Biaxial + or -, first order gray to very pale straw yellow interference colors; low relief; polysynthetic twinning is very common, as in the photos on the left, which helps distsinguish plagioclase from orthoclase; twins are typically continuous, with parallel sides, unlike microcline. Plagioclase also may be strongly zoned, as in the photos at the right above: normal zoning when the center of the grain is more Ca-rich than the edges, and oscillatory zoning when the composition alternates from Ca- to Na-richer zones.
Plagioclase composition may be determined by several optical techniques. Perhaps the easiest technique to apply is the Michel-Levy method.
Plagioclase is the most abundant mineral in Earth's crust, occurring in most igneous and metamorphic rocks.
Sericite (fine-grained muscovite) flakes altering plagioclase grain in the center of the picture; zoned plagioclase in the lower left; twinned plagioclase upper right.
