Böhmite

bohmite

kaolinite

gibbsite

diaspore

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Formula: AlO(OH)
Oxide containing hydroxyl, a major constituent of the aluminium ore bauxite, paramorph of diaspore (both minerals are orthorhombic)
Specific gravity: 3.03
Hardness: 3½
Streak: White
Colour: White to grey
Environments:

Pegmatites
Hydrothermal environments

Böhmite is formed from aluminosilicates during tropical weathering, commonly a major constituent of bauxite, laterite or fire clay; it is a late product of low-temperature hydrothermal decomposition of corundum, in nepheline pegmatites, syenite and ocean-ridge basalt. Associated minerals include kaolinite, gibbsite and diaspore, in bauxite, and nepheline, gibbsite, diaspore, natrolite and analcime in nepheline pegmatites (HOM, Mindat).

Localities

At lots 10 and 11 of concession 1, Bathurst Township, Lanark County, Ontario, Canada (DeWitts corner), the deposit is located in the Grenville Geological Province, which consists mostly of marble, gneiss, and quartzite. Syenite-migmatite was also reported in the area where the vein-dikes are located. Characteristic features of the vein-dikes include the fact that perfectly formed euhedral crystals of different minerals can often be found floating in calcite with no points of contact with the walls. Sometimes these crystals have inclusions of calcite, irregular or rounded in shape. It has been argued that at least some of the vein-dikes were formed as a result of melting of Grenville marble.
Böhmite is found as thin yellow to cream-coloured crusts on spinel (R&M 97.6.556-564).

At the type locality, Mas Rouge, Les Baux-de-Provence, Bouches-du-Rhône, Provence-Alpes-Côte d'Azur, France, in the bauxite deposit böhmite is associated with kaolinite, gibbsite and diaspore (Mindat).

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