Albite

Albite
Albite from Crete, scale = 1 in.
General
Category	plagioclase, feldspar, tectosilicate
Formula (repeating unit)	NaAlSi3O8 or Na1.0–0.9Ca0.0–0.1Al1.0–1.1Si3.0–2.9O8
Strunz classification	9.FA.35
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	C1
Unit cell	a = 8.16, b = 12.87 c = 7.11 [Å]; α = 93.45° β = 116.4°, γ = 90.28°; Z = 4
Identification
Color	White to gray, blueish, greenish, reddish; may be chatoyant
Crystal habit	Crystals commonly tabular, divergent aggregates, granular, cleavable massive
Twinning	Common giving polysynthetic striae on {001} or {010}also contact, simple and multiple
Cleavage	Perfect on {001}, very good on {010}, imperfect on {110}
Fracture	Uneven to conchoidal
Tenacity	Brittle
Mohs scale hardness	6 – 6.5
Luster	Vitreous, typically pearly on cleavages
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.60 - 2.65
Optical properties	Biaxial (+)
Refractive index	nα = 1.528 – 1.533 nβ = 1.532 – 1.537 nγ = 1.538 – 1.542
Birefringence	δ = 0.010
2V angle	85–90° (low); 52–54° (high)
Dispersion	r < v weak
Other characteristics	Low- and high-temperature structural modifications are recognized
References

Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi₃O₈. It is a tectosilicate. Its color is usually pure white, hence its name from Latin albus. It is a common constituent in felsic rocks.

Albite crystallizes with triclinic pinacoidal forms. Its specific gravity is about 2.62 and it has a Mohs hardness of 6 - 6.5. Albite almost always exhibits crystal twinning often as minute parallel striations on the crystal face. Albite often occurs as fine parallel segregations alternating with pink microcline in perthite as a result of exolution on cooling.

There are two variants of albite, which are referred to as low albite and high albite; the latter is also known as analbite. Although both variants are triclinic, they differ in the volume of their unit cell, which is slightly larger for the high form. The high form can be produced from the low form by heating above c. 750 °C (1382 °F). Upon further heating to more than c. 1050 °C the crystal symmetry changes from triclinic to monoclinic; this variant is also known as monalbite.

It occurs in granitic and pegmatite masses, in some hydrothermal vein deposits and forms part of the typical greenschist metamorphic facies for rocks of originally basaltic composition.