Chromite

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Iron chromium oxide
Chemical Formula: Fe2+Cr2O4
Method(s) of Verification: Holy Island - optical microscopy (Greenly, 1919; Maltman, 1977); Great House – optical microscopy (Boulton, 1911).

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Igneous
Introduction: chromite is a member of the spinel group, it is dimorphous with donathite (they share the same chemical composition but the atoms are arranged into different crystal lattices) and forms a series with magnesiochromite (MgCr2O4) and with hercynite (FeAl2O4). Chromite is typically found in ultrabasic igneous rocks such as peridotite or its altered equivalent serpentinite.
Occurrence in Wales: two occurrences of chromite are know from Wales, one from Holy Island, Anglesey from serpentinite and associated rocks related to the alteration of basic and ultrabasic igneous rocks; the other from South Wales from peridotite nodules within the Great House diatreme (volcanic pipe) near Usk.

Key Localities:

  • Great House, Usk, Monmouthshire: Boulton (1911) identified chromite in magnesian peridotite nodules within the volcanic rocks at this locality.
  • Holy Island, Anglesey: Greenly (1919) reported chromite from various localities on Holy Island in chlorite-chromite-magnetite schist, talc schist, marble and serpentinite found within the New Harbour Groups metasediments. Maltman (1977) confirmed these occurrences and described serpentinite containing chromite associated with magnetite, minor picotite (a variety of hercynite), leucoxenized ilmenite and zircon. Typical specimens are held in the National Museum of Wales collection.

References:

  1. Boulton, W.S., 1911. On a monchiquite intrusion in the Old Red Sand stone of Monmouthshire. Quarterly Journal of the Geological Society, London, 67, 460-476.
  2. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  3. Maltman, A.J., 1977. Serpentinites and related rocks of Anglesey. Geological Journal, 12, 113-128.