The variable composition of cordierite in the Dartmoor granites

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Abstract
The recognition of pronounced varietalism in both the garnet and the basic mica of the Dartmoor granites prompted the authors to inquire whether the cordierite associated with these species was similarly variable in composition.The inquiry was directed in the first instance to possible variation in the optic axial angle 2V, which is determinable with accuracy on the unaltered parts of cordierite in thin rock sections. Determinations made on the sector-twinned cordierites in the garnetiferous granite of Sweltor a gave the following peculiar results, which Mr. Rama Rao verified repeatedly, using a Fedorov microscope in perfect adjustment.

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