Albiteanorthite

Albiteanorthite is a term used in mineralogy and petrology to describe plagioclase feldspar crystals whose composition falls between the endmembers albite and anorthite. It denotes the continuous solid solution series between Na-rich albite (NaAlSi3O8) and Ca-rich anorthite (CaAl2Si2O8). The composition is commonly expressed as An, the molar percentage of anorthite in the crystal; pure albite corresponds to 0% An, pure anorthite to 100% An. Most plagioclases in rocks are intermediate and are described as albiteanorthite rather than as a definite endmember.

Occurrence and significance: Albiteanorthite plagioclase is widespread in many igneous rocks, from felsic granites and rhyolites to mafic basalts and gabbros, and in metamorphic rocks as well. The Na–Ca ratio varies with the original melt composition and cooling history, and the An content provides a record of crystallization conditions. Higher An contents are common in more mafic, higher-temperature assemblages, while lower An contents occur in sodic, felsic rocks.

Structure and identification: Plagioclase is a feldspar with a framework of SiO4 tetrahedra in which Na and Ca occupy large-cation sites and Al substitutes for Si in the tetrahedral framework. Albiteanorthite crystals commonly display twinning (such as albite or Carlsbad twinning) and may show zoning or exsolution textures in intermediate compositions. Optical properties vary with composition, reflecting changing Ca–Na and Al–Si ratios.

Naming and scope: The term highlights the continuous nature of the plagioclase solid solution Ab–An. In petrographic work, compositions are often described along the Ab–An range (e.g., oligoclase, andesine, labradorite, bytownite) to indicate approximate An contents.