Metallic iron is virtually unknown on the surface of the Earth except as iron-nickel alloys from meteorites and very rare forms of deep mantle xenoliths. Some iron meteorites are thought to have originated from accreted bodies 1,000 km in diameter or larger. The origin of iron can be ultimately traced to formation through nuclear fusion in stars and most of the iron is thought to have originated in dying stars that are large enough to collapse or explode as supernovae. Although iron is the fourth-most abundant element in the Earth's crust, composing about 5%, the vast majority is bound in silicate or more rarely carbonate minerals (for more information, see iron cycle). The thermodynamic barriers to separating pure iron from these minerals are formidable and energy-intensive, therefore all sources of iron used by human industry exploit comparatively rarer iron oxide minerals, primarily hematite.