Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Locally Abundant
Chemical Composition: sodium calcium iron magnesium aluminium silicate hydroxide
Chemical Formula: •(NaCa)Mg4(AlFe3+)Si8O22(OH)2
Method(s) of Verification: Llanfairpwllgwyngyll - EMPA (Gyopari, 1984).
Introduction: winchite is a member of the sodic-calcic amphibole group and forms a series with ferrowinchite (•(NaCa)Fe2+4(AlFe3+)Si8O22(OH)2). It is a metamorphic rock-forming mineral, and has been recorded from blueschists, greenschists and skarns. It is commonest in blueschist facies rocks where it may form an intermediary phase between barroisite and glaucophane. Winchite has also been described from carbonatites (unusual igneous rocks containing more than 50% carbonate minerals).
Occurrence in Wales: winchite is recorded from just one locality in Wales, from belt of poorly exposed blueschist in southeast Anglesey (Horák & Gibbons, Gibbons & Gyopari, 1986). Adye (1906) illustrated green cores to the blue amphiboles 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 Gyopari (1984), with the aid of further analyses, extended this to the range barroisite-ferrobaroisite-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 at conditions intermediate between greenschist and blueschist facies metamorphism.
Although winchite in blueschist is likely to occur widely throughout the Blueschist Belt (or Aethwy Zone) in southern Anglesey, this mineral has only been identified from a locality near Llanfairpwllgwyngyll.
- 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 assemblage a transition of dark green barroisite-ferrobaroisite-winchite to rims of blue amphibole. Gyopari (1984) provided an analysis of this occurrence.
- Adye, E.H., 1906. The Twentieth Century Atlas of Microscopical Petrography. Murby (London).
- 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.
- Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
- Gyopari, M., 1984. A study of blueschist mineral chemistry and a new look at the Penmynydd-Gwna Boundaey in SE Anglesey. Unpublished M.Phil thesis, University of Wales, 123pp.
- Horak, J.M. & Gibbons, W., 1986. Reclassification of blueschist amphiboles from Anglesey, North Wales. Mineralogical Magazine, 50, 532-535.
- Leake, B.E., et al., 1997. Nomenclature of amphiboles: report of the subcommittee on amphiboles of the international mineralogical association, commission on new minerals and mineral names. The Canadian Mineralogist, 35, 219-246.