The natural substances in which the metal or their compounds occurin the earth are called minerals. The mineral has a definite composition. Itmay be a single compound or complex mixture. The minerals from whichthe metals can be conveniently and economically extracted are knownas ores. All the ores are minerals but all minerals cannot be ores, e.g.,both bauxite (Al2O3.xH2O) and clay (Al2O3.2SiO2.2H2O) are minerals ofaluminium. It is bauxite which is used for extraction of aluminium andnot clay. Thus bauxite is an ore of aluminium.