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 Mg 2+, the Aluminium ion Al 3+ and the Silicon ion Si 4+ combine with the Oxygen O2- ion to form Cu2O (Cuprite), MgO (Periclase), Al2O3 (Corundum) and SiO2 (Silica) respectively.

As a rule, the chemical bond is strongly ionic, and hence most primary 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 large O ions with the small matal ions contained in the spaces between. Primary oxides usually form deep in the crust directly from molten rock which has crystallised. Examples include Corundum var. Ruby whose hardness and transparency makes it a very tough and attractive gemstone.

The commonest Oxides, however, are those that form from other minerals (like the sulfides) by weathering and oxidation near the surface. They frequently form thick "rusts" which protect unreacted material laying beneath. Significantly these secondary oxides may also contain water as well as Oxygen and are thus soft, opaque and of low density.

The Oxide group is therefore one of the most varied mineral classes in chemical and physical properties - ranging from very soft to very hard, transparent to opaque, soluble and insoluble. Notably, some metals, including 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.

The Hydroxides are similar to oxides. In their case the metals are combined with the hydroxyl ion (OH)- rather than O2-. Hydroxides easily dissociate on heating. The (OH)- ions are driven off as water leaving behind the metal oxide. The structure of Hydroxides is strongly dependent on the radii of the constituent ions.

The Oxides and Hydroxides are divided into the following groups:

• Periclase Group - includes all oxides that have bivalent (two positive charges) metallic ions and the halite structure. In this arrangement each metallic ion is shielded by siz oxygen ions around it. Only metallic ions of medium size adopt this structure. Smaller ions are surrounded by four oxygen ions and larger ones by six. In oxides where the bonding is more covalent the crystal structures are determined by both the configuration of the electrons and the size of the ion. Periclase minerals, like most oxides, are chemically reactive and therefore found in unusual environments. For example Manganosite MnO forms in marbles from the breakdown of dolomite. Amongst minerals in this group only Periclase is common.
• Zincite Group - includes Zincite and Bromellite. Both are rare. The Zn2+ or Be2+ are small enough to be adequately shielded by four oxygen ions. Each oxygen in turn is surrounded by four Zn or Be ions. The resulting hexagonal crystal adopts a hemimorphic habit. In other words, the two terrminations have differing faces, indicating a lacking centre of symmetry. Bromellite is much harder than Zincite because the smaller Be ion forms a more strongly covalent bond with its neighbouring oxygens.
• Cuprite Group - in this group the metal is surroundeb by only two oxygen ions. Minerals, including Cuprite, crystallise in the cubic system, often also forming octahedra, dodecahedra or a combination of all three. Because the bonds with oxygen have a large degree of covalency the minerals tend to be brittle.
• Corundum Group - also commonly called the Hematite Group. Includes the simple oxides Corundum (Al2O3), Hematitie (Fe2O3) and other oxides with the general formula ABO3. All have hexagonal structures with the metal surrounded by six oxygen ions. The metals may be trivalent (eg. Al3+) or may be a mixture of bivalent and tetravalent metals such as Fe2+ and Ti4+, as in Ilmenite. The possibility of metal substitution means that the group is prone to form solid solutions . An example is the Ilmenite- Pyrophanite series. Members of the group typically form at high temperatures in igneous rocks or during high-temperature metamorphism of silica-deficient rocks. Hematite and Corundum are the only common species.
• Spinel Group - probably the largest and most complex group of oxides. Many exist as members of several solid solution series. They are mixed oxides containing a combination of metals with a general formula AB2O4. Some metals are stabilised by four oxygen atoms in a tetraherdal structure whilst others are stabilised by six in an octahedral arrangement. Still others can occur in both these positions. The oxygen ions are held in a cubic close packed arrangment with the metals. The combinations gives rise to very complex crystal lattice structures. Most minerals in the group form at high temperature in igneous and metamorphic rocks, usually those lacking silica.
• Rutile Group - includes all oxides with the general formula MO2, in which the metal is tetravalent (carries four positive charges). The metal is surrounded by six oxygen ions, giving rise to the rutile structure. Typically the metals involved are titanium, manganese, tin and lead. However, because of the significant differences between these metals their oxides do not form solid solutions and the minerals have widely different origins. Brookite, Anatase and Rutile are polymorphs of titanium dioxide (TiO2) having the same composition but differening arrangement of ions in the rutile structure.
• Uraninite Group - members include Uraninite and Thorianite, oxides of uranium and thorium respectively. They have cubic structures resembling Fluorite with each metal ion surrounded by eight oxygen ions. Because of the chemical and physical similarity of the metals they form a complete solid solution series, producing a wide range of minerals of intermediate composition. Nearly all crystallise in the cubic system as octahedra, cubes or combinations.
• Diaspore Group - includes oxyhydroxides of trivalent metals including Al3+, Fe3+ and Mn3+. The general chemical formula is MO(OH). Each metal ion is surrounded by six negative ions, three O2- and three (OH)-. Group minerals are important sources of key industrial metals and are associated with other hydrous minerals. Some, like Bauxite, do not form crystals but occur as noncrystalline colloidal precipitates. Bauxite is usually the result of prolonged weathering of aluminous rocks where the silica content has dissolved leaving behind the aluminium hydroxide as residue.
• Brucite Group - includes the hydroxides of divalent metals including Magnesium Mg2+ and Manganese Mn2+. The arrangement is octahedral with the metal at the centre. The structure is usually layered, consisting of six hydroxyl (OH)- ions surrounding the metal ion. The layers are stacked upon each other and held together by weak hydrogen bonds. The minerals therefore adopt plate-like habits, have very pronounced cleavage and are very soft.