The Mode of Occurrence of Sillimanite in the Glen District, Co. Donegal

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Abstract
Sillimanite produced by the contact action of the Donegal Granite shows varied textural and time relationships with the other minerals of the schists in which it occurs. Early Sillimanite was formed in the main stage of metamorphism by destruction of biotite, K, Fe, and Mg being expelled in the process. A later influx of new K from the granite led to the growth of muscovite metacrysts. This was associated with destruction of part of the early Sillimanite, recrystallization of part, growth of a new crop of Sillimanite needles, and formation of muscovite-sillimanite intergrowths.