Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Aluminium silicate
Chemical Formula: Al2SiO5
Method(s) of Verification: all occurrences recorded - optical identification.
- Sedimentary: allogenic (detrital)
Introduction: andalusite is a metamorphic mineral with two polymorphs, sillimanite and kyanite. It develops under conditions of lower pressure than the other two polymorphs, and is typical of contact metamorphism of aluminium-rich rocks.
Occurrence in Wales: the presence of andalusite has been recorded from the ‘contact aureole’ of the Coedana Granite by Greenly (1919), from the contact aureole of the Tan-y-Grisiau Granite by Bromley (1969) and from that of the St. David’s Head Intrusion by Roach (1969). However in all these instances only pseudomorphs remain (usually made up of white mica). Fresh andalusite is only known from Wales as a detrital mineral in sedimentary rocks. Thomas (in Greenly, 1919) summarizes the occurrence of andalusite in glacial sands and tills as being present from Bagillt, Clwyd to Pembrokeshire where it is characteristically associated with augite, ‘green hornblende’ and pink garnet. Thomas (1909) provides details of andalusite from several localities in Pembrokeshire and Billinghurst (1929) from glacial deposits in north-east Wales. The significance of andalusite in these deposits is that it is considered an ‘exotic mineral’ derived from the Irish Sea drift, with an original source much further afield. More specifically Griffiths (1939) used the presence of a suite of detrital minerals, including andalusite, to indicate the former advance of the Irish Sea ice into South Wales.
There are no key localities for this specimen.
- Billinghurst, B.B., 1929. Mineral analyses of some Ordovician Rocks from Carnarvonshire. Geological Magazine, 66, 289-301.
- 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.
- 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.
- 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.
- Thomas, H.H., 1909b. Detrital andalusite in Tertiary and post-Tertiary sands. Mineralogical Magazine, 15, 241-244.