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).
- Metamorphic: low-grade
- Hydrothermal: alpine type veins
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.
- Afon Wen, Gwynedd: The Afon Wen Intrusive Complex, Gwynedd contains, pinkish twinned crystals up to 3 mm in size associated with chlorite and epidote in small cavities in altered microtonalite (J.S. Mason, unpublished data).
- Brynyrafr Mine, Ponterwyd, Ceredigion: an unusual occurrence of albite, with apatite, rutile and quartz, is found in small (<10 mm long), open cavities in sandstone and mudstone clasts within a quartz-cemented breccia (British Micromount Society Newsletter 35). The albite is clearly post-brecciation and a mechanism to explain their formation has yet to be derived. The assemblage occurs as euhedral crystals (<1 mm), and has also been found, in a less well-developed form, at Castell, Darren, Erglodd, Ystrad Einion and Bwlchysgellyn mines, and although widespread, is restricted paragenetically to the early or 'A1' assemblages (J.S. Mason, unpublished data).
- Cae Mawr Mine, Gwynedd: the tips from this small mine contain common albite, as small pink veinlets in altered microtonalite, associated with quartz, chlorite and epidote.
- Coed Llyn-y-Garnedd, near Maentwrog, Gwynedd: albite is present as small pinkish-buff crystals in Alpine fissure-type veins exposed along forest roads, cutting both sedimentary and igneous rocks (including part of the Tanygrisiau Microgranite). It is associated with anatase, clinochlore and quartz, and was discovered during the Gwynedd phase of the Minescan project in 1997 (Bevins & Mason, 1998).
- Cwmorthin Quarry, Blaenau Ffestiniog, Gwynedd: albite forms small tabular crystals associated with quartz on open vein-fractures cutting turbidite-tuffs, discovered during Minescan follow-up work, 2001 (NMW/Manchester Museum, unpublished data).
- Gloddfa Ganol Quarry, Blaenau Ffestiniog, Gwynedd: albite is common as complex crystals up to 2 mm in size in an Alpine fissure-type vein association (anatase, rutile, synchysite, clinochlore, apatite) in a brecciated slate matrix. This was discovered during Minescan follow-up work, 2001 (NMW/Manchester Museum unpublished data).
- Hendre Quarry, Glyn Ceiriog, Clwyd: small (<2 mm) colourless to white albite crystals have been described from an Alpine fissure-type assemblage on joints cutting dolerite (Starkey et al., 1991).
- Manod Quarry, Blaenau Ffestiniog, Gwynedd: tabular, colourless to white albite crystals, up to 10 mm in size, form extensive coatings on fractured quartz-latite, associated with quartz, anatase, brookite, apatite, synchysite and clinochlore. Some fine specimens have been recovered from this locality.
- Minffordd Quarry, near Penrhyndeudraeth, Gwynedd: pink complexly-twinned crystals have been collected from Alpine-type veins at this (still working) quarry. Specimens in the National Museum of Wales Collection (e.g. NMW 27.111.GR.374) were collected prior to 1927 by G.J. Williams but a more recent (1995) search failed to locate any further material (J.S. Mason, unpublished data).
- Prenteg, Tremadog, Gwynedd: this locality is famous for its splendid brookite crystals, but the outcrop at Prenteg formerly produced rich hand specimens of albite composed of aggregates of 5-10 mm, white, tabular crystals forming extensive areas on dolerite (Starkey & Robinson, 1992). The locality is now protected and not accessible to mineral collectors.
- Tanygrisiau Station, Blaenau Ffestiniog, Gwynedd: albite is common at this locality as extensive coatings of small (<3 mm) twinned tabular crystals (Green & Middleton, 1996).
- 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.
- Bevins, R.E. & Rowbotham, G., 1983. Low-grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal, 18, 141-167
- 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.
- 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.
- Starkey, R.E. & Robinson, G.W., 1992. Famous mineral localities, Prenteg, Tremadog, Gwynedd, Wales. Mineralogical Record, 23, 391-399.
- 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.