Labradorite

Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Sodium calcium aluminium silicate
Chemical Formula: (Na,Ca)Al(Al,Si)Si2O8
Method(s) of Verification: Tal y Fan - EMPA (Merriman et al., 1986); St. David’s Head - EMPA (Bevins et al., 1994).

Chemical Group:

  • Silicates

Geological Context:

  • Igneous
Introduction: labradorite, once a valid species name, is now an informal term used for plagioclase feldspar with anorthite/albite ratios in the range 50:50 to 70:30. It occurs as a major rock-forming mineral in basic igneous rocks.
Occurrence in Wales: labradorite is widely distributed throughout Wales in basic igneous rocks. As with other calcic plagioclase, primary igneous feldspar may be altered by low-grade alteration or metamorphism to feldspar of a lower calcium and higher sodium content. Image caption:

Key Localities:

  • St. David's Head, Pembrokeshire: Bevins et al. (1994) describe labradorite from all units of the St. David’s Head Intrusion in northern Pembrokeshire.The most calcium-rich compositions are found in the laminated olivine gabbros and range from labradorite up to bytownite (An71). Compositional zoning in the labradorite is reflected by more calcium-rich cores.
  • Tal y Fan, Conway: relict crystals of primary labradorite (An53-69) are present within dolerite in the centre of the Tal y Fan Intrusion (Merriman et al., 1986). The feldspar occurs as tabular crystals intergrown with clinopyroxene, and shows evidence of compositional zoning and replacement by albite and hydrous calcium-aluminium silicates such as prehnite.

References:

  1. Bevins, R.E., 1982. Petrology and geochemistry of the Fishguard Volcanic Complex, Wales. Geological Journal, 17, 1-21
  2. 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
  3. 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.