File:Iron (Tertiary; Disko Island, Greenland) 2 (17150419318).jpg

Native terrestrial iron from the Tertiary of Greenland. (public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA)

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 rock shown above is a mass of native terrestrial iron from the famous Disko Island, Greenland locality. During the Tertiary, basaltic dikes intruded Cretaceous and lower Tertiary sedimentary rocks of Disko Island. These rocks included coals and carbonaceous shales. The meeting of mafic magma and carbon resulted in a reducing environment - a natural smelter. Native iron formed in the magma as small blebs. When erupted as basalt lava flows or intruded as basalt sills, the iron blebs settled to flow bases and sill bases to form iron masses.

Chemical analysis has shown that Disko Island iron masses contain native iron (Fe) and cohenite (iron carbide) plus various inclusions of silicates, sulfides, and oxides (see list in Bird et al., 1981).

Locality: Disko Island, offshore western Greenland

Disko Island geology synthesized from: