The curved shape of the isogyre is characteristic of biaxial minerals-though the degree of curvature will change as the microscope stage is rotated, and at some orientations the pattern will resemble the " maltese cross " pattern of a uniaxial mineral.
2.
One or two curved isogyres ( sometimes called " brushes " ) will be visible, one of which will have its point of maximum curvature perfectly centred . ( The figure shows an example with a single isogyre visible . ) If two isogyres are visible, they will be positioned back-to-back.
3.
Rotating the stage will cause the isogyres to move and change shape strikingly-moving from a position where the isogyres curve smoothly and are widely separated at their closest point, then gradually becoming more tightly curved / squarer at their midpoints as they approach each other ( a second isogyre appearing from out of the field of view if it was absent before ), then merging to form a maltese cross pattern very much like that of a uniaxial mineral.
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