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Cymraeg

Xenotime-(Y)

Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
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

Chemical Group:

Geological Context:

Cream-coloured prismatic xenotime-(Y) associated with bipyramidal anatase, Cwmorthin Quarry, Blaenau Ffestiniog, Gwynedd. Photo D.I. Green, © National Museum of Wales.
Scanning electron micrograph of sub-millimetre euhedral 'cleopatra's-needle'-shaped xenotime-(Y) from Cwmorthin Quarry near Blaenau Ffestiniog, Gwynedd. © National Museum of Wales.
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in igneous rocks is in Ordovician rhyolitic tuffs on Ramsey Island off the W Pembrokeshire coast (R.E. Bevins, unpublished data).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.

Key Localities:

References:

  1. Bennett, M.A., 1987a. The Cambrian manganese deposits of North Wales. Unpublished Ph.D. thesis, University of Leeds.
  2. 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.
  3. 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.