Plagioclase

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. 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
A photomicrograph of a plagioclase crystal under cross polarized light. The plagioclase crystal shows a distinct banding effect called polysynthetic twinning.
General
CategoryFeldspar mineral group, tectosilicate
Formula
(repeating unit)
NaAlSi3O8 – CaAl2Si2O8
IMA symbolPl[1]
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupC1
Identification
ColorWhite, gray, or bluish white
Mohs scale hardness66.5
LusterVitreous
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.62 (albite) to 2.76 (anorthite)[2]
Optical propertiesBiaxial (+) albite, biaxial (-) anorthite[2]
Refractive indexAlbite: nα 1.527, nβ 1.532 nγ 1.538
Anorthite: nα 1.577 nβ 1.585 nγ 1.590[2]
SolubilityAlbite insoluble in HCl, anorthite decomposed by HCl [2]
References[3]
Plagioclase displaying cleavage. (unknown scale)
In volcanic rocks, fine-grained plagioclase can display a 'microlitic' texture of many small crystals.

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 Moon. Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase is the most abundant mineral in the crust of Mars.[4]

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


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