What is "plagioclase"

Plagioclase \Pla"gi*o*clase\, n. [Gr. pla`gios oblique + kla^n to break.] (Min.) A general term used of any triclinic feldspar. See the Note under Feldspar.

"triclinic feldspar," 1868, from German, coined 1847 by German mineralogist Johann Friedrich August Breithaupt (1791-1873) from plagio- + Greek klasis "fracture," from stem of klan "to break" (see clastic). Related: Plagioclastic.

n. (context mineralogy English) Any of a group of aluminum silicate feldspathic minerals ranging in their ratio of calcium to sodium.

n. any of a rock-forming series of triclinic feldspars [syn: oligoclase]

Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series (from the Greek "oblique fracture", in reference to its two cleavage angles). This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSiO to CaAlSiO), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or 'record-groove' effect.

Plagioclase is a major constituent mineral in the Earth's crust, and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks. Plagioclase is also a major constituent of rock in the highlands of the Earth's moon. Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase is the most abundant mineral in the crust of Mars.