Plagioclase

Plagioclase
A photomicrograph of a plagioclase crystal under cross polarized light. The plagioclase crystal shows a distinct banding effect called a Polysynthetic twinning.
General
Category	Feldspar mineral group, tectosilicate
Formula (repeating unit)	NaAlSi3O8 – CaAl2Si2O8
Crystal symmetry	Triclinic pinacoidal H-M symbol: (1) Space group: C1
Identification
Color	White, gray, bluish white, reddish white, greenish white
Crystal habit	Massive to granular
Crystal system	Triclinic
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 an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a 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 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 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.

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
% An = % CaAl₂Si₂O₈
% Ab = % NaAlSi₃O₈

Name	% An	% Ab
Anorthite	90–100	10–0
Bytownite	70–90	30–10
Labradorite	50–70	50–30
Andesine	30–50	70–50
Oligoclase	10–30	90–70
Albite	0–10	100–90

Endmembers

• Anorthite: Anorthite was named by Rose in 1823 from the Greek meaning oblique, referring to its triclinic crystallization. Anorthite is a comparatively rare mineral but occurs in the basic plutonic rocks of some orogenic calc-alkaline suites.

• Albite: Albite is named from the Latin albus, in reference to its unusually pure white color. It is a relatively common and important rock-making mineral associated with the more acid rock types and in pegmatite dikes, often with rarer minerals like tourmaline and beryl.

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.

• Bytownite: Bytownite, named after the former name for Ottawa, Canada (Bytown), is a rare mineral occasionally found in more basic rocks.

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

• Labradorite: Labradorite is the characteristic feldspar of the more basic rock types such as diorite, gabbro, andesite, or basalt and is usually associated with one of the pyroxenes or amphiboles. Labradorite frequently shows an iridescent display of colors due to light refracting within the lamellae of the crystal. It is named after Labrador, where it is a constituent of the intrusive igneous rock anorthosite which is composed almost entirely of plagioclase. A variety of labradorite known as spectrolite is found in Finland.

• Andesine: Andesine is a characteristic mineral of rocks such as diorite which contain a moderate amount of silica and related volcanics such as andesite.

• Oligoclase: Oligoclase is common in granite, syenite, diorite, and gneiss. It is a frequent associate of orthoclase. The name oligoclase is derived from the Greek for little and fracture, in reference to the fact that its cleavage angle differs significantly from 90°. Sunstone is mainly oligoclase (sometimes albite) with flakes of hematite.

See also

• Hypersolvus
• List of minerals
• Subsolvus

References

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