Study of minerals from the pegmatites of the Nellore mica-belt, Andhra Pradesh, India Part II. Muscovite

SUMMARY. The muscovites are 2M 1 polymorph type; minimum temperature of crystallization of 435 ~ is obtained from the molecular percentage of paragonite in muscovite. THE Nellore pegmatites are mined mainly for muscovite. Generally, pegmatites rich in microcline perthite are uneconomical for muscovite mining. Though the muscovite is not an important mineral of the wall zone, in some of the pegmatites muscovite lodes are observed in the wall zone; it is an essential mineral constituent of inter-mediate zone, is observed in the quartz core margin, and occurs as fracture fillings. Concentration of muscovite around xenoliths of country rock in the pegmatites i common. Muscovite occurs in shades of green and ruby. Swaminathan (I93O), who gave analyses, reported books of mica measuring as much as ' IO feet along the basal planes, and 15 feet along the folia'. But usually all gradations of size from a few square feet of area down to scales of microscopic dimensions are observed. Sp. gr. 2.9o~o-o5; 2V ~ 38-44 ~ varies from 1"58o to 1"59o. The inclusions in muscovite are divided into primary inclusions (those formed durin