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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)
Crystal System: Orthorhombic
Specific gravity: 3.02 to 3.05 measured, 3.08 calculated
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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