Na-biotite in Dalradian pelitic schists, Angus

Synopsis Na-biotites have been found within pelitic schists in lower amphibolite facies Dalradian metasediments. These micas are optically very similar to normal K-biotites which they partially replace, although the Na-biotites do contain a more prominent 001 cleavage. The formation of the Na-biotites is linked to the local infiltration of a Na-rich fluid during retrogression and was dependent on late deformation opening fluid pathways within existing biotite porphyroblasts. Micas of such unusual compositions (up to 3 wt% Na 2 O) have not previously been reported from pelitic rocks.