Chemistry XI

Metals occur in nature sometimes in free or native state, but most of these occur in nature in the form of chemical combination, i.e., in the form of their stable compounds which are associated with gangue or matrix. The earthy impurities such as sand, clay, rocks etc. associated with ore are collectively known as gangue or matrix. Thus a large number of metals in nature occur in the combined form along with other elements, but some metals, such as Ag, Au, Pt etc. occur in the native form (metallic state) in some regions. Ag occurs in native (or free) as well as in the form of compounds. The natural material in which the metal or their compounds occur in the earth is known as mineral.

Thus a mineral is a naturally occurring material present in earth’s crust which contains metal in the native (or free state) or in combined state. A mineral may be a single compound or a complex mixture of various compounds. When a mineral contains sufficient amount of a metal in combined state, from which it can be readily and profitably separated on commercial scale, then the mineral is said to be an ore of the metal. Thus all ores are minerals, but all minerals are not ores. A mineral from which a metal can be profitably extracted is called an ore.

For example, clay (Al2O3 2 SiO2 2 H2O) and bauxite (Al2 O3 2H2O) are two minerals of aluminium, but aluminium, can be profitably extracted only from bauxite and not from the clay. Hence bauxite is an ore of aluminium, while clay is a mineral. The biggest source of metal is the earth’s crust and the process of taking out the ores from the earth crust is called mining.

In the combined state ores are generally found in the form of oxides, sulphides, carbonates, sulphates, chlorides and silicates. The important ores are given in Table 2.1.
Table 2.1 Classification of ores