Thorite

Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Thorium uranium silicate
Chemical Formula: (Th,U)SiO4
Method(s) of Verification: all listed occurrences - EMPA

Chemical Group:

  • Silicates

Geological Context:

  • Igneous
  • Sedimentary: diagenetic
Introduction: thorite is dimorphous with huttonite (that is they have the same chemical composition but different arrangements of atoms in the crystal lattice). It occurs as a primary accessory mineral in felsic igneous rocks and granitic pegmatites and may subsequently be preserved as a detrital mineral in sediments. It is often associated with rare-earth element minerals such as zircon, monazite, gadolinite and fergusonite and niobium minerals such as yttrialite and pyrochlore or uraninite.
Occurrence in Wales: thorite has two modes of occurrence in Wales. In igneous rocks it forms either an accessory mineral (see Ramsey Island entry) or an alteration product of other primary accessory minerals, such as that described by Howells et al., (1991) from rhyolitic rocks in Snowdonia. The other documented occurrences of thorite are diagenetic in origin, having formed during the early stages of consolidation and burial of the sediment in which they are now found (Parnell & Eakin, 1989; A.T. Kearsley, unpublished data).

Key Localities:

  • Aberangell area, Gwynedd: thorite has been recognised forming overgrowths on xenotime crystals in Ordovician mudstones (A.T. Kearsley, unpublished data).
  • Presteigne, Powys: thorite has also been located as minute inclusions, less than 2 μm across associated with thorianite within spherical bitumen nodules in the Folly Sandstone, of Llandovery age, exposed in the Presteigne area of Powys (Parnell & Eakin, 1989). The nodules formed within the sediment although the source of the thorium is uncertain, a possible candidate being monazite nodules within the Ordovician and Silurian strata of the Welsh Basin. Although the thorium-bearing nodules are minute Parnell & Eakin (1989) calculate that over a two kilometre area the host sandstone may contain up to 500 tonnes of thorium.
  • Ramsey Island, Pembrokeshire: thorite has identified by electron microscope (in both back-scatter and energy-dispersive modes) as narrow overgrowths on primary zircons in a rhyolitic ash-flow tuff of Ordovician age (R.E. Bevins, unpublished data).

References:

  1. 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.
  2. Parnell, J. & Eakin, P., 1989. Thorium-bitumen mineralization in Silurian sandstones, Welsh Borderland. Mineralogical Magazine, 53, 111-116.