Fer natif mis en évidence par polissage dans le trapp basaltique sur le plateau Putoran, Sibérie (âge de la limite permien-trias), détail
Native terrestrial iron in basalt/dolerite from the ~Permian-Triassic boundary of Russia. (cut surface, 1.5 cm across) A mineral is a naturally-occurring, solid, inorganic, crystalline substrance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates. Elements are fundamental substances of matter - matter that is composed of the same types of atoms. At present, 118 elements are known (four of them are still unnamed). Of these, 98 occur naturally on Earth (hydrogen to californium). Most of these occur in rocks & minerals, although some occur in very small, trace amounts. Only some elements occur in their native elemental state as minerals. To find a native element in nature, it must be relatively non-reactive and there must be some concentration process. Metallic, semimetallic (metalloid), and nonmetallic elements are known in their native state as minerals. Iron is quite rare in its elemental state at the Earth's surface. In the presence of oxygen, iron turns to rust (iron oxide). Many meteorites that fall to Earth from the asteroid belt are composed of native iron (plus impurities). Very few localities on Earth have native iron of terrestrial origin. These occurrences are along natural smelting zones, where magma or lava has come in direct contact with coals or other carbonaceous sedimentary rocks. In such smelting zones, reducing conditions are created, and metallic iron can crystallize. Famous localities where this has happened include Disko Island in Greenland and the Siberian Traps Flood Basalt Province. Iron has a metallic luster, a silvery-gray color, is somewhat hard (H = 4 to 4.5), is strongly magnetic, has no cleavage, is malleable, and is moderately heavy for its size. Native iron is always alloyed with nickel in meteorites. In terrestrial iron occurrences, the Fe is also alloyed with a little Ni. The remarkable rock shown above is from the Putoran Plateau of Siberia. The silvery-gray colored material is vesicle-filling native iron. The black-colored material is the basalt/dolerite host rock, which contains labradorite plagioclase feldspar & bytownite plagioclase feldspar & pyroxene. Available chemical information indicates that the metal is about 94% Fe, 4% Ni, plus impurities. This Siberian rock formed during the largest outpouring of flood basalt in Earth’s history. The Siberian Traps Flood Basalts erupted through crustal fissures as the large head of a new hotspot (mantle plume) surfaced. This happened at Permian-Triassic boundary times (~251 Ma), and the timing suggests that Siberian Traps volcanism is related to the Permian-Triassic mass extinction in some way, and it appears to be antipodal to the Wilkes Land Impact Crater in Antarctica. The Late Paleozoic sedimentary succession of Siberia contains coal beds, as do many Late Paleozoic successions on Earth. The magmas came into direct contact with the coal beds, resulting in reducing conditions, which permitted the formation of elemental iron. Locality: Putoran Plateau, Siberia, Russia