NATIVE ELEMENT

any of a number of chemical elements that may occur in nature uncombined with other elements. The elements that occur as atmospheric gasses are excluded. Of the 92 chemical elements found in nature only 19 are known to occur as minerals. These native elements are commonly divided into three groupsnamely, metals (platinum, iridium, osmium, iron, zinc, tin, gold, silver, copper, mercury, lead, chromium); semimetals (bismuth, antimony, arsenic, tellurium, selenium); and nonmetals (sulfur, carbon). In metals the mineral structure is usually either cubic close-packed or hexagonal close-packed. The semimetals and nonmetals have more complex structures. Several native elements (e.g., carbon) have one or more polymorphic forms whose occurrence depends on the conditions of formation. It is virtually impossible to make generalizations as to the occurrence of the native elements. They form under greatly contrasting physicochemical conditions and in all types of rocks. Even a single native element can occur in widely diverse environments. Native iron ( kamacite), for example, is found primarily in meteorites. The iron meteorites called hexahedrites are almost completely composed of kamacite, and in those called octahedrites it is the principal constituent. Although terrestrial native iron is a great rarity, it has been found in igneous rocks (basalts), in carbonaceous sedimentary rocks, and in petrified wood. Many of the other metals and certain non-metals are sufficiently abundant to form deposits of commercial importance. Native gold and silver, for example, are the principal ores of these metals.