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in the magnesian peridoties in the diatreme exposed at Great House, and also at Glen Court, near Usk (D.T. Moffat, unpublished data).

There are no key localities for this specimen.

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.

There are no references for this specimen.