Plagioclase

Plagioclase
A photomicrograph of a plagioclase crystal (gray) under cross polarized light. The plagioclase crystal shows a distinct banding effect called polysynthetic twinning.
General
Category	Feldspar mineral group, tectosilicate
Formula (repeating unit)	NaAlSi3O8 – CaAl2Si2O8
IMA symbol	Pl[1]
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	C1
Identification
Color	White, gray, or bluish white
Mohs scale hardness	6–6.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.62 (albite) to 2.76 (anorthite)[2]
Optical properties	Biaxial (+) albite, biaxial (-) anorthite[2]
Refractive index	Albite: nα 1.527, nβ 1.532 nγ 1.538 Anorthite: nα 1.577 nβ 1.585 nγ 1.590[2]
Solubility	Albite insoluble in HCl, anorthite decomposed by HCl[2]
References	[3]

Plagioclase (/ˈplædʒ(i)əˌkleɪs, ˈpleɪdʒ-, -ˌkleɪz/ PLAJ-(ee)-ə-klayss, PLAYJ-, -⁠klayz)^[4] 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. 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 NaAlSi₃O₈ to CaAl₂Si₂O₈), 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 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 Moon. Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase is the most abundant mineral in the crust of Mars.^[5]

Its name comes from Ancient Greek πλάγιος (plágios) 'oblique', and κλάσις (klásis) 'fracture', in reference to its two cleavage angles.