Muscovite with unusual optical and structural characteristics is described. The size of the optic axial angle (2V) varies from about 15° for areas of highest birefringence in the cleavage plane to about 3° for areas of lowest birefringence. X-ray studies show the muscovite to be three layered monoclinic with Z = a and thus dimorphous with previously described muscovites. Variation in the observed optic axial angle is attributed to the coalescence of the optical effects of superposed twin elements.

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