Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium yttrium rare earths iron aluminium silicate hydroxide
Chemical Formula: (Ce,Ca,Y)2(Al,Fe3+)3(SiO4)3(OH)
Method(s) of Verification: Ffestiniog Granite Quarry - chemical analysis (Bromley, 1964); all other occurrences by optical or visual means.
Allanite crystals (up to 3 cm long) in alteration veins in the Ffestiniog Granite, Blaenau Ffestiniog. Specimen National Museum of Wales (NMW 27.111.GR.435), ex G.J. Williams collection, © National Museum of Wales.
Introduction: allanite-(Ce) (formerly known as orthite) is a widespread accessory mineral in igneous rocks of both acidic and basic compositions. It is also found in aplite and pegmatite veins. It is a member of the epidote mineral group, distinguished from epidote and clinozoisite by its relatively high rare earths content which may result in it being weakly radioactive.
Occurrence in Wales: the first account of allanite from Wales was that by Thomas (1909) and Fearnsides (1910), concerning the discovery, in the Ffestiniog Granite Quarry, of an 'unknown and very interesting mineral' in 1908, which was identified a year later as allanite. Subsequently, it was found to occur in the Cregennen granophyre, an intrusion outcropping to the south-west of Dolgellau, as an accessory mineral (Cox & Wells, 1920). Williams (1927) described further probable occurrences from dolerites in Central Snowdonia and further details of occurrences in rhyolites, intrusives and pyroclastic rocks from the same area were given by Howells et al. (1991) and in the Tal y Fan intrusion by Merriman et al. (1986). An occurrence within 'hornblende gneisses' on Anglesey was mentioned by Greenly (1919) without reference to specific localities: however, Horak (1993) confirmed its occurrence as inclusions in amphiboles and as an accessory groundmass mineral from Y Werthyr and Craig yr Allor.
- Ffestiniog (Tanygrisiau) Granite Quarry, Cefn Bychan, Blaenau Ffestiniog, Gwynedd: allanite is present in two modes of occurrence at this locality (Bromley, 1964). Firstly it occurs as an accessory mineral in the groundmass of the microgranite where it forms small (1 mm) euhedral crystals associated with chlorite and interstitial m
- Bromley, A.V., 1964. Allanite in the Tan-y-Grisiau Microgranite, Merionethshire, North Wales. American Mineralogist, 49, 1747-1752.
- Cox, A.H. & Wells, A.K., 1920. The Lower Palaeozoic rocks of the Arthog-Dolgelley district. Quarterly Journal of the Geological Society of London, 76, 254-324.
- Fearnsides, W.G., 1910. Excursion to North Wales. Proceedings of the Geologists' Association, 21, 368-390.
- Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
- 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.
- Howells, M.F., Reedman, A.J. & Campbell, S.D.G., 1991. Ordovician (Caradoc) marginal basin volcanism in Snowdonia (north-west Wales). HMSO for the British Geological Survey, 191pp.
- 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.
- Thomas, H.H., 1909a. Orthite in North Wales. Nature, 81, 487.
- Williams, H., 1927. The geology of Snowdon (North Wales). Quarterly Journal of the Geological Society of London, 83, 346-431.