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
Crystal symmetry	Triclinic H–M Symbol 1
Unit cell	a = 8.16 Å, b = 12.87 Å, 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
Crystal system	Triclinic Pinacoidal
Twinning	Coomon 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

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.

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.

It was first reported in 1815 for an occurrence in Finnbo, Falun, Dalarna, Sweden.

References

• Mineral galleries
•  This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press.