokeshire: Ixer (in Thorpe et al., 1991) noted the presence of spinel in altered dolerite from the Carn Menyn, in the Mynydd Preseli area of Pembrokeshire.
  • Rhiw, Llŷn, Gwynedd: Hawkins (1970) noted the presence of small, euhedral to subhedral spinel in the hornblende-picrite unit of the Rhiw Intrusion, on Llŷn. They form small (0.05-0.20 mm) octahedral or cubes, typically enclosed by larger olivine crystals and semi-quantitative analysis indicates the presence of iron and titanium, suggesting a possible intergrowth with ilmenite.
  • Usk, Monmouthshire: spinel has also been recorded as a minor component in the magnesian peridoties in the diatreme exposed at Great House, and also at Glen Court, near Usk (D.T. Moffat, unpublished data).

References:

  1. Hawkins, T.R.W., 1970. Hornblende gabbros and picrites at Rhiw, Caernarvonshire. Geological Journal, 7, 1-24.
  2. Thorpe, R.S., Williams-Thorpe, O., Jenkins, D.G. & Watson, J.S., 1991. The geological sources and transport of the bluestones of Stonehenge, Wiltshire, U.K. Proceedings of the Prehistoric Society, 57, 103-157.