The metamorphism of amygdales at 'S Airde Beinn, northern Mull

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Abstract
Summary: Amygdales found metamorphosed in the aureole of the volcanic plug at 'S Airde Beinn, northern Mull, may be divided into three classes by their behaviour on metamorphism. The first class, characterized by the original dominant presence of zeolites, forms feldspar readily. It is related to a type, composed to a greater degree or entirely of ferromagnesian minerals, which seems to have formed by the infilling of originally void amygdales during the recrystallisation of the basalts. The second class, originally containing dominant gyrolite, forms first reyerite and then wollastonite on metamorphism, the wollastonite-filled amygdales being enveloped in a rim of aegirine-augite from the earliest stages owing to reaction between the wollastonite and the basalt. The third class originally contained calcite, now represented by the anhydrous calcium silicates larnite, rankinite and wollastonite. These minerals form concentric monomineralic zones decreasing in Ca/Si ratio outwards. Later reaction has led to the replacement of both basalt and calcium silicates by melilite. Thin aegirine-augite rims surround the amygdales.

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