English: A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5600 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The oxide minerals all have one or more oxide anions (O2 -2).

Goethite is an iron hydroxy-oxide mineral, FeO(OH). It has a nonmetallic to metallic 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.

This goethite sample from Gilbert, Minnesota is hosted in rocks of the ~1.878 billion year old Biwabik Iron-Formation. The Biwabik is correlative with the Ironwood Iron-Formation and Negaunee Iron-Formation of northern Michigan.

Locality: Schley Mine (Schley Pit; now flooded), southeastern side of the town of Gilbert, Mesabi Iron Range, central St. Louis County, northeastern Minnesota, USA (47° 28 51.92" North latitude, 92° 27' 17.86" West longitude)

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Photo gallery of goethite:

www.mindat.org/gallery.php?min=1719