OFF CENTRED OPTIC AXIS FIGURE

The interference figure is produced when the optic axis is not vertical, resulting in the interference figure, i.e. the melatope, no longer being centred in the field of view.

The isogyres still form a cross, with the melatope at the centre.

Because the figure is off centred, the melatope (optic axis) does not appear in the field of view, on rotation the melatope swings in a circle around the center of the field of view.

Isogyres will retain their basic NS & EW orientations and sweep across the field of view centred on the melatope, always moving parallel to the crosshairs.

If the melatope is just in the field of view the optic sign can easily be determined, using the technique outlined above.


If the melatope is well outside the field of view the isogyres sweep across the field of view in sequence as the stage is rotated - with the isogyres always remaining parallel to the crosshairs.

By noting the direction and sequence of how the isogyres pass through the field of view, as the stage is rotated, it is possible to identify which quadrant is being viewed and therefore the optic sign may be determined, knowing the vibration directions of omega & epsilon, in the NE quadrant of the interference figure.

A grain which produces an off centred optic axis figure will exhibit a birefringence intermediate to the maximum and minimum birefringence for that mineral in the thin section.