Sometimes, mineral compositions that are stable at high temperature become unstable when temperature decreases. They may exsolve ("unmix") so that a grain that was once uniform contains blebs, patches or stringers of two minerals. Exsolution is generally only visible in XP views.
Exsolution in Feldspar and Pyroxene
The views above, both XP, show two examples of exsolution. The one on the left shows a large K-feldspar grain with a single twin down its center. Included in the K-feldspar are stringers of plagioclase. K-feldspar with unmixed plagioclase is called perthite. The K-feldspar and plagioclase existed as a single solid solution mineral at high temperature but unmixed due to cooling.
The view on the right is of a clinopyroxene grain. At high temperatures, pyroxenes with intermediate Ca/Mg values are stable. Upon cooling they may unmix into two pyroxenes -- one Ca-rich and the other Ca-poor -- yielding a striped grain like the one shown.
The view of perthite (left)
comes from the Unversity in Lille, France: http://www.univ-lille1.fr/geosciences/cours/cours_mineralo/cours_mineralo_3.html.
The view on the right comes from the University of Melbourne: