A plagioclase feldspar with composition Ab₅₀An₅₀ to Ab₃₀An₇₀ (Ab = NaAlSi₃O₈; An = CaAl₂Si₂O₈), occurring in basic igneous rocks, that is, basalts, gabbros and anorthosites. Its hardness is 6–6.5; mean refractive index, 1.56; and specific gravity, 2.70. Labradorite crystals are translucent to dark or cloudy (from inclusions) but often display brilliant colours like those in peristerites and Huttenlocher intergrowths. In nature, most plagioclases throughout the composition range Ab₅₄An₄₆ to Ab₄₀An₆₀ are exsolved into two e-plagioclases that have compositions Ab_56±4An_44∓4 and Ab_42±6An_58∓6 and are coherently intergrown as somewhat irregular lamellar slabs 50 to 20 nanometers thick and known as Boggild intergrowths (see illus.). Intense colours may result from constructive interference of selected wavelengths of light reflected at successive interfaces between the alternating Ab- and An-rich lamellae. In fact, interference colours are so commonly observed in this composition range that the phenomenon has been called labradorescence. Gemologists often use the nonmineralogical term spectrolite in describing them. Iridescent colours in hummingbird feathers and the wings of Morpho butterflies are physically similar phenomena.