They are the minerals
that combine their cations with a hydroxyl (OH) anion.
They form as
a result of a
chemical reaction between water and an
oxide - that is, they are weathering products of oxide
are usually softer and less dense than oxides, and like oxides, they
are often important ore minerals.
Gibbsite. By Mary Harrsch via Flickr.com
Gibbsite is an aluminium
hydroxide. It is an alteration product of aluminium silica
minerals after the silica has leached out. This happens through
chemical weathering mainly in warm and wet conditions (in tropical
environments). It can also be formed deep under the surface in
hydrothermal veins. It can form tabular crystals or be massive. It is
most often white but can be coloured by impurities.
Diaspore. By fluor_doublet via Flickr.com
Diaspore is also a hydrated aluminium oxide.
It can form elongated, platy, acicular or tabular crystals; or be
granular or massive. It can be white, brownish, greyish, greenish,
yellowish, pink or purple. It is relatively widespread and found in marbles
and altered igneous
associated with corundum, chlorite, dolomite, spinel and magnetite.
mixture of minerals such as diaspore, boehmite, gibbsite,
and some aluminium
oxides. It is most often massive in habit and has a reddish-brownish
colour. It forms as a weathering product of aluminium silicates
process often taking place in tropical conditions.
Goethite is a hydrated iron oxide.
It is reddish brown to black in colour and can form prismatic, striated
crystals, but is most often earthy, massive, or botryoidal in habit. It
forms as a weathering product by oxidation of iron minerals such as
magnetite, siderite and pyrite. It becomes magnetic when heated.
Limonite. By Eelco
Limonite is a hydrated iron oxide.
It is a mineral like bauxite, which could actually be called a rock
because it contains many different minerals. Limonite can contain
goethite, lepidocrocite and unidentified iron oxides and hydroxides. It
does not form crystals and is usually massive in habit. It is reddish
yellow, orange or brownish in colour. It forms in oxidised iron
Brucite. By Kotomi_ via Flickr.com
Brucite is a magnesium
hydroxide. It is white when pure but can be grey, bluish or pale green;
or coloured reddish-yellow by impurities. It can form tabular crystals
or be massive or fibrous in habit. It is transparent, and vitreous or
waxy. It forms in low-temperature hydrothermal veins, and in metamorphic
such as schist.
Manganite. By arcticpenguin via Flickr.com
Manganite is a hydrated manganese oxide.
It is a widespread mineral and an important manganese ore. It can form
prismatic crystals (often twinned), or be massive, granular, columnar
or fibrous in habit. It is black or dark grey in colour. It forms in
low temperature hydrothermal veins, in shallow marine or freshwater
environments, and in replacement deposits.