|How are metals found in nature?|
Metals are an integral part of our planet and are found in almost all rocks and soils. Most metals form compounds, called minerals, which are naturally occurring, inorganic solids with regular chemical compositions and crystal structures. Although most metal-bearing mineral compositions comprise several elements, there are a few exceptions such as gold, which is found in its elemental form as a mineral called native gold.
Metals can form, or be part of, many different minerals. The number of metals (over 70 in the periodic table) and their compounds results in an enormous array of minerals. Iron, for instance, which is very abundant in nature, is found in over 1100 minerals*. The brilliant colours frequently associated with gems such as emerald, ruby and sapphire reflect the variety of metal-containing minerals. Chalcopyrite, an important copper-bearing mineral, is bright yellow, while the copper-phosphate mineral, turquoise, has a blue colour.
Minerals combine to become the rocks that make up our planet. Most rocks form considerably below the surface of the earth under the influence of pressure and heat. Geologic processes can cause them to move upward toward the surface. There, in the presence of oxygen and water, they break down, releasing elements - including metals - into solutions, and forming new minerals. This process, known as weathering, forms our soils. From soil, metals are taken up by plants and then by animals and humans in food. As soils are eroded, metal-bearing sediment is carried into streams and rivers, and eventually into the ocean. These sediments contribute to new rocks through ongoing geologic processes.
Metals are ubiquitous in nature, and their distribution in the earth depends on geologic processes that have taken place. Some processes may form minerals with high metal contents; rocks containing these minerals may be so enriched that they can be mined at a profit - becoming ore deposits. Rocks that contain lower enrichments are known simply as mineral deposits. The metal content of deposits can range from a few parts per million (ppm) to as much as 650,000 ppm (65%) in the case of some iron ores. Mining companies employ special technologies to extract metals from complex ores in the production of pure metals such as iron, aluminum, copper and gold.
While high concentrations of metals may lead to the formation of deposits, in many cases where the concentration of a metal is low, the metal may simply replace, or substitute for, another element in the crystal structure of common minerals. For example, rocks that make up the sea floor contain high concentrations of the metal magnesium, as well as smaller concentrations of nickel which substitute for some of the magnesium. Similarly, rocks that make up the continents can contain lead, which substitutes for the more abundant metallic element, potassium. This substitution phenomenon leads to the wide distribution of many metals at low concentrations throughout the rocks of the earth.
Natural geologic processes continue at a very slow pace to concentrate and disperse metals, forming large zones of elevated metal concentrations and constantly releasing metals into the environment. A full understanding of these natural processes and the resulting metal dispersion patterns is important in the discovery and recovery of metals and for determining the impact of metals in the environment.