What does plagioclase mean?
Plagioclase is a group of feldspar minerals that forms much of the mineral composition of common igneous rocks. Its members vary in chemical composition: sodium-rich varieties occur toward the albite end of the series, while calcium-rich varieties occur toward the anorthite end. Individual plagioclase minerals are recognized by their triclinic crystal structure and characteristic cleavage, which produces angled fracture surfaces. In geology, the term may refer to the whole mineral group or to a particular mineral within it. Plagioclase is especially important in the study of igneous rocks because its composition and crystal structure can indicate the conditions under which a rock formed. The name combines Greek roots meaning “oblique” and “cleavage,” referring to the mineral group’s characteristic geometry. Despite being a specialist term, plagioclase is widely encountered in descriptions of granite, basalt, and other common rocks.
nounPlagioclase is a group of rock-forming feldspar minerals with a triclinic crystal structure. The group includes minerals ranging in composition from albite, rich in sodium, to anorthite, rich in calcium.
- Any mineral of the plagioclase feldspar group, especially the series extending from albite to anorthite.
"The basalt contains conspicuous crystals of plagioclase, some of which reflect light from their cleavage surfaces."
"The granite contains pale plagioclase crystals interlocked with quartz and mica."
"Geologists identified plagioclase in the lava sample using its characteristic cleavage and optical properties."
Used for multiple plagioclase minerals, grains, or specimens; in many geological descriptions, the singular form may also name the mineral group collectively.
"Several plagioclases occur in the granite, including varieties with both sodium- and calcium-rich compositions."
Plagioclase literally means “oblique cleavage,” a name that points to its distinctive angled crystal fractures.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does plagioclase come from?
The name was formed in the nineteenth century from Greek plagios, meaning “oblique,” and klasis, meaning “cleaving” or “cleavage.” The terminology reflects the angled cleavage characteristic of the mineral group. Its components also lie behind related words such as oblique, plagiarize, and cleavage, all of which trace to Greek roots concerned with sloping, taking, or splitting.
Related word forms
How plagioclase is actually used
The term is primarily used in geology, mineralogy, petrology, and related Earth sciences. It may refer either to the entire mineral group or, in more specific contexts, to a particular member of that group.
Easily confused with plagioclase
Plagioclase is a sodium- and calcium-rich feldspar group with triclinic symmetry, whereas orthoclase is a potassium feldspar mineral with monoclinic symmetry.