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Cymraeg

Albite

Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Sodium aluminium silicate
Chemical Formula: NaAlSi3O8
Method(s) of Verification: Tanygrisiau - XRD (Manchester Museum).

Chemical Group:

Geological Context:

Backscatter-mode SEM image of a complex albite crystal with anatase (pale grey, top L) from an alpine-type vein association at Gloddfa Ganol Quarry, Blaenau Ffestiniog. © National Museum of Wales.
Introduction: albite is the sodic end-member of the plagioclase feldspar series. It is present commonly in acid intrusive and eruptive igneous rocks such as granites and rhyolites, and is also a characteristic mineral in low-grade metamorphic rocks particularly those derived from basic igneous precursors. However, to the collector, the most important occurrences are the well-formed crystals that are commonly found in cavities in Alpine fissure-type vein associations. Here, the association with anatase, brookite, rutile, apatite and clinochlore is typical.
Occurrence in Wales: albite is present in many areas of Wales as an important rock-forming mineral of both primary and secondary origin. Primary occurrences are to be found in many of the acidic igneous rocks of Wales: examples include the St. David's Granophyre in Pembrokeshire (Bloxam & Dirk, 1988). Secondary occurrences, in which albite has replaced original calcic plagioclase, are frequently observed in low-grade metabasites (Bevins & Rowbotham, 1983), in such cases electron microprobe analyses have shown the albite to be virtually pure. In specimen terms, there are a number of important occurrences in Alpine fissure-type veins at localities scattered across North Wales, perhaps the best known being from Prenteg, near Tremadog (Starkey & Robinson, 1992). These specimen localities are listed below, along with an unusual occurrence from the Central Wales Orefield.

Key Localities:

There are no key localities for this specimen.

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. Bevins, R.E. & Rowbotham, G., 1983. Low-grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal, 18, 141-167
  3. Bloxam, T.W. & Dirk, M.H.J., 1988. The petrology and geochemistry of the St. David's granophyre and the Cwm Bach rhyolite, Pembrokeshire, Dyfed. Mineralogical Magazine, 52, 563-575.
  4. 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.
  5. Starkey, R.E. & Robinson, G.W., 1992. Famous mineral localities, Prenteg, Tremadog, Gwynedd, Wales. Mineralogical Record, 23, 391-399.
  6. 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.

There are no references for this specimen.