Albite

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Albite
Albite - Crete (Kriti) Island, Greece.jpg
Albite from Crete, scale = 1 in.
General
Categoryplagioclase, feldspar, tectosilicate
Formula
(repeating unit)
NaAlSi3O8 or Na1.0–0.9Ca0.0–0.1Al1.0–1.1Si3.0–2.9O8
Strunz classification9.FA.35
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupC1
Unit cella = 8.16, b = 12.87
c = 7.11 [Å]; α = 93.45°
β = 116.4°, γ = 90.28°; Z = 4
Identification
ColorWhite to gray, blueish, greenish, reddish; may be chatoyant
Crystal habitCrystals commonly tabular, divergent aggregates, granular, cleavable massive
TwinningCommon giving polysynthetic striae on {001} or {010}also contact, simple and multiple
CleavagePerfect on {001}, very good on {010}, imperfect on {110}
FractureUneven to conchoidal
TenacityBrittle
Mohs scale hardness6–6.5
LusterVitreous, typically pearly on cleavages
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.60–2.65
Optical propertiesBiaxial (+)
Refractive indexnα = 1.528–1.533 nβ = 1.532–1.537 nγ = 1.538–1.542
Birefringenceδ = 0.010
2V angle85–90° (low); 52–54° (high)
Dispersionr < v weak
Melting point1100-1120 °C
Other characteristicsLow- and high-temperature structural modifications are recognized
References[1][2][3]

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.[4] 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.

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).[5] Upon further heating to more than c. 1050 °C the crystal symmetry changes from triclinic to monoclinic; this variant is also known as monalbite.[6] Albite melts at 1100-1120 °C.[7]

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.[2]

It is used as a gemstone.[8]

References[edit]

  1. ^ http://rruff.geo.arizona.edu/doclib/hom/albite.pdf Handbook of Mineralogy
  2. ^ a b http://www.mindat.org/min-96.html Mindat.org
  3. ^ http://www.webmineral.com/data/Albite.shtml Webmineral data
  4. ^  One or more of the preceding sentences incorporates text from a publication now in the public domainChisholm, Hugh, ed. (1911). "Albite". Encyclopædia Britannica (11th ed.). Cambridge University Press.
  5. ^ O.F. Tuttle, N.L. Bowen (1950): High-temperature albite and contiguous feldspars. J. Geol. 58(5), 572–583, https://www.jstor.org/stable/30068571
  6. ^ Monalbite on Mindat
  7. ^ J.P. Greenwood, P.C. Hess (1998): Congruent melting of albite: theory and experiment. J. Geophysical Research. 103(B12), 29815-29828
  8. ^ Loose Gemstone Guide – Secrets of the Gem Revealed By Pao Nipperkin

External links[edit]