Hyroxides

Hydroxides are a common mineral group that is formed by the weathering and hydration of other minerals. They area combination of a metallic element and water (H2O) or hydroxyl (OH). They are often found as aggregates of fine-grained material that includes other minerals and clays. Two hydroxide minerals, in particular, are important because their structure serves as a starting point for describing silicates. Both gibbsite and brucite have two sheets of OH- anions, in between which is a layer of cations (Mg or Al). The result is a series of neutrally charged octahedral sheets that are held together by relatively weak electrostatic bonds.
- Iron hydroxides are most commonly produced through weathering and hydrothermal alteration. They may often appear to be a yellowish brown or rust colored stain in weathered rocks. In fact, they're very fine-grained goethite, lepidocrocite, or hematite crystals that have formed through the weathering of biotite, amphibole, proxene, magnetite, and other iron bearing minerals. Through the heavy weathering of iron bearing materials lateritic soils are formed which are rich in iron hydroxide minerals.
- Aluminum hydroxides are often found in soils and emery deposits as weathering products of corundum, the aluminum silicates, and other aluminum bearing minerals. The most common aluminum hydroxides are diaspore, boehmite, and gibbsite. The term bauxite is often used when referring to aluminum hydroxides. It, however, consists of a mixture of minerals of uncertain identity of a rock type that contains a large amount of aluminum hydroxides. Bauxite forms in old, tropical soils that have been subject to intense weathering. As a result, it is often impossible to distinguish aluminum hydroxide minerals from the surrounding clays and other fine-grained minerals in a hand sample or thin section.

Continue...