Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Calcium sodium aluminium silicate
Chemical Formula: (Ca,Na)Al(Al,Si)3O8
Method(s) of Verification: all occurrences - polarizing-light microscopy (Greenly, 1919; Hawkins, 1970; Cattermole, 1976; Archer & Elliott, 1965; Roach, 1969): St David’s Head Intrusion - EMPA (Bevins et al., 1994).

Geological Context:

  • Igneous
Introduction: bytownite, once a valid species name, is now an informal term used for plagioclase feldspar with anorthite/albite ratios in the range 70:30 to 90:10. It is an important rock-forming mineral in basic composition igneous rocks such as gabbros and basalts.
Occurrence in Wales: the effects of low-temperature alteration have resulted in much bytownite being replaced by more sodic plagioclase, most notably albite. Greenly (1919) reported the mineral from the ‘post-Carboniferous’ (Palaeogene) dykes from Anglesey, although no specific localities are referred to. Hawkins (1970) & Cattermole (1976) both describe bytownite in the Rhiw Intrusion, Llŷn, although identifications were made on the basis of optical properties of the mineral. Similar optical determination of bytownite has been made by Archer & Elliott (1965) from olivine dolerite dykes from Parc Mine, near Llanrwst and by Roach (1969) in some gabbros of the St. David’s Head Intrusion. The latter occurrence has been confirmed by microprobe analyses.

Key Localities:

There are no key localities for this specimen.


  1. Archer, A.A.& Elliot, R.W., 1965. The occurrence of olivine-dolerite dykes near Llanrwst, North Wales. Bulletin of the Geological Survey of Great Britain, 23, 145-152.
  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. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  4. Hawkins, T.R.W., 1970. Hornblende gabbros and picrites at Rhiw, Caernarvonshire. Geological Journal, 7, 1-24.
  5. Roach, R.A., 1969. The composite nature of the St. David's Head and Carn Llidi intrusions of North Pembrokeshire. In: Wood, A. (ed.) The Pre-Cambrian and Lower Palaeozoic rocks of Wales, University of Wales Press, Cardiff, 409-433.