Goethite. Credit: Mineral Information Institute.
Goethite is a brown hydroxide mineral, iron oxyhydroxide, of composition FeO(OH). It is a major iron ore of widespread occurrence, formed in bogs or by weathering of other iron minerals. Goethite is very similar to limonite, but is crystalline in the orthorhombic system as slender plates and velvety needles. X-ray diffraction analysis has shown that much supposed limonite is in fact goethite. Hardness: 5–5.5; relative density: 3.3–4.3.