Albite

Albites
Albite from Crete
General
Category	plagioclase, feldspar, tectosilicate
Formula (repeating unit)	NaAlSi ₃O ₈ or Na _1.0–0.9Ca _0.0–0.1Al _1.0–1.1Si _3.0–2.9O ₈
IMA symbol	Ab^[1]
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	n_α = 1.528–1.533 n_β = 1.532–1.537 n_γ = 1.538–1.542
Birefringence	δ = 0.010
2V angle	85–90° (low); 52–54° (high)
Dispersion	r < v weak
Melting point	1,100–1,120 °C (2,010–2,050 °F)
Other characteristics	Low- and high-temperature structural modifications are recognized
References	^[2]^[3]^[4]

Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula Na Al Si
₃O
₈. It is a tectosilicate. Its color is usually pure white, hence its name from Latin, albus.^[5] It is a common constituent in felsic rocks.

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Properties

Albite crystallizes with triclinic pinacoidal forms. Its specific gravity is about 2.62 and it has a Mohs hardness of 6 to 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 750 °C (1,380 °F)^[6] High albite can be found in meteor impact craters such as in Winslow, Arizona.^[7] Upon further heating to more than 1,050 °C (1,920 °F) the crystal symmetry changes from triclinic to monoclinic; this variant is also known as 'monalbite'.^[8] Albite melts at 1,100–1,120 °C (2,010–2,050 °F).^[9]

Oftentimes, potassium can replace the sodium characteristic in albite at amounts of up to 10%. When this is exceeded the mineral is then considered to be anorthoclase.^[10]

Occurrence

It occurs in granitic and pegmatite masses (often as the variety cleavelandite),^[11] in some hydrothermal vein deposits, and forms part of the typical greenschist metamorphic facies for rocks of originally basaltic composition. Minerals that albite is often considered associated with in occurrence include biotite, hornblende, orthoclase, muscovite and quartz.^[12]

Discovery

Albite was first reported in 1815 for an occurrence in Finnbo, Falun, Dalarna, Sweden.^[3]

Use

Albite is used as a gemstone, albeit semiprecious. Albite is also used by geologists as it is identified as an important rock forming mineral. There is some industrial use for the mineral such as the manufacture of glass and ceramics.^[13]^[14]