Hydroxides

Brucite

Hexagonal.
Commonly foliated, massive.
H = 2.5
SG = 2.39
Transparent to translucent.
Pearly (base) and vitreous to waxy (elsewhere) luster.
Color = white, gray, light green.
Associated with serpentine, dolomite, magnesite, and chromite as an alteration product of Mg-silicates.

Gibbsite

One of the 3 hydroxides of Al that are the main constituents of bauxite.
Same structure as brucite.

Romanechite

Orthorhombic.
Massive, botryoidal, stalactitic; appears amorphous.
H = 5-6
SG = 3.7-4.7
Submetallic luster.
Color = black.
Streak = brownish black.
Transforms to holandite (Ba2Mn8O16) at 600 degrees Celsius.
Occurs along pyrolusite.

Psilomelane

Mixture of Mn oxides.
Romanechite + Cryptomelane + Manjiroite + Todorokite.

Diaspore

Orthorhombic.
Bladed, foliated, massive, disseminated.
H = 6.5-7
SG = 3..35-3.45
Transparent to translucent.
Vitreous and pearly (cleavage face) luster.
Color = white, gray, yellowish, greenish.
Associated with corundum, dolomite, and chlorite schist.

Goethite

Orthorhombic.
Acicular, massive, reniform, stalactitic, in radiating fibrous aggregate (bog ore).
H = 5-5.5
SG = 4.37
Sub-translucent.
Adamantine to dull, silky (some scaly varieties) luster.
Color = yellowish-brown to dark brown.
Streak = yellowish brown.
Gossan over metalliferous veins.

Lepidocrosite

Polymorph of goethite.
Platy mineral associated with goethite.
Isostructural with boehmite.

Bauxite

Mixed crystallography.
Pisolitic, massive, earthy, claylike.
Gibbsite + diaspore + boehmite.
H = 1-3
SG = 2-2.55
Translucent.
Dull to earthy luster.
Color = white, yellow, gray, red.
Supergene origin.