Limonite (Ironstone)

Physical Properties

Hardness: 4.0 - 5.5 on Mohs scale; Color: Yellowish-brown to dark brown; Luster: Dull to earthy; Crystal Structure: Amorphous (often pseudomorphous); Cleavage: None; Specific Gravity: 2.7 - 4.3.

Formation & Geological History

Formed as a secondary material through the oxidation and hydration of iron-rich primary minerals (like pyrite or magnetite). It is commonly found in the gossan (oxidized outcrop) of ore deposits and as a chemical precipitate in shallow marine or bog environments.

Uses & Applications

Historically used as a major iron ore and for pigments (brown ochre). Currently used as a low-grade iron source and in soil remediation.

Geological Facts

Limonite is not a true mineral species but a mixture of hydrated iron oxide minerals, primarily goethite. It has been used as a pigment for cave paintings since prehistoric times.

Field Identification & Locations

Identify in the field by its characteristic yellowish-brown streak on a porcelain plate and its earthy appearance. It is found globally, often in weathered sedimentary layers or near mine tailings.

Identified on: 4/26/2026

Mode: Standard