Plagioclase is the most common mineral group building the crust of our planet.

Nature · Published 3 October 2026 Swipe more like this →

In short: Look closely at a piece of igneous rock and you are likely staring at plagioclase. This glassy, brittle mineral is not a single substance but a shifting spectrum of chemistry. It is so abundant that it helps define the very crust we walk on, yet it remains a geological chameleon.

Plagioclase is not a single mineral species but a continuous solid-solution series within the broader feldspar group. It is defined as a mixture of two endmembers: albite, a sodium aluminosilicate, and anorthite, a calcium aluminosilicate. Because these two components can substitute for one another in the crystal lattice, plagioclase exists in any proportion between pure albite and pure anorthite. This flexibility makes it a vital tool for petrologists, who use its specific composition to trace the origin and evolution of igneous rocks.

The name plagioclase is derived from the Ancient Greek words for oblique and fracture, referencing the mineral's distinct cleavage angles. While it often appears as light-colored, glassy, or translucent crystals, it is frequently identified in the field by its characteristic polysynthetic twinning—a system of fine, parallel striations that look like record grooves on the crystal surface. These striations serve as a key diagnostic feature, allowing geologists to distinguish plagioclase from other common feldspars like the alkali group.

Beyond Earth, plagioclase is a major constituent of the lunar highlands. On Mars, thermal emission spectra suggest it is the most abundant mineral in the Martian crust. Back on our own planet, feldspars as a whole comprise more than half of the Earth's crust. Because these minerals are so pervasive, they serve as the primary basis for naming most rocks exposed on the surface. Identifying them usually requires only macroscopic examination and simple tests, though precise chemical analysis can be performed using more sophisticated equipment.

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