Abstract

Several types of pegmatites are distributed in zones, thought to reflect differences in temperature of formation. In general, plagioclase predominates over microcline, and crystallized first; the amount of microcline varies with the composition of the plagioclase.Abundant structures suggesting replacement of microcline by plagioclase are the result of metamorphism of the pegmatitic-magmatic constituents; very minor amounts of albite are meta-somatic replacements; much of the commercial mica is primary. Accessory minerals include compounds of columbium, uranium, and rare-earths, allanite, beryl, and others.180 pegmatites were investigated in the field and the plagioclase was determined for more than 100 of these. The paragenesis is shown by banded structures, filter pressing, textures, and other field and microscopic evidence.A survey of recent literature suggests that the conclusions for the Spruce Pine district---the largest producer of muscovite and feldspar in North America---may be applied to other districts, and shows that economic deposits of feldspar and mica that contain plagioclase predominating over microcline are found elsewhere.

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