Fig. 10. Albite-twinned plagioclase (tan) showing interior replacement by irregular islands of K-feldspar (gray; orthoclase that was formerly microcline ?). The K-feldspar cross-cuts the albite twinning. Such replacement occurs only rarely in the thin section and, thus, is atypical of antiperthite in granitic rocks crystallized from a melt in which the majority of the plagioclase grains would have K-feldspar islands. From Popple Hill gneiss near Colton.