Ferrobarroisite

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

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic
Introduction: ferrobarroisite is an member of the amphibole group, and is more specifically a sodic-calcic amphibole, and the iron (Fe2+)-rich member of the barroisite-ferrobarroisite series. It occurs typically in metamorphosed basic igneous rocks.
Occurrence in Wales: ferrobarroisite 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 describe 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 ferrobarroisite 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: crags below the Marquis of Anglesey’s Column (The Monument), 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 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 (http://www.nmgw.ac.uk/geology/ask.en.shtml).

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. Horak, J.M. & Gibbons, W., 1986. Reclassification of blueschist amphiboles from Anglesey, North Wales. Mineralogical Magazine, 50, 532-535.