Plagioclase

Plagioclase is a continuous solid-solution series of feldspars ranging from sodium-rich albite to calcium-rich anorthite, with named intermediate members oligoclase, andesine, labradorite, and bytownite. It is one of the most abundant minerals in Earth's crust and a defining component of countless igneous rocks.

The series is classified by the percentage of the anorthite (calcium) component, written as An values: albite is An0-10, anorthite An90-100. Plagioclase crystallizes in the triclinic system and is recognized by fine parallel striations from repeated albite twinning.

Gem and ornamental varieties include labradorite (with its colorful labradorescence), sunstone, peristerite, and moonstone-type material, making the group important to both geologists and lapidaries.

Plagioclase crystallizes directly from silicate magmas and is a primary mineral in basalt, andesite, diorite, gabbro, and granite, as well as in many metamorphic rocks. Its composition tracks magma chemistry: calcium-rich plagioclase forms early from hot mafic magmas, while sodium-rich plagioclase crystallizes later as magmas cool and evolve.

This predictable behavior is captured in Bowen's reaction series, where plagioclase continuously changes composition from anorthite toward albite during cooling. It is abundant on the Moon (lunar highlands are rich in anorthite) and forms the bulk of the rock anorthosite. Localities are essentially worldwide.

Plagioclase is white to gray with a vitreous luster, hardness 6-6.5, white streak, and two cleavages meeting at close to 90 degrees. The single most diagnostic feature is fine, parallel striations (albite twin lamellae) on a cleavage face, which potassium feldspar lacks.

Distinguish it from potassium feldspar (orthoclase, microcline) by these striations and by the general absence of salmon-pink color; plagioclase is typically white or gray. Labradorite and sunstone are recognized by their schiller. It is harder than calcite and shows cleavage absent in quartz.

Plagioclase-rich rocks are quarried as construction aggregate and dimension stone, and feldspar is a key raw material in ceramics, glass, and glazes. Anorthosite is even studied as a potential source of aluminum.

Gem and ornamental plagioclase varieties - labradorite, spectrolite, sunstone, and moonstone - are widely cut for jewelry and carvings. Scientifically, plagioclase composition is a powerful tool for interpreting the origin and cooling history of igneous and metamorphic rocks, and lunar plagioclase records the Moon's early crust.