Albite
The sodium end-member of plagioclase feldspar, NaAlSi3O8, a triclinium tectosilicate forming white to colorless crystals.
Tectosilicate framework: Three-dimensional frameworks of corner-sharing SiO4 and AlO4 tetrahedra that define feldspar structures. Albite's properties follow from how Na sits in cavities of this aluminosilicate framework.
Anorthite: The calcium end-member of plagioclase, CaAl2Si2O8, contrasting with albite in charge balance and melting behavior. The other endpoint of the solid solution from which the reader descends; the couple is defined together.
Cleavelandite: A bladed, platy variety of albite found in granitic pegmatites. The pegmatite habit of albite, prized by mineral collectors and pegmatite petrologists.
Johan Gottlieb Gahn: Swedish chemist who described albite from the Uto mine, Sweden, around 1815. The discoverer whose analysis fixed albite as a species distinct from other feldspars.
Aluminum–silicon ordering: The distribution of Al among tetrahedral sites in a framework, changing from disorder to order as temperature falls. Low albite is fully ordered on one tetrahedral site; high albite is disordered, giving a continuous structural spectrum.
Oligoclase: Plagioclase of composition An10–An30, the first member past albite in the series. The immediate neighbor across the peristerite gap; the boundary of the albite field in thin section.
Peristerite iridescence: Blue-white play of color from exsolution lamellae scattering light in certain albite–oligoclase feldspars. The gemological payoff of the peristerite gap, seen in moonstone-quality feldspar.
Plagioclase twinning laws: The named twin laws—albite, pericline, Carlsbad and others—systematized during 19th-century optical mineralogy. The albite law of twinning was named for this mineral and became its optical fingerprint.
Pericline twinning: Twinning in triclinic feldspars about the b axis, producing parallel lamellae visible under cross-polars. A diagnostic twinning law of albite and other plagioclase, used to identify compositions in thin section.
Sanidine: The high-temperature monoclinic form of potassium feldspar, typical of rapidly cooled volcanic rocks. The alkali-feldspar counterpart at the other side of the alkali exchange vector Na↔K.