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

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