Anatase

Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Titanium dioxide
Chemical Formula: TiO2
Method(s) of Verification: all of the following by XRD: Tremadoc - Natural History Museum, London (ref. x9286); Hendre Quarry - NHM (7021F) & EMPA (National Museum of Wales, unpublished data); Grib Goch ridge - NHM (x13477); Cwm Meillionen, Beddgelert - NHM (x15630 & x15631).

Geological Context:

  • Igneous
  • Hydrothermal: alpine type veins
Inky-blue tabular anatase crystal, Tanygrisiau, Blaenau Ffestiniog, Gwynedd. Photo D.I. Green, © National Museum of Wales.
Black, bipyramidal anatase crystals with creamy prismatic xenotime, Cwmorthin Quarry, Blaenau Ffestiniog, Gwynedd. Photo D.I. Green, © National Museum of Wales.
Reddish-brown bipyramidal crystals of anatase (up to 1.5 mm in size) from Hendre Quarry, Glyn Ceiriog. N. Hubbard Collection (NHMM). Photo M.P. Cooper, © M.P. Cooper.
SEM backscatter image of bipyramidal anatase crystal (0.5 mm) on quartz from an alpine-type vein, Cwmorthin Quarry, Blaenau Ffestiniog. Specimen: National Museum of Wales, © National Museum of Wales.
Introduction: anatase typically occurs in alpine fissure-type veins with the two other TiO2 polymorphs, brookite and rutile plus albite, quartz and a range of other minerals. It is a frequent accessory mineral in igneous rocks, particularly those which have undergone hydrothermal alteration. Anatase is readily identified visually when well-crystallized by its morphology, colour and paragenetic association.
Occurrence in Wales: anatase is a widespread accessory mineral in intrusive igneous rocks throughout Wales. For example, fine-grained orange anatase occurs in association with disseminated copper sulphides in highly altered diorite in the Coed-y-Brenin porphyry-copper deposit near Dolgellau. However, in specimen terms, it is best known from the alpine fissure-type veins of North Wales. In such assemblages, it occurs variably associated with albite, brookite, rutile, clinochlore, apatite, synchysite, monazite, xenotime and quartz. Crystals tend to be small (up to 5 mm) but euhedral; the colour varies from reddish-brown through black to sapphire-blue. Two habits occur: at some localities it forms sharp bipyramids while at others it forms single or multiply-twinned tabular crystals.

Key Localities:

  • Coed Llyn y Garnedd, near Maentwrog, Gwynedd: deep-blue tabular crystals, up to 2 mm across, are associated with quartz, albite, and clinochlore in exposures along forestry roads. These were discovered during the Gwynedd phase of the Minescan project in 1997 (Bevins & Mason, 1998).
  • Cwmorthin Quarry, Blaenau Ffestiniog, Gwynedd: black bipyramids (up to 1 mm across) associated with quartz, albite, brookite, rutile, synchysite and xenotime, were discovered during the Minescan follow-up work in 2001 (National Museum of Wales/Manchester Museum, unpublished data).
  • Gloddfa Ganol Quarry, near Blaenau Ffestiniog, Gwynedd: blue tabular crystals up to 0.5 mm in size are associated with quartz, clinochlore, albite, apatite and synchysite. These were discovered during Minescan follow-up work in 2001 (NMW/Manchester Museum, unpublished data).
  • Hendre Quarry, Glyn Ceiriog, Clwyd: reddish-brown bipyramids, up to 1.5 mm in size, are associated with quartz, albite, apatite, brookite, xenotime and monazite (Starkey et al., 1991).
  • Manod Quarry, Blaenau Ffestiniog, Gwynedd: blue twinned tabular crystals occur at this locality in stacked groups up to 5 mm in size. They are associated with quartz, clinochlore, albite, apatite, brookite, rutile and synchysite. A well-known locality since the mid-20th Century.
  • Moel Ysgyfarnogod, Eisingrug, Gwynedd: dark bipyramidal crystals in quartz are known from this locality (P. Haas Collection).
  • Prenteg, Tremadog, Gwynedd: at this, the classic Welsh alpine-mineral site, anatase forms black to brown bipyramids, often several mm in length, associated with quartz, albite, apatite, clinochlore and brookite. Exceptionally crystals to 5.5 mm long have been collected at this locality, which has been known about since the early 19th Century, but which is now relatively inaccessible (Starkey & Robinson, 1992).
  • Tanygrisiau Station, Blaenau Ffestiniog, Gwynedd: blue tabular crystals, exceptionally reaching 3 mm, are associated with quartz, clinochlore, albite, apatite and brookite (Green & Middleton, 1996).

References:

  1. Bevins, R.E. & Mason, J.S., 1998. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Gwynedd. National Museums of Wales, Cardiff.
  2. Green, D.I. & Middleton, D., 1996. Alpine-type vein minerals from Tanygrisiau, Gwynedd. U.K. U.K. Journal of Mines and Minerals, 16, 30-33.
  3. Starkey, R.E. & Robinson, G.W., 1992. Famous mineral localities, Prenteg, Tremadog, Gwynedd, Wales. Mineralogical Record, 23, 391-399.
  4. 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.