Ilmenite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Iron titanium oxide
Chemical Formula: FeTiO3
Method(s) of Verification: all by electron microprobe analyses (EMPA). Anglesey - (Horák, 1993); Rhiw - (Cattermole, 1976); Snowdonia - (R.J. Merriman, unpublished data); St. David’s Head - (Bevins et al., 1994); Tal y Fan - (Merriman et al., 1986)

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Igneous
  • Metamorphic
Introduction: ilmenite is a common accessory mineral in many igneous and metamorphic rocks. It forms a series with both geikielite (Mg,Fe2+)(Ti,Mn)O3 and pyrophanite (MnTiO3).
Occurrence in Wales: ilmenite is widely distributed throughout Wales being present in many basic igneous rocks (both lavas and intrusions) and within the high-grade Precambrian metamorphic rocks of Anglesey. Where these rocks have been heavily altered, the ilmenite may be partially or totally replaced by leucoxene, titanite or one of the TiO~2 polymorph minerals (rutile, brookite or anatase). In almost all instance ilmenite can only identified by means of a petrological microscope.

Key Localities:

  • Anglesey: ilmenite is the only opaque mineral present within the Anglesey amphibolites, basic igneous rocks metamorphosed in the amphibolite facies. It may constitute up to 5% of the rock (by volume) and forms irregular-shaped grains and crystals aggregates up to 1.5 mm long. It is most commonly associated with amphibole-rich areas of the rock and shows variable alteration to titanite (Horák, 1993).
  • Rhiw, Llŷn, Gwynedd: ilmenite-plagioclase mesocumulates are present in the Rhiw Intrusion, on Llŷn (Cattermole, 1976).
  • Snowdonia, Gwynedd: spectacular ilmenite-bearing tuffs of Ordovician age are developed in Central Snowdonia, occurring in the Lower Rhyolitic Tuff Formation, exposed near Beddgelert, and in the Bedded Pyroclastic Formation at various localities including Cwm Meirch, near Lliwedd, in the crags of Gallt-y-llyn and Coed Pant Agored, and also near Snowdon summit (Williams, 1927). The ilmenite forms platy crystals up to 0.3 mm in length and 0.1 mm wide. Electron microprobe analyses indicate that they are manganese-rich, with up to 10 wt% MnO, (R.J. Merriman, unpublished data).
  • St. David's Head, Pembrokeshire: ilmenite is present throughout the St. David’s Head Intrusion, but is particularly abundant in the laminated quartz ferrogabbros where it constitutes up to 12% of the rock by volume (Bevins et al., 1994).

References:

  1. Bevins, R.E., Lees, G.J. Roach, R.A. Rowbotham, G. & Floyd, P.A., 1994. Petrogenesis of the St David’s Head Layered Intrusion, Wales: a complex history of multiple magma injection and in situ crystallisation. Transactions of the Royal Society of Edinburgh: Earth Sciences, 85, 91-121
  2. Cattermole, P.J., 1976. The crystallization and differentiation of a layered intrusion of hydrated alkali olivine-basalt parentage at Rhiw, North Wales. Geological Journal,11,45-70.
  3. Horák, J.M., 1993. The Late Precambrian Coedana and Sarn Complexes, Northwest Wales - a Geochemical and Petrological study. Unpublished Ph.D. thesis, University of Wales, 415pp.
  4. Merriman, R. J., Bevins, R.E. & Ball, T.K., 1986. Petrology and geochemical variation within the Tal y Fan Intrusion: a study of element mobility during low-grade metamorphism with implications for petro-tectonic modelling. Journal of Petrology, 27, 1409-1436.
  5. Williams, H., 1927. The geology of Snowdon (North Wales). Quarterly Journal of the Geological Society of London, 83, 346-431.