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Wüstite is a mineral with the chemical formula FeO. It is the high-temperature polymorph of iron(II) oxide and is the lowest-valent iron oxide mineral under standard conditions. Wüstite is a black to dark gray opaque mineral. It has a metallic to submetallic luster and a black streak. The mineral is relatively soft, with a Mohs hardness of approximately 5 to 5.5. Its density is around 5.74 g/cm³.

Wüstite crystallizes in the cubic crystal system, typically forming as anhedral grains. It is not commonly found in large, well-formed crystals. It is an important constituent of certain iron meteorites, particularly those that have undergone high-temperature events. In terrestrial environments, wüstite is found in association with other iron oxides and minerals in some reduced igneous and metamorphic rocks. It can also form as a product of high-temperature oxidation or reduction processes involving iron-bearing materials.

The stability of wüstite is strongly dependent on temperature and oxygen partial pressure. At atmospheric pressure, wüstite is stable above 575 degrees Celsius. Below this temperature, it tends to invert to a mixture of metallic iron and magnetite (Fe3O4). This instability at lower temperatures contributes to its relative rarity in surface geological environments. Its presence often indicates specific formation conditions involving high temperatures and low oxygen availability.