Foitite

Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Sodium iron aluminium hydroxyborosilicate
Chemical Formula: (□,Na)(Fe2+,Al)3Al6Si6O18(BO3)3(OH)4
Method(s) of Verification: Cwm Bychan - EMPA (National Museum of Wales & A.G. Tindle, unpublished data).

Chemical Group:

  • Silicates

Geological Context:

  • Igneous

SEM backscatter image of zoned tourmaline from Cym Bychan. Largest crystal 0.2 mm diameter. Whereas this crystal is composed of schorl with an outer rim of dravite, the other crystals are composed of schorl alone or schorl and foitite. © NMW

Introduction: foitite is a tourmaline family mineral and belongs to the 'vacancy' group, based on the absence of atoms in the X site. The mineral can be further divided into fluor-, hydroxyl- or oxy varieties, depending on the filling of the W site in the crystals lattice (see tourmaline entry for more details).
Occurrence in Wales: although several occurrences of tourmaline have been reported from Wales (see tourmaline entry), a lack of detailed optical or analytical data has precluded most of these from being classified to mineral species level. New data (National Museum of Wales & A.G. Tindle, unpublished data) confirms the presence of foitite in at least one of these occurrences. As more microprobe data becomes available it is anticipated that a more widespread distribution of foitite in Wales will be demonstrated.

Key Localities:

  • Cwm Bychan, Beddgelert, Gwynedd: samples of Caradocian sandstone studied by Beavon (1963) (e.g. NMW 80.30G), held in the National Museum of Wales, contain aggregates of subhedral basal sections and prisms, showing concentric optical zoning. Microprobe analyses reveal these to be crystals of schorl and foitite, with outer rims of dravite.

References:

  1. Beavon, R.V., 1963. The succession and structure east of the Glaslyn River, North Wales. Quarterly Journal of the Geological Society of London, 119, 479-512.
  2. Hawthorne, F. C. & Henry, D. J., 1999. Classification of the minerals of the tourmaline group. European Journal of Mineralogy, 11, 201-215.