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- The interference figure that is obtained in convergent light when an optic axis of the mineral being observed in thin section or as a fragment coincides with the axis of the polarizing microscope. When a thin section of a uniaxial mineral that was cut at right angles to an optic axis is examined between crossed nicols (that is, between two polarizers, the polarization planes of which are at right angles to each other) an equal-armed shadowy cross and a series of spectrally colored, circular bands are seen. If the mineral is biaxial, two shadowy parabolic curves called isogyres and opening away from each other in a series of spectrally colored, oval bands appear. Hess
- In polarized light microscopy, an interference figure in which an optic axis is centered in the field of view.
Source: Dictionary of Mining, Mineral, and Related Terms