Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Locally Abundant
Chemical Composition: Sodium magnesium iron aluminium silicate hydroxide
Chemical Formula: Na2(Mg,Fe2+)3Al2Si8O22(OH)2
Method(s) of Verification: Marquis of Anglesey’s Column - optical (Blake, 1888), EMPA (Horák & Gibbons, 1986; Gibbons & Gyopari, 1986).
Introduction: glaucophane is a sodic amphibole; it does not occur in its colourless pure state but forms a series with ferroglaucophane by the addition of iron (Fe2+). It should be noted that many analyses previously classified as crossite (abandoned term) are now identified as glaucophane/ferroglaucophane. Glaucophane occurs in blueschists, metamorphic rocks which are commonly thought to be diagnostic of former subduction zone settings, because they imply relatively high pressure conditions relative to the temperature (compared to normal geothermal gradients). Glaucophane/ferroglaucophane provide the blue colour characteristic of blueschists and are typically found with a combination of the following minerals: lawsonite, chlorite, garnet, albite, zoisite, phengite and paragonite mica.
Occurrence in Wales: Blake (1888) provided the first description of glaucophane from the British Isles, reporting its occurrence in rocks of his ‘Monian System’ [now known as the Aethwy Terrane or Blueschist Belt (Gibbons & Horák, 1990)] from near Llanfairpwllgwyngyll, Anglesey. Blake’s identification was confirmed by Greenly (1919) who also noted zoning in the amphiboles with green ‘hornblende’ core passing outwards into blue rims. Holgate (1951) provided a chemical analysis on separated rims, but suggested that as the blue amphibole contained a higher Fe3+ to Aliv content, it was crossite and not glaucophane. Macpherson (1983) recalculated Holgate’s analysis and, applying the new IMA amphibole classification, deemed them to be magnesio-arfvedsonites. New microprobe data (Horák & Gibbons, 1986; Gibbons & Gyopari, 1986) showed that the rims fall within the glaucophane-ferroglaucophane series. The Anglesey blueschists occur as poorly-exposed scattered outcrops forming a NW-SW oriented belt in the southern part of the island. Although blue amphibole can be found throughout this zone in rocks of suitable composition, only two areas near Llanfairpwllgwyngyll have been described in any detail (Blake, 1888; Greenly, 1919; Horák & Gibbons, 1996; Gibbons & Gyopari, 1986). They have particular significance in that they are Precambrian in age (Dallmeyer & Gibbons, 1987) and represent some of the oldest blueschists in the World, having formed during subduction on the Proterozoic margin of the supercontinent Gondwanaland (Gibbons & Horák, 1996).
- Llanfairpwllgwyngyll, Anglesey: crags below the Marquis of Anglesey’s Column are composed of fine-grained, dark-grey schist. These amphibole-epidote schists contain minor quartz, chlorite, titanite hematite and magnetite, but most of these minerals are not clearly discernible in hand specimen. In thin section the blue amphibole forms a mass of beautiful, euhedral, lavender-blue crystals, typically less than 1 m in size, which has overgrow euhedral or anhedral green amphibole cores (see barroisite-ferrobarroiste-winchite entries). The Marquis of Anglesey’s Column area is a protected site and no collecting or access with hammers is permitted. Those wishing to study or view specimens should contact the National Museums & Galleries of Wales (http://www.nmgw.ac.uk/geology/ask.en.shtml). Road cuttings on the A5 at Llanfairpwllgwyngyll provide exposures of blueschist, many showing three stages of amphibole development (actinolite core rimmed by barroisite and in turn rimmed by glaucophane/ferroglaucophane), were described by Gyopari (1984) and Gibbons & Gyopari (1986).
- The occurrence of glaucophane-bearing rocks in Anglesey. Geological Magazine, 5, 125-127.
- The age of blueschist metamorphism in Anglesey, North Wales: evidence from 40Ar/39Ar mineral dates of the Penmynydd schists. Journal of the Geological Society, London, 144, 843-850.
- A greenschist protolith for blueschist on Anglesey, U.K. In: Evans, B.W. & Brown, E.H. (eds), Blueschist and Eclogites. Geological Society of American Memoir, 164, 217-228.
- Contrasting metamorphic terranes in NW Wales. In D'Lemos, R.D., Strachan, R.A., Topley, C.G. and Beckinsale, R.D. eds, The Cadomian Orogeny, Geological Society of London, Special Publication No. 51, 315-327.
- The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
- On crossite from Anglesey. Mineralogical Magazine, 29, 792-798.
- Reclassification of blueschist amphiboles from Anglesey, North Wales. Mineralogical Magazine, 50, 532-535.
- References for, and updating of L.J. Spenser’s 1st and 2nd supplimentary list of British Minerals. Mineralogical Magazine, 47, 243-257.
- A FORTRAN program for the classification of amphiboles according to IMA (1978). IGS Petrographic Report 560124 (unpublished).