Mica fish

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Thin section of a mica fish in mylonitic quartzite from the Italian Alps

Mica fish are lenticular, elongate lozenge, parallelogram-shaped, or lens-shaped (so roughly fish-shaped) single mica crystals that are often used as shear sense indicators.[1] They commonly occur in micaceous quartzitic mylonites. They characteristically lie with their longest dimension at a small angle to the mylonitic foliation.