straight extinction

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straight extinction (parallel extinction) In optical mineralogy, the phenomenon which occurs when the vibration direction of the light ray is parallel to the crystal face or cleavage traces within the mineral. As the mineral thin section is rotated between crossed polars, the vibration direction is brought parallel to the plane of the polarizer. The light passes through the polarizer and is eliminated by the analyser. Extinction occurs four times in a 360° rotation of the mineral.