Oxides, Natural

a class of minerals that consists of naturally occurring chemical compounds in which elements are combined with oxygen, a hydroxyl group (in hydroxides), or both O and OH. A natural oxide may include as many as 40 cationic elements, most of which are lithophile, for example, Si, Ti, Nb, Ta, Mn, Al, Mg, Sn, and Zr; many natural oxides, however, are formed from chalcophile and siderophile elements.

Natural oxides can be divided into the following types: (1) simple oxides, for example, silica minerals, cuprite (CU₂O), corundum (Al₂O₃), hematite (Fe₂O₃), and cassiterite (SnO₂); (2) multiple oxides, which are mixed isodesmic oxides whose crystalline structure is made up of oxygen and one or two metal atoms with different oxidation numbers, for example, magnetite (Fe²⁺Fe₂³⁺O₄) and ilmenite (Fe²⁺Ti⁴⁺O₃); (3) a special group of compounds that are formed from Nb, Ta, and Ti, for example, columbite [(Fe, Mn)²⁺(Ta, Nb)₂⁵⁺O₆], brannerite (U⁴⁺Ti₂⁴⁺-O₆), and perovskite (CaTiO₃); (4) hydroxides, for example, brucite [Mg(OH)₂] and gibbsite [Al(OH)₃]; (5) and oxides that contain a hydroxyl group and oxygen, for example, boehmite [AIO(OH)]. Within the last type, hydrogen may form the proton H⁺ with coordination number 2, usually between two oxygen atoms, as in geothite (FeOOH) and diaspore (AIOOH). Molecular water in the form of a solid solution is known to occur in certain natural oxides, for example, hydrogoethite (FeOOH·nH₂O). The crystalline structure of many natural oxides is coordinated, as in corundum, NaCl, and fluorite; chainlike, as in rutile; or skeletal, as in quartz and anatase.

The formation of natural oxides is primarily associated with supergene and lithogene processes on land and in shallow coastal waters. An environment with high oxygen and water contents and a high oxidation potential is typically conducive to natural oxide formation. Hydroxides are converted into simple or multiple oxides during the metamorphism of rocks, for example, limonite alters to hematite and magnetite, and diaspore to boehmite and corundum. Furthermore, a number of simple and multiple oxides, including chromite, magnetite, ilmenite, cassiterite, uraninite, and columbite, are formed upon solidification of magma and crystallization of minerals from residual melts and from solutions primarily in pegmatites and hydrothermal veins. Many natural oxides are commercially valuable metalliferous and nonmetalliferous minerals, for example, the oxide ores of Mn, Fe, and Al.