The kyanite isograd in Glen Clova, Angus, Scotland

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Abstract
Summary: Mineral data from Glen Clova, Angus, suggest that the development of kyanite in graphite-bearing staurolite mica-schists, treated in terms of a Ca-free system, involved decrease of staurolite and a decrease in biotite MgO:FeO ratio. A kyanite isograd may be defined in Glen Clova as the locality where kyanite joins the assemblage staurolite-garnet-biotite-muscovite-plagioclase-quartz-graphite, the biotite having mol. , although the persistence of this identical assemblage over some 600 metres introduces some uncertainty of interpretation. It is suggested that the reaction staurolite+quartz → garnet+kyanite is unlikely to be a common reaction in nature and that its use to subdivide the amphibolite facies is unjustified.