Limonite (Ironstone)

Physical Properties

Hardness: 4-5.5; Color: Yellowish-brown to dark brown; Luster: Dull/Earthy; Crystal Structure: Amorphous (often pseudomorphic after other minerals); Streak: Yellow-brown.

Formation & Geological History

Formed as a secondary mineral through the oxidation and weathering of iron-rich minerals such as pyrite, magnetite, and siderite. Commonly found in gossans (iron hats) above ore deposits and in bog iron deposits.

Uses & Applications

Historically used as a minor ore of iron; primarily used today as a natural earth pigment (yellow ochre) in paints and dyes. Significant in soil science for indicating oxidation states.

Geological Facts

Limonite is not a true mineral but a mineraloid composed of varying mixtures of goethite and other iron oxides. It was once a major source of iron for early civilizations and bog iron production.

Field Identification & Locations

Identify by its distinctive yellow-brown streak, earthy appearance, and lack of cleavage. Found globally in weathered zones/areas with high iron content. Tips for collectors: check for pseudomorphs of pyrite (iron pyrites) which are more valuable.

Identified on: 4/20/2026

Mode: Standard