What is Goethite?
Goethite is an iron oxyhydroxide common in weathering zones, soils and ore deposits, where it forms earthy, botryoidal and stalactitic material.
Mineralogy and structure
Reference formula: FeO(OH). Goethite is often fine-grained or poorly crystalline in natural iron-rich coatings. It commonly occurs with hematite and limonite, a field term for mixed hydrated iron oxides. Goethite belongs to the oxides and has a orthorhombic crystal system. Typical hardness is 5–5.5 on the Mohs scale and specific gravity is approximately 3.3–4.3.
Colour and appearance
Typical colours include brown, yellow-brown, black, red-brown. The usual botryoidal, fibrous, stalactitic habit and earthy, sub-metallic, vitreous lustre are useful descriptive features, but colour alone is not diagnostic.
Formation and localities
Goethite forms in or is associated with weathering profiles, iron ore deposits, hydrothermal veins, caves and soils. Important documented localities include Brazil, France, Spain, the United States, Australia and South Africa.
Collecting and care
Keep surfaces dry and protected from abrasion; record whether the specimen is a confirmed goethite crystal or an earthy iron-oxide mixture.
