Boehmite or böhmite is an aluminium oxide hydroxide (γ-AlO(OH)) mineral, a component of the

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

ore

bauxite Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO ...

. It is dimorphous with

diaspore Diaspore , also known as diasporite, empholite, kayserite, or tanatarite, is an aluminium oxide hydroxide mineral, α-AlO(OH), crystallizing in the orthorhombic system and isomorphous with goethite. It occurs sometimes as flattened crystals, but ...

. It crystallizes in the

orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with ...

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 The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by ...

of 3 to 3.5 and a

specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its dens ...

of 3.00 to 3.07. It is colorless in thin section, optically biaxial positive with

refractive indices In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

of nα = 1.644 - 1.648, nβ = 1.654 - 1.657 and nγ = 1.661 - 1.668. Boehmite occurs in tropical

laterite Laterite is both a soil and a rock type rich in iron and aluminium and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. They develop by ...

s and bauxites developed on alumino-silicate bedrock. It also occurs as a

hydrothermal alteration Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical co ...

product of

corundum Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the pr ...

and

nepheline Nepheline, also called nephelite (), is a rock-forming mineral in the feldspathoid groupa silica-undersaturated aluminosilicate, Na3 K Al4 Si4 O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmati ...

. It occurs with

kaolinite Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahed ...

gibbsite Gibbsite, Al(OH)3, is one of the mineral forms of aluminium hydroxide. It is often designated as γ-Al(OH)3 (but sometimes as α-Al(OH)3.). It is also sometimes called hydrargillite (or hydrargyllite). Gibbsite is an important ore of aluminiu ...

and

in bauxite deposits; and with nepheline, gibbsite, diaspore, natrolite and analcime in nepheline

pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silici ...

s. Industrially, it is used as an inexpensive

flame retardant The term flame retardants subsumes a diverse group of chemicals that are added to manufactured materials, such as plastics and textiles, and surface finishes and coatings. Flame retardants are activated by the presence of an ignition source and ...

additive for

fire-safe polymers Fire-safe polymers are polymers that are resistant to degradation at high temperatures. There is need for fire-resistant polymers in the construction of small, enclosed spaces such as skyscrapers, boats, and airplane cabins.Zhang, H. ''Fire-Safe P ...

. It was first described by J. de Lapparent in 1927 for an occurrence in the bauxites of Mas Rouge,

Les Baux-de-Provence Les Baux-de-Provence (; oc, label= Provençal, Lei Bauç de Provença), commonly called Baux, is a commune in the Bouches-du-Rhône department in the Provence-Alpes-Côte d'Azur region of Southern France. It is located in the Alpilles mountai ...

France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...

, 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).

References

Steven Dutch, 2002, Lepidocrocite and Boehmite Structure, University of Wisconsin - Green Bay
{{Authority control Hydroxide minerals Aluminium minerals Orthorhombic minerals Minerals in space group 63

See also

References