Plagioclase

Plagioclase
A photomicrograph of a plagioclase crystal 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
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	C1
Identification
Color	White, gray, bluish white, reddish white, greenish white
Crystal habit	Massive to granular
Twinning	Common, albite law striations on {001}
Cleavage	Good on {001} and {010}
Tenacity	Brittle
Mohs scale hardness	6 - 6.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.62 (albite) to 2.76 (anorthite)
Optical properties	Biaxial (+) albite, biaxial (-) anorthite
Refractive index	Albite: nα 1.527, nβ 1.532 nγ 1.538 Anorthite: nα 1.577 nβ 1.585 nγ 1.590
Solubility	Albite insoluble in HCl, anorthite decomposed by HCl
References

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 Ancient Greek for "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 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 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.

The composition of a plagioclase feldspar is typically denoted by its overall fraction of anorthite (%An) or albite (%Ab), and readily determined by measuring the plagioclase crystal's refractive index in crushed grain mounts, or its extinction angle in thin section under a polarizing microscope. The extinction angle is an optical characteristic and varies with the albite fraction (%Ab). There are several named plagioclase feldspars that fall between albite and anorthite in the series. The following table shows their compositions in terms of constituent anorthite and albite percentages.