Exsolution lamellae in moonstone

Cleavage plane {001} of plagioclase feldspar var. moonstone; 8 µm scan size, 4 nm color scale; AC mode image on Cypher. The narrow lines that run from front to back in this perspective view are thin layers (lamellae) of Na-rich and Ca-rich mineral phases that formed as the crystal cooled from the melt. On average the layers are 50 nm thick. When the crystal is viewed from the correct angle, light scatters from the layers, causing a blue flash of opalescence for which this mineral is valued. Also visible are step edges and oriented inclusions.