An example of a sharp change in the magnitude of birefringence and the angle of optical axes in a layered epidote grain

Abstract

In the quartz-epidote tuff schist, from the Akmola region, there are voids filled either with quartz and epidote, or with turbid grains of feldspar. In the latter, epidote sometimes develops, replacing feldspar from the center to the periphery. One of these epidote grains, shown in the attached figure , has a belt fold. The core of the grain, polarizing with 2nd order yellow and clearly pleochroic, is bordered by hexagonal zones of lesser birefringence (blue and red), almost non-pleochroic; to the periphery of them there remains one sector with colorless layers of low birefringence. With such a difference in birefringence, all these zones are simultaneously extinguished and brightened in crossed nicols, especially clearly when observed in red light, which eliminates dispersion.