Andalusite

Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5. This mineral was called andalousite by Delamétehrie,[3] who thought it came from Andalusia. It soon became clear that it was a locality error, and that the specimens studied were actually from El Cardoso de la Sierra, in the province of Guadalajara, not Andalusia.[4]

Andalusite
Andalusite, Tyrol Austria.
General
CategoryNesosilicates
Formula
(repeating unit)		Al2SiO5
Strunz classification9.AF.10
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPnnm
Unit cella = 7.7980 Å, b = 7.9031 Å
c = 5.5566 Å; Z = 4
Identification
ColorPink, violet, yellow, green, white, gray; in thin section, colorless to pink or green
Crystal habitAs euhedral crystals or columnar aggregates having nearly square cross sections; fibrous compact to massive
TwinningOn {101}, rare
CleavageGood on {110}, poor on {100}
Fractureuneven to subconchoidal
Mohs scale hardness6.5 - 7.5
LusterVitreous
StreakWhite
DiaphaneityTransparent to nearly opaque with inclusions
Specific gravity3.17 (+/- .04)
Optical propertiesdouble refractive, biaxial negative; chiastolite has anomalous aggregate reaction.[1]
Refractive indexnα = 1.629 - 1.640 nβ = 1.633 - 1.644 nγ = 1.638 - 1.650
Birefringenceδ = 0.009 - 0.010
Pleochroismstrongly trichroic
2V angle71 - 86°
Dispersionr < v strong
Ultraviolet fluorescencenon-fluorescent
References[1][2]

Andalusite is trimorphic with kyanite and sillimanite, being the lower pressure mid temperature polymorph. At higher temperatures and pressures, andalusite may convert to sillimanite. Thus, as with its other polymorphs, andalusite is an aluminosilicate index mineral, providing clues to depth and pressures involved in producing the host rock.[5]

Phase diagram of Al2SiO5
(nesosilicates).[7]

Varieties

The variety chiastolite commonly contains dark inclusions of carbon or clay which form a cruciform pattern when shown in cross-section. This stone was known at least from the sixteenth century, being taken to many European countries, as a souvenir, by pilgrims returning from Santiago de Compostela.[8]

A clear variety found in Brazil and Sri-Lanka can be cut into a gemstone.[9] Faceted andalusite stones give a play of red, green, and yellow colors that resembles a muted form of iridescence, although the colors are actually the result of unusually strong pleochroism.

Occurrence

Andalusite is a common metamorphic mineral which forms under low pressure and low to high temperatures. The minerals kyanite and sillimanite are polymorphs of andalusite, each occurring under different temperature-pressure regimes and are therefore rarely found together in the same rock. Because of this the three minerals are a useful tool to help identify the pressure-temperature paths of the host rock in which they are found. It is particularly associated with mica schist.

Uses

Andalusite is used as a refractory in furnaces, kilns and other industrial processes. South Africa possesses by far the largest portion of the world's known andalusite deposits.

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See also

References
  1. Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
  2. http://rruff.geo.arizona.edu/doclib/hom/andalusite.pdf Handbook of Mineralogy
  3. Delamétherie, Jean Claude (1798). "Sur une pierre de l'Andalousie". Journal de Physique, de Chimie d'Histoire Naturelle et des Arts. 46: 386–387.
  4. Calvo, Miguel (2018). Minerales y Minas de España. Vol. IX. Silicatos. Madrid, Spain: Escuela Técnica Superior de Ingenieros de Minas de Madrid. Fundación Gómez Pardo. pp. 91–94. ISBN 978-84-8321-883-9.
  5. Whitney, D.L. (2002). "Coexisting andalusite, kyanite, and sillimanite: Sequential formation of three Al2SiO5 polymorphs during progressive metamorphism near the triple point, Sivrihisar, Turkey". American Mineralogist. 87 (4): 405–416. Bibcode:2002AmMin..87..405W. doi:10.2138/am-2002-0404.
  6. Whitney, D.L. (2002). "Coexisting andalusite, kyanite, and sillimanite: Sequential formation of three Al2SiO5 polymorphs during progressive metamorphism near the triple point, Sivrihisar, Turkey". American Mineralogist. 87 (4): 405–416. doi:10.2138/am-2002-0404.
  7. Whitney, D.L. (2002). "Coexisting andalusite, kyanite, and sillimanite: Sequential formation of three Al2SiO5 polymorphs during progressive metamorphism near the triple point, Sivrihisar, Turkey". American Mineralogist. 87 (4): 405–416. doi:10.2138/am-2002-0404.
  8. Calvo, Miguel (2016). "El "lapis crucifer", "piedra de cruz de Compostela": un elemento importante de los patrimonios geológico y cultural del NW de España". De Re Metallica. 6: 67–79.
  9. "International Colored Gem Association: Anadalusite". Archived from the original on 2006-07-17. Retrieved 2006-07-13.
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