Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Magnesium aluminium silicate
Chemical Formula: Mg2Al4Si5O18
Method(s) of Verification: All occurrences cited - verification by polarizing microscope.
Introduction: cordierite forms a series with sekaninaite ((Fe,Mg)2Al4Si5O18) and is dimorphous with indialite. It is a rock-forming mineral found in both regional and contact metamorphic rocks. In contact metamorphic environments it most commonly forms in argillaceous rocks (mudstones) but in regional metamorphism settings is found in rocks with both arenaceous (sandy) and argillaceous (muddy) precursors.
Occurrence in Wales: cordierite with a regional metamorphic origin is not found in Wales as the grade of metamorphism is too low for its formation, however contact metamorphic cordierite is recorded as having formed in two areas, although preserved at only one of these. Bromley (1969) described cordierite from the contact metamorphic aureole of the Tan-y-Grisiau microgranite, although no specific localities are mentioned. He also reports ovoid, light to dark spots usually 1 mm to 5 mm in diameter composed of radial, concentric or sectoral aggregates of sericitic mica and chlorite in the sedimentary rocks of the contact zone of the granite. He interpreted these as representing the replacement of cordierite and andalusite. Similarly Roach (1969) noted ovoid chlorite-rich pseudomorphs after cordierite in sedimentary rocks adjacent of the St. David’s Head Intrusion, in Pembrokeshire. Heard (1922) identified colourless grains of cordierite, partly altered to chlorite and containing inclusions of magnetite and zircon, as detrital grains in sandstones of the Pennant Series (Upper Coal Measures), of Carboniferous age, exposed to the east of the Taff River, in South Wales. Once again no specific localities are given.
There are no key localities for this specimen.
- Bromley, A.V., 1969. Acid plutonic igneous activity in the Ordovician of North Wales. In: Wood, A. (ed.) The Pre-Cambrian and Lower Palaeozoic rocks of Wales. University of Wales Press, 387-408.
- Heard, A., 1922. The petrology of the Pennant Series, east of the River Taff. Geological Magazine, 59, 83-94.
- 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.