Goethite

Goethite is an iron oxide hydroxide mineral with the chemical formula FeO(OH). It is a common constituent of iron ore deposits and a principal component of limonite. Goethite forms as a weathering product under oxidizing near-surface conditions and can also form by hydrothermal processes in soils and sedimentary rocks. It is named after Johann Wolfgang von Goethe, who studied the mineral in the early 19th century.

Properties: Goethite crystallizes in the orthorhombic system and commonly occurs as yellow-brown to reddish-brown needles, fibers, or earthy masses. The luster is dull to earthy; the streak is yellowish brown; hardness is 5 to 5.5; and the specific gravity is about 4.3.

Occurrence and associations: It is widespread in soils and sedimentary rocks and forms during iron oxidation or hydration of iron minerals such as magnetite or hematite. It often coexists with hematite, limonite, and other iron oxides. It is a major ore of iron in some deposits and can impart brown colors to rocks and soils.

Uses and significance: Goethite is used as an earth pigment (yellow-brown ochre) and serves as an iron ore in some deposits. In geology, its presence indicates historical oxidative conditions and can influence ore processing.

Notes: Dehydration converts goethite to hematite upon heating, releasing water.