Goethite is an iron hydroxy-oxide mineral, FeO(OH). It has a nonmetallic luster, is usually brown to dark brown in color, has a brownish streak, no cleavage, and is moderately hard (H=5 to 5.5). It typically forms by the oxidation of iron-rich minerals. Goethite forms crystals or fibrous masses of varying morphology, but many samples are massive.
Goethite (3.3 cm across)
Goethite from a bog iron mine in Lancaster County, Pennsylvania, USA (public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA.
Goethite that has replaced a mass of gypsum crystals (hydrous calcium sulfate) from the Cave of Swords, Naica Mine, eastern Chihuahua State, northern Mexico.
Goethite from the White Desert of Egypt.
Left: goethite replacing marcasite crystals.
Right: goethite replacing pyrite crystals.