Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Calcium titanium silicate
Chemical Formula: CaTiSiO5
Method(s) of Verification: all localities cited except Fron Oleu - polarizing microscope; Anglesey - EMPA (Horák, 1993)
- Hydrothermal: alpine type veins
Scanning electron micrograph image of a detached titanite crystal from Benallt Mine. Image T.F. Cotterell, © National Museum of Wales.
Introduction: titanite, the International Mineralogical Association (IMA) approved name for the mineral also known as sphene, is found as an accessory mineral in acidic igneous rocks, it also develops in contact and low-grade regional metamorphic rocks and is a common alteration product of ilmenite.
Occurrence in Wales: titanite is widely distributed across Wales as a low-grade metamorphic mineral in rocks of basaltic composition. It typically forms anhedral dusty granules up to 0.1 mmacross associated with chlorite in the groundmass of the rocks, or pseudomorphing primary titanium-oxides. Electron microprobe analyse show a characteristic substitution of aluminium and iron for titanium, as reported elsewhere in titanite occurring in low-grade metamorphic rocks (Bevins & Rowbotham, 1983; Bevins,& Merriman, 1988).
- Anglesey: titanite also occurs as a secondary phase within basic gneisses (amphibolites), in central Anglesey, forming large rhombic crystals and less distinct crystal aggregates after ilmenite (Horák, 1993).
- Prenteg, Tremadog, Gwynedd: at Fron Oleu Starkey & Robinson (1992) recorded titanite in alpine-type veins generated by hydrothermal activity.
- Snowdonia, Gwynedd: titanite is present as an accessory phase in various rhyolitic rocks in Snowdonia, Gwynedd (Campbell et al., 1987; Howells et al., 1991) where it forms euhedral crystals up to 200 μm across, with some crystals showing well-developed sector zoining, with tin-rich cores with up to 3.5 wt% SnO2 (A.T. Kearsley, unpublished data, quoted in Bevins, 1994).
- Bevins, R.E. & Rowbotham, G., 1983. Low-grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal, 18, 141-167
- Bevins,R.E. & Merriman, R. J., 1988. Compositional controls on co-existing prehnite-actinolite and prehnite-pumpellyite assemblages in the Tal y Fan metabasite intrusion, North Wales: implications for Caledonian metamorphism field gradients. Journal of Metamorphic Geology, 6, 17-39.
- Campbell, S.D.G., Reedman, A.J., Howells, M.F. & Mann, A.C., 1987. The emplacement of geochemically distinct groups of rhyolities during the evolution of the Lower Rhyolitic Tuff Formation caldera (Ordovician), North Wales, U.K. Geological Magazine, 124, 501-511.
- 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.
- Starkey, R.E. & Robinson, G.W., 1992. Famous mineral localities, Prenteg, Tremadog, Gwynedd, Wales. Mineralogical Record, 23, 391-399.