Barroisite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Sodium calcium magnesium iron aluminium silicate hydroxide
Chemical Formula: NaCa(Mg,Fe2+)3Al2(Si7Al)O22(OH)2
Method(s) of Verification: electron microprobe analyses (Horák & Gibbons, 1986; Gibbons & Gyopari, 1986).

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: low-grade
Photomicrograph of zoned amphibole (0.45 mm across) showing; inner pale core of actinolite; middle zone of barroisite; and outer rim of glaucophane. Llanfaipwllgwyngyll, Anglesey. Image copyright: Dr W. Gibbons.
Introduction: barroisite is a sodic-calcic amphibole, and the magnesium-rich member of the barroisite-ferrobarroisite series. It typically occurs in metamorphosed basic igneous rocks.
Occurrence in Wales: barroisite is recorded from a belt of poorly exposed blueschist in SE Anglesey, North Wales (Horák & Gibbons, 1986; Gibbons & Gyopari, 1986). Although Blake (1888) was the first to identify blue amphibole from Anglesey, (see entries for ferroglaucophane/glaucophane) he made no mention of the green core to these crystals. Adye (1906) illustrated these in The Twentieth Century Atlas of Microscopical Petrography, and it was left to Greenly (1919) to formerly described them as green 'hornblende'. Microprobe analyses enabled Horák & Gibbons (1986) to classify the green rims as barroisite and Gibbons & Gyopari (1986), with the aid of further analyses, extended this to the range barroisite-ferrobarroisite-winchite. The presence of green amphibole in the core to blue amphibole is unusual, as the reverse relationship is more common. Gibbons & Gyopari (1986) interpreted the green amphibole to have formed by ocean floor metamorphism prior to blueschist metamorphism. Although barroisite in blueschist is likely to occur widely throughout the Blueschist Belt (or Aethwy Zone) in southern Anglesey, details have only been published from two localities near Llanfairpwllgwyngyll.

Key Localities:

  • Llanfairpwllgwyngyll, Anglesey: sections along the A5 at Llanfairpwllgwyngyll cut through blueschist facies metabasites. The best examples were exposures during the construction of the road and showed three stages of amphibole growth from early greenschist actinolite, through a transition of dark green barroisite to rims of blue amphibole. Gyopari (1984) and Gibbons & Gyopari (1986) provided analyses of this occurrence. Crags below the Marquis of Anglesey’s Column at Llanfairpwllgwyngyll (also described under the glaucophane/ferroglaucophane entry), are composed of fine-grained, amphibole-epidote schist. In thin section the euhedral, blue amphibole is seen to overgrow euhedral or anhedral dark green amphibole cores, typically less than 0.1 mm in length. The Marquis of Anglesey’s Column area is a protected site and no collecting or access with hammers is permitted. Those wishing to view or borrow specimens should contact the National Museum of Wales at http://www.nmgw.ac.uk/geology/ask.en.shtml.

There are no key localities for this specimen.

References:

  1. Adye, E.H., 1906. The Twentieth Century Atlas of Microscopical Petrography. Murby (London).
  2. Blake, J.F., 1888. The occurrence of glaucophane-bearing rocks in Anglesey. Geological Magazine, 5, 125-127.
  3. Gibbons, W. & Gyopari, M., 1986. 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.
  4. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  5. Gyopari, M., 1984. A study blueschist mineral chemistry and a new look at the Penmynydd-Gwna Boundary in SE Anglesey. Unpublished M.Phil thesis, University of Wales, 123pp.
  6. Horak, J.M. & Gibbons, W., 1986. Reclassification of blueschist amphiboles from Anglesey, North Wales. Mineralogical Magazine, 50, 532-535.

There are no references for this specimen.