Open Adit™ - Oxides

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Oxides are compounds that combine metals or semi-metals with oxygen. The simple oxides have only one metal, which combines with oxygen in proportion to balance the charges on their ions. Thus the copper ion Cu⁺, the magnesium ion Mg²⁺, the aluminium ion Al³⁺ and the silicon ion Si⁴⁺ combine with the oxygen O^2- ion to form Cu₂O (cuprite), MgO (periclase), Al₂O₃ (corundum) and SiO₂ (silica) respectively. As a rule, the chemical bond is strongly ionic, and hence Oxides are characterised by great hardness and high density. This is helped by the fact that the minerals are normally built up as close packings of the large O ions with the small matal ions contained in the spaces in between. Some of the rarest gems like rubies are simple oxides

The primary oxides form deep in the crust, most often when liquid rock is solidifying. The most common oxides, however are those that form from other minerals (like the sulfides) by weathering on the surface. These often form thick "rusts" protecting the still unreacted underlying rocks. Significantly these secondary oxides often also contain water as well as oxygen and are thus soft and of low density.

The oxides class is pehaps the most varied in it's chemical properties - ranging from very soft to very hard, transparent to opaque, soluble and insoluble. Notably, some metals, like silver and gold, do not combine chemically with oxygen and thus have no minerals in this class.

Mixed oxides have more than one type of metal ion, examples being MgAl₂O₄ (spinel) and FeTiO₃ (ilmenite). Significantly these have more complex structures than simple oxides and are usually much less common.

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