Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Iron magnesium aluminium silicate hydroxide
Chemical Formula: (Fe,Mg)4Al17(Si,Al)8O45(OH)3
Method(s) of Verification: all cited occurrences - polarizing microscope
- Sedimentary: allogenic (detrital)
Introduction: staurolite is a metamorphic mineral which is found in pelitic schists and gneisses formed under medium grade (amphibolite facies) conditions. Staurolite is typically Fe-rich in composition, with magnesium-rich staurolite only being stable under a restricted range of high P-T conditions in quartz-free rocks (Deer et al., 1992). Staurolite is a durable minerals and is consequently found as detrital grains in sediments.
Occurrence in Wales: no primary metamorphic occurrences of staurolite are known from Wales as amphibolite-grade metamorphic rocks are limited in distribution, being restricted to the Coedana Complex gneisses of Anglesey (Horák, 1993). The presence of detrital staurolite grains has been reported from glacial sands from several separate localities around Wales (Greenly, 1919, Griffiths, 1939, Griffiths & Stuart, 1940), indicating that the source of staurolite lay in a metamorphic terrain outside Wales.
- Ludchurch, Pembrokeshire: Griffiths & Stuart (1940) reported detrital staurolite from from the glacial deposits at Ludchurch in Pembrokeshire.
- South Wales: staurolite was identified as a major component of the Irish Sea Drift in the region between Afon Neath and Afon Tywi (Griffiths, 1939).
- Ty’n-ycaeau, near Menai, Anglesey: Greenly (1919) reported staurolite in the heavy residues from glacial sands exposed around Ty’n-ycaeau, near Menai, on Anglesey.
- Deer, W.A., Howie, R.A. & Zussman, J., 1992. An Introduction to Rock-Forming Minerals. Longman Scientific & Technical, 696pp.
- Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
- Griffiths, J.C., 1939. The mineralogy of the glacial deposits of the region between the Rivers Neath and Towy, South Wales. Proceedings of the Geologists’ Association, 50, 433-462.
- Griffiths, J.C. & Stuart, A., 1940. An occurrence of detrital diaspore in South Wales. Geological Magazine, 77, 74-76.
- 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 Unpublished Ph.D. thesis, University of Wales, 415pp.