Double refraction

Uniaxial minerals like calcite and quartz with hexagonal or tetragonal structures have two indices of refraction; one index is related to the velocity of light parallel to the unique c-axis, the other is related to the velocity at right angles to the c-axis. Calcite (pictured above) is the best known example.

Biaxial minerals like cryolite have two planes of almost equal refractive indices, and a third that differs from the other two.

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