Goethite is a very common mineral. It is formed by hydrothermal alteration or weathering of iron-bearing minerals, mostly oxides.
The most characteristic feature of goethite is brown color. Its composition is FeO(OH). It may resemble hematite but hematite is either gray or reddish. Powdered hematite is reddish brown but goethite is brown. Both of these minerals are good paint pigments. Goethite may form mixtures with other similar minerals which may alter the color. Limonite is a mixture of several iron oxides and hydroxides, goethite usually being one of the most important constituents.
Sand sample from Martha's Vineyard, Massachusetts, USA. Brown rounded pellets are goethite grains. The width of the view is 10 mm.
