Ribbe, Paul H. Formerly, Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, Virginia.
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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 colors like those in peristerites and Huttenlocher intergrowths (Fig. 1). In nature, most plagioclases throughout the composition range Ab54An46 to Ab40An60 are exsolved into two e-plagioclases that have compositions Ab56±4An44∓4 and Ab42±6An58∓6 and are coherently intergrown as somewhat irregular lamellar slabs 50 to 20 nanometers thick and known as Boggild intergrowths (Fig. 2). Intense colors 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 colors 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 colors in hummingbird feathers and the wings of Morpho butterflies are physically similar phenomena. See also: Albite; Andesine; Anorthosite; Basalt; Feldspar; Gabbro; Igneous rocks
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