It is the result of the mineral dividing a light wave into two mutually perpendicular components. The two waves travel through the mineral with unequal velocities and unequal wavelengths. The amount of light transmitted through the upper analyzer is function of the phase differnce between the two transmitted waves. This phase difference is determined by crystal thickness, mineral orientation, birefringence, and the nature of the mineral.
Interference color is often a key diagnostic criteria for mineral identification.
In thin section , the thickness is uniform, thus the interference color is dependent on the birefringence and orientation of the mineral. The result if that portions of the mineral are in different orientation. Twins consist of segments of the mineral that usually go extinct at different positions with stage rotation. Twinning is a diagnostic feature used to identify many minerals. Feldspars are the most commonly observed minerals in soils that are twinned.