Boehmite

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Boehmite
Böhmite-89904.jpg
Böhmite and Natrolite from Sagåsen (Strandåsen), Mørje, Porsgrunn, Telemark, Norway (Field of view 10 mm)
General
Category Oxide mineral
Formula
(repeating unit)
γ-AlO(OH)
Strunz classification 04.FE.15
Dana classification 6.1.2.1
Crystal symmetry Orthorhombic 2/m 2/m 2/m
Unit cell a = 3.693 Å, b = 12.221 Å, c = 2.865 Å; Z = 4
Identification
Color White, pale greyish brown; yellowish or reddish when impure; colorless in thin section
Crystal habit Tabular crystal rare, fine grained in pisolitic aggregates or disseminated
Crystal system Orthorhombic dipyramidal
Cleavage Very good on {010}, good on {100}, and poor on {001}
Fracture Uneven
Tenacity Brittle
Mohs scale hardness 3.5
Luster Vitreous, pearly on {010}
Streak White
Diaphaneity Translucent
Specific gravity 3.02 - 3.05
Optical properties Biaxial (+)
Refractive index nα = 1.644 - 1.648 nβ = 1.654 - 1.657 nγ = 1.661 - 1.668
Birefringence δ = 0.017 - 0.020
2V angle Measured: 74° to 88°, Calculated: 80°
Dispersion weak
References

[1][2][3]

[4]

Boehmite or böhmite is an aluminium oxide hydroxide (γ-AlO(OH)) mineral, a component of the aluminium ore bauxite. It is dimorphous with diaspore. It crystallizes in the orthorhombic dipyramidal system and is typically massive in habit. It is white with tints of yellow, green, brown or red due to impurities. It has a vitreous to pearly luster, a Mohs hardness of 3 to 3.5 and a specific gravity of 3.00 to 3.07. It is colorless in thin section, optically biaxial positive with refractive indices of nα = 1.644 - 1.648, nβ = 1.654 - 1.657 and nγ = 1.661 - 1.668.

Boehmite occurs in tropical laterites and bauxites developed on alumino-silicate bedrock. It also occurs as a hydrothermal alteration product of corundum and nepheline. It occurs with kaolinite, gibbsite and diaspore in bauxite deposits; and with nepheline, gibbsite, diaspore, natrolite and analcime in nepheline pegmatites.[3]

It was first described by J. de Lapparent in 1927 for an occurrence in the bauxites of Mas Rouge, Les Baux-de-Provence, France, and named[5] for the Bohemian-German chemist Johann Böhm (1895–1952) who carried out X-ray studies of aluminium oxide hydroxides in 1925[6] (and not for the German geologist Johannes Böhm (1857–1938) as often stated).[1][2]

References[edit]

  1. ^ a b "Boehmite". Webmineral data. Retrieved 2010-07-10. 
  2. ^ a b "Boehmite". Mindat with location data. Retrieved 2010-07-10. 
  3. ^ a b Mineral Data Pub. Handbook of Mineralogy
  4. ^ "The Mineral Boehmite". minerals.net. Retrieved June 10, 2014. 
  5. ^ http://www.springerlink.com/content/u2005q723h61km44/
  6. ^ http://onlinelibrary.wiley.com/doi/10.1002/zaac.19251490114/abstract