Limonite
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Limonite is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as FeO·nH₂O, although this is not entirely accurate as the ratio of oxide to hydroxide can... Wikipedia
Cleavage: Absent
Color: Various shades of brown and yellow
Crystal habit: Fine grained aggregate, powdery coating
Density: 2.7–4.3 g/cm3
Diaphaneity: Opaque
Formula (repeating unit): FeO(OH)·nH2O
Fracture: Uneven