Boehmite

Boehmite
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	4.FE.15
Dana classification	6.1.2.1
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Amam
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
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

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.

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 for the Bohemian-German chemist Johann Böhm (1895–1952) who carried out X-ray studies of aluminium oxide hydroxides in 1925 (and not for the German geologist Johannes Böhm (1857–1938) as often stated).