Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Yttrium phosphate
Chemical Formula: YPO4
Method(s) of Verification: Hendre Quarry - XRD (Manchester Museum); Cwmorthin Quarry - XRD (Manchester Museum); all other igneous occurrences were identified by electron microscope
- Hydrothermal: alpine type veins
- Sedimentary: diagenetic
Introduction: xenotime commonly contains a range of rare-earth elements (REE).It is found typically in acidic igneous rocks, such as granites and rhyolites and acidic pegmatites, where it is present as an accessory mineral. It is also found in alpine fissure-type veins, especially (but not exclusively) those which are hosted by acidic rocks. Associated minerals are diverse, but include commonly other REE-bearing species.
Occurrence in Wales: the first possible report of xenotime from Wales was by Bennet (1987) who recorded a yttrium-rich phosphate mineral in pyritic mudstones belonging to the Hafotty Formation (Cambrian) of the Harlech Horst area of Gwynedd. Another reported sedimentary occurrence was in association with diagenetic microconcretions in Ordovician mudstones from the Aberangell area of NW Powys (as in National Museum of Wales specimens NMW 85.13G.M1-M3). Occurrences in altered rhyolites in Snowdonia were described by Howells et al (1991), while a further minor occurrence in igneous rocks is in Ordovician rhyolitic tuffs on Ramsey Island off the W Pembrokeshire coast (R.E. Bevins, unpublished data). In specific specimen locality terms, xenotime is rarely found within Alpine Fissure-type veins in North Wales, with two widely-spaced localities currently known. In both cases, however, the crystals are microscopic. With a good binocular microscope, the distinctive crystal habit of xenotime aids identification, but chemical analysis is required in order to establish which variety is present.
- Cwmorthin Quarry, Blaenau Ffestiniog, Gwynedd: xenotime is present as a minor phase associated with anatase and synchysite-(Ce) on quartz in uncommon thin alpine fissure-type veins (National Museum of Wales/Manchester Museum, unpublished data). These are hosted by rhyolitic tuff-turbidites which form distinctive grey-white banded horizons within the slaty mudstones of Lower Ordovician age worked at the quarry. Crystals reach 0.5 mm in size and may be aggregated together in rather chaotic bunches: they are transparent and golden-buff in colour. Fully quantitative analyses have yet to be obtained, but preliminary semiquantitative work at the National Museum of Wales has given consistent indications of the significant presence of the heavy REE Gd, Dy, Er and Yb (and particularly Dy). Work is continuing in order to fully determine this unusual chemical composition.
- Hendre Quarry, Glyn Ceiriog, Clwyd: Starkey et al. (1991) record sharp, golden prismatic crystals of xenotime rarely occurring on albite and associated with anatase from alpine fissure-type veins. A specimen held by the National Museum of Wales (NMW 87.36G.M.1), donated by N. Hubbard, is a typical example.
- Bennett, M.A., 1987a. The Cambrian manganese deposits of North Wales. Unpublished Ph.D. thesis, University of Leeds.
- 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., Hubbard, N. & Bayley, M.P., 1991. Mineralization at Hendre Quarry, Glyn Ceiriog, Clwyd, Wales. U.K. Journal of Mines and Minerals, No. 10, 48-51.