Plagioclase Feldspar (thin section)

Key Characteristics: Low relief, colorless nature, polysynthetic twinning and oscillatory zoning are diagnostic of plagioclase 2V: 2Vx = 45 - 102° Relief: Low negative or positive relief Cleavage: Cleavages intersect at 93 °, usually not observed in thin section Colour and Plechroism: Colorless in thin section; Grains may appear cloudy due to alteration Optical Orientation: Inclined extinction; Oscillatory zoning common; Optical orientation varies with composition and thus can be used to determine composition Twinning: Polysynthetic twinning (albite, pericline twin laws), penetration twinning (simple twin law) Optical character: Biaxial (+) or (-)%value%value%valu