Plagioclase feldspar minerals occur in the solid solution series between albite [Ab: Na(Si₃Al)O₈] and anorthite [An: Ca(Si₂Al₂)O₈] end-members. Ordering between Al and Si is very sluggish in the plagioclase feldspars and requires a coupled substitution between (Si + Na) and (Al + Ca). The plagioclase feldspar minerals are the most abundant minerals in the earth’s crust. Plagioclase mineral names are divided into albite (An_0-10), oligoclase (An_10-30), andesine (An_30-50), labradorite (An_50-70), bytownite (An_70-90), and anorthite (An_90-100), but it is increasingly common to designate molecular percentages (i.e., Or₁₀Ab₆₀An₃₀, where Or = orthoclase and Ab and An are defined above). Labradorite feldspar may show iridescent colors at compositions of An₅₀–₆₀ and with Or₂–₅ where semi-periodic exsolution lamellae occur with periodicity of ~100 nm to ~300 nm. Labrodorite commonly occurs in gabbroic rocks or anorthosites (plagioclase-dominated rocks) that experienced extremely slow cooling history. Plagioclase feldspars are used in glass making and ceramics, paints, rubber, and plastics, although the alkali feldspars are more commonly used in industry.
Cf., alkali feldspar