The earth’s crust is the main source of metals. The occurrence of metal in native or in combined state in the earth’s crust along with a number of rocky and other impurities depends upon the chemical nature of metals.
Metals having less electropositive character have less affinity for oxygen, moisture and occur in free or metallic or native state i.e., in uncombined state e.g. Au, Pt , Ag etc.
On the other hand metals with higher electropositive character occurs in combined state i.e., as compounds.
The compound of a metal found in nature is called a mineral. A mineral may be a single compound or a complex mixture. Those minerals from which metal can be economically extracted are called ores.
Thus all ores are minerals but all minerals are not ores. For e.g. copper occurs in nature in the form of several minerals like Cu2O, Cu2S, CuFeS2 , but copper pyrites is considered as the most economical mineral for the extraction of the metal. Hence copper pyrites is the chief ore of copper.
Ores may be divided into four groups
(i) Native Ores: These ores contain the metal in free state eg. Silver gold etc. These are usually formed in the company of rock or alluvial impurities like clay, sand etc.
(ii) Oxidised Ores: These ores consist of oxides or oxysalts (eg. carbonates, phosphate) and silicate of metal. Important oxide ore includes, Fe2O3 , Al2O3.2H2O etc. and important cabonate ores are limestone (CaCO3), Calamine (ZnCO3) etc.
(iii) Sulphurised Ores: These ores consist of sulfides of metals like iron, lead, mercury etc. Examples are iron pyrites (FeS2). galena (PbS), Cinnabar (HgS)
(iv) Halide ores: Metallic halides are very few in nature. Chlorides are most common examples include horn silver (AgCl) carnallite KCl. MgCl2.6H2O and fluorspar (CaF2) etc.