Perthite forms in microcline K-feldspar when the mineral plagioclase, contained within, becomes unstable and "exolves" out of the microcline lattice as wormy streaks of albite plagioclase. Found in granites which crystalized at temperatures above the instability zone (Hypersolvus granites) or in very high grade Granulite Facies metamorphic rocks again associated with recrystallization above and cooling down past the feldspar istability zone.
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