A member of the plagioclase feldspar solid-solution series with a composition ranging from Ab₃₀An₇₀ to Ab₁₀An₉₀ (Ab = NaAlSi₃O₈ and An = CaAl₂Si₂O₈) (see illus.). In the high-temperature form, bytownite has the high-albite structure type in which Al and Si atoms are completely disordered; that is, neither element shows a preference for any of the four distinct tetrahedral sites. During cooling, natural material changes its Al/Si distribution, tending toward an ordered state in which Al and Si concentrate in different sites which alternate with one another in all directions. Bytownite is very abundant in basic igneous rocks, where it is the first plagioclase to crystallize under plutonic conditions; it forms the cores of zoned plagioclase phenocrysts in basaltic volcanics; and it sometimes occurs in anorthosites.