Limonite (Iron Ore)

Physical Properties

Hardness: 4-5.5; Color: Yellowish-brown to dark brown; Luster: Dull to earthy; Crystal structure: Amorphous; Cleavage: None; Specific gravity: 2.7-4.3

Formation & Geological History

Formed as a secondary material through the oxidation and weathering of primary iron-bearing minerals like pyrite, magnetite, and hematite in oxygen-rich environments.

Uses & Applications

Historically used as an iron ore; currently used as a natural earth pigment (yellow ochre) in paints and dyes, and for soil amendment.

Geological Facts

Limonite is not a true mineral but a mixture of related hydrated iron oxide minerals. It was the primary source of iron for ancient civilizations and is what gives many soils their yellow-brown color.

Field Identification & Locations

Identify by its characteristic yellowish-brown streak on a porcelain plate and dull, rusty appearance. Often found in bog deposits, gossans (weathered rock caps), and as a staining agent in sedimentary rocks.

Identified on: 4/17/2026

Mode: Standard