Mineral Information and Data:

AlO(OH) [CNMNC approved formula]

Named for Johannes Böhm (1857–1938), German geologist, who first studied the species.

IMA status : Valid species (Pre-IMA 1927) Grandfathered
(8th edition) :  
  (06) Hydroxides and Oxides Containing Hydroxyl
  (06.01) where X+++O OH
  (06.01.02) Boehmite Group
(10th edition) :
  (04) Oxides and Hydroxides
  (04.F) Hydroxides (without V or U)
  (04.FE) Hydroxides with OH, without H2O; sheets of edge-sharing octahedra
Crystal system: Orthorhombic System
Point group (H-M): mmm (or 2/m 2/m 2/m) — dipyramidal
Space group: Amam
Unit cell: a = .693(1)Å, b = 12.221(2)Å, c = 2.865(1)Å, Z=4
Crystal Habit: Rarely as tabular to stout prismatic crystals, to 2 mm, showing {001}, {110}, {101}, {111}, {221}, and in groups; commonly extremely fine-grained, in pisolitic aggregates or disseminated.
Color: White, pale grayish brown; colorless in thin section
Diaphaneity: Semitransparent
Luster: Vitreous, pearly on {010}
Hardness (Mohs): 3.5
Measured Density: 3.02 – 3.05 g/cm3
Cleavage: {010}, very good; {100}, well-developed; {001}, poor
Polymorphism and Series: Dimorphous with diaspore.
Geologic Setting: Formed from aluminosilicates during tropical weathering, commonly a major constituent of bauxites, laterites, or fireclays; a late product of low-temperature hydrothermal decomposition of corundum, nepheline in nepheline pegmatites, syenites, or ocean-ridge basalts.
Mineral Association: Kaolinite, gibbsite, diaspore (bauxites); nepheline, gibbsite, diaspore, natrolite, analcime (nepheline pegmatites)
Ore deposits: A major constituent of the aluminium ore Bauxite - mined from rocks that have been exposed to weathering in a tropical environment.
Synonyms/varieties: Boehmite, bohmeite


Flattened dipyramidal crystals up to 1 mm in length,
usually in intergrown aggregates, in vugs in fibrous natrolite
Origin: Saga 1, Tvedalen, Norway
Sample size: micromount
Photo: Diederik Visser