Plagioclase feldspar solid solution series
The continuous solid solution between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), covering most feldspars in igneous rocks.
Albite: The sodium-rich end member of the plagioclase series, NaAlSi3O8, stable at lower temperatures. The low-temperature endpoint toward which plagioclase compositions drift as melt cools.
Norman L. Bowen: Petrologist whose 1910s–1920s melting experiments founded experimental igneous petrology and the reaction series. Bowen demonstrated the continuous plagioclase reaction experimentally and framed it in his reaction series.
Labradorite: An intermediate plagioclase around An50–An70, famous for blue iridescent play of color. The best-known mid-series composition, showing the continuum made visible in hand specimens.
Alkali feldspar: The Na–K feldspar solid solution, which is complete at high temperature but unmixes on cooling. The parallel series of the alkali feldspars, limited by the large potassium ion.
Feldspar crystal structure: A framework of corner-sharing AlO4 and SiO4 tetrahedra with large cations filling cavities. The framework's tolerance of size mismatch underlies solid solution in feldspars.
Anorthite: The calcium-rich end member of the plagioclase series, CaAl2Si2O8, crystallizing at the highest temperatures. The high-temperature endpoint where the series begins in basaltic melts.
Bowen's reaction series: Bowen's scheme ordering silicate minerals by the temperatures at which they crystallize from magma. The plagioclase branch is the continuous arm of Bowen's two-branch scheme.
Andesine: An intermediate plagioclase roughly An30–An50, named for andesite lavas. One of the named mid-series ranges that subdivides the continuum for field use.
Perthite: An intergrowth of sodium-rich and potassium-rich feldspar formed by exsolution from alkali feldspar. Perthite records broken solid solution, the failure mode the plagioclase series largely avoids.
Solid solution: A mineral phase in which one or more components vary continuously in composition without changing structure. The general phenomenon the plagioclase series exemplifies in its purest common form.