Limonite (Iron Ore)

Physical Properties

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

Formation & Geological History

Formed as a secondary mineral through the weathering and oxidation of iron-rich minerals such as pyrite, magnetite, and siderite in the presence of water.

Uses & Applications

Historically used as a major source of iron ore (though largely superseded by hematite and magnetite) and used as a source of yellow ochre pigment since ancient times.

Geological Facts

Limonite is not a true mineral but a mineraloid mixture of various hydrated iron oxides; it is the material that gives many soils their yellow or rusty-brown color.

Field Identification & Locations

Identify by its yellowish-brown streak on a porcelain plate and its rusty appearance. Often found in marshes (bog iron) or as gossans (caps) over metallic ore deposits.

Identified on: 4/27/2026

Mode: Standard