A photomicrograph of a plagioclase crystal under cross polarized light. The plagioclase crystal shows a distinct banding effect called polysynthetic twinning.
Category Feldspar mineral group, tectosilicate
(repeating unit)
NaAlSi3O8 – CaAl2Si2O8
Crystal system Triclinic
Crystal class Pinacoidal (1)
(same H-M symbol)
Space group C1
Color White, gray, bluish white, reddish white, greenish white
Crystal habit Massive to granular
Twinning Common, albite law striations on {001}[1]
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)[1]
Optical properties Biaxial (+) albite, biaxial (-) anorthite[1]
Refractive index Albite: nα 1.527, nβ 1.532 nγ 1.538
Anorthite: nα 1.577 nβ 1.585 nγ 1.590[1]
Solubility Albite insoluble in HCl, anorthite decomposed by HCl [1]
References [2]
Plagioclase displaying cleavage. (unknown scale)
In volcanic rocks, fine-grained plagioclase can display a 'microlitic' texture of many small crystals.

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.[3]

Plagioclase series members

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.

Plagioclase minerals and their compositions
Name % CaAl2Si2O8
% NaAlSi3O8 Image


Intermediate members

The intermediate members of the plagioclase group are very similar to each other and normally cannot be distinguished except by their optical properties.

Labradorite displaying typical iridescent effect termed labradorescence. (unknown scale)

See also


  1. 1 2 3 4 5 Klein, Cornelis and Cornelius S. Hurbut, Jr.; Manual of Mineralogy, Wiley, 20th ed., 1980, pp.454-456 ISBN 0-471-80580-7
  2. Webmineral data
  3. Milam, K. A.; et al. (2010). "Distribution and variation of plagioclase compositions on Mars". Journal of Geophysical Research: Planets. 115 (E9). doi:10.1029/2009JE003495.
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