The interference colour and the optic indicatrix
The variation in interference colour which the grains of a certain mineral present depends on the orientation of the grain.
The interference colours of each cut of a mineral can be deduced from its optic indicatrix.
The figure shows how the interference colour can change for a uniaxial crystal according to the inclination of the plane of the cut. The interference colour varies from nil for the horizontal cut (perpendicular to the optic axis) to a maximum value represented by the vertical cut (parallel to the optic axis), passing through a series of intermediate colours corresponding to the inclined cuts (the more inclined the cut, the higher the colour).
In order to evaluate the interference colour of a mineral, and thus its birefringence, we must always find the one which corresponds to its maximum value.
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