As a result of hydrothermal alteration or low-grade metamorphism, the Ca-component of plagioclase can be altered to epidote-group minerals such as epidote or zoisite.  This replacement, resulting in fine-grained to extremely fine-grained epidote enclosed in Na-rich plagioclase, a greenschist facies assemblage, is referred to as saussurite, and the process is saussuritization.  In A, fine-grained, high relief, yellowish epidote grains occur in low relief, colourless plagioclase.  In B, the bright interference colours of the epidote stand out in contrast to the first order grey/white of the polysynthetically twinned plagioclase.  Sample is from the Robertson River Batholith, Virginia.  Both views are 2.2 mm across.  A ppl, B x-nicols.