limonite

limonite (līm`ənīt) or brown hematite (hĕm`ətīt, hē`–), yellowish to dark brown mineral, a hydrated oxide of iron, FeO(OH)·nH₂O, occurring commonly in deposits of secondary origin, i.e., those formed by the alteration of minerals containing iron. Both iron rust and bog iron ore are limonite. It serves as a pigment (see ocher) and as an ore of iron. It is found mainly in Austria and England.

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limonite

Limonite (left) from Ironwood, Mich., and (right) from Montgomery, Pa.

(credit: Courtesy of the Field Museum of Natural History, Chicago; photograph, John H. Gerard/EB Inc.)

One of the major iron minerals, a hydrous ferric oxide of variable composition. Often brown and earthy, it is formed by alteration of other iron minerals, such as the hydration of hematite or the oxidation and hydration of siderite or pyrite.