böhmite

{{Infobox mineral | name = Boehmite | category = Oxide mineral | image = Böhmite-89904.jpg | imagesize = 260px | alt = | caption = Böhmite and Natrolite from Sagåsen (Strandåsen), Mørje, Porsgrunn, Telemark, Norway (Field of view 10 mm) | formula = γ-AlO(OH) | IMAsymbol=Bhm | molweight = | strunz = 4.FE.15 | dana = 6.1.2.1 | system = Orthorhombic | class = Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) | symmetry = Amam | unit cell = a = 3.693 Å, b = 12.221 Å, c = 2.865 Å; Z = 4 | color = White, pale greyish brown; yellowish or reddish when impure; colorless in thin section | colour = | habit = Tabular crystal rare, fine grained in pisolitic aggregates or disseminated | twinning = | cleavage = Very good on {010}, good on {100}, and poor on {001} | fracture = Uneven | tenacity = Brittle | mohs = 3.5 | luster = Vitreous, pearly on {010} | streak = White | diaphaneity = Translucent | gravity = 3.02–3.05 | density = | polish = | opticalprop = Biaxial (+) | refractive = nα = 1.644 – 1.648 nβ = 1.654 – 1.657 nγ = 1.661 – 1.668 | birefringence = δ = 0.017 – 0.020 | pleochroism = | 2V = Measured: 74° to 88°, Calculated: 80° | dispersion = weak | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | 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.