goethite

goethite, a widespread iron oxide mineral [α-FeO(OH)] and the most common ingredient of iron rust. It was named in 1806 for J.W. von Goethe, a German poet and philosopher with a keen interest in minerals. The name was originally applied to lepidocrocite [γ-FeO(OH)], a less common mineral with the same chemical composition as goethite but with a different crystal structure. In goethite, oxygen and hydroxyl anions are closely packed in hexagonal arrays, while in lepidocrocite they are arranged in cubic arrays; in both structures, however, iron cations occupy the octahedral interstices.

In terms of relative abundance, goethite is second only to hematite (α-Fe₂O₃) among iron oxides. Goethite is normally formed under oxidizing conditions as a weathering product of iron minerals (e.g., pyrite, magnetite). Because it is formed near the surface, goethite is the major component of the gossan of iron sulfide deposits in locations such as Arizona and of lateritic deposits in places such as Cuba. It also occurs as a direct precipitate in marine and meteoric waters, and deposits accumulate in springs and marshes.

Goethite is the source for the pigment known as yellow ochre; it is also the primary mineral in some important iron ores, such as those in the Alsace-Lorraine basin in France. Other important goethite deposits are found in the southern Appalachians, U.S.; Brazil; South Africa; Russia; and Australia.

Goethite varies in colour from yellow-brown to red. It is composed of about 80 to 90 percent Fe₂O₃ and approximately 10 percent water. When dehydrated, goethite forms hematite; upon hydration, goethite becomes limonite. For detailed physical properties, see oxide mineral (table).