Sources of metalsMost metals are reactive and therefore are not found in the Earth’s crust, as elements, but rather present combined with other elements as compounds. Examples of uncombined elements include gold, platinum and silver. The reactivity of a given metal with substances in the lithosphere, atmosphere and hydrosphere determines the likelihood of finding the metal in the Earth’s crust in the uncombined elemental form. Most metals are reactive and are therefore usually found combined with other elements in naturally occurring compounds called minerals. Highly reactive metals such as potassium and sodium are never found as the free element but unreactive metals such as gold are much more likely to be found as the free element. Minerals are naturally occurring substances, usually compounds, with a definite composition or a range of compositions and crystal structures. They usually contain metals in the form of oxides, carbonates, silicates, sulfides and sulfates. Silicates are the most common class of minerals, but it is very difficult to extract metals from these compounds. Occurrence of metals in the earth’s crust Gold, platinum and silver are almost always found in the free elemental form, while metals such as mercury, copper and lead are occasionally found in the free elemental form. Metals such as tin, nickel, iron and zinc are rarely found in the free elemental form, while metals such as aluminium, magnesium, calcium, sodium and potassium are never found in the free elemental form. In order to obtain metals in their elemental form they must be extracted from their minerals, as they usually occur in nature as mixtures with other minerals. Factors affecting the use of metals Their uses are determined by their physical and chemical properties. Other factors that affect the commercial price and extent to which particular metals are used include: the abundance of the metal in the ore deposit, and in the Earth’s crust the cost of extraction of the metal which is dependent on how accessible the mineral is and how much energy input is required to extract the pure metal the demand for the metal More important than the overall abundance is the occurrence of ore deposits that contain a sufficient proportion of a particular mineral to make the mining of the ore and extraction of the metal economical. The cost of mining an ore and metal extraction is another important factor that affects the uses of metals. Despite being the most abundant metal in the Earth’s crust, aluminium was once so expensive to extract that its use was severely limited, even though it had very many useful properties. As extracted methods improved, metals became easier and cheaper to extract, resulting in more metals being available today than 200 years ago. Extraction of metals from ores: If there is metal present in sufficient quantity to make the mining and extraction of metal economically viable, it is called an ore. An ore is therefore defined as a naturally occurring deposit that is a mixture of minerals from which the substance usually a metal can be economically extracted. The mineral in the ore requires energy to extract to be separated from the other rocks. The pure metal requires energy to break the bonds between each element. This energy input adds to the cost of producing the pure metal.