Limonite (Iron Ore)

Physical Properties

Hardness: 4-5.5 (Mohs scale); Color: Yellowish-brown to rusty orange; Luster: Dull, earthy; Crystal Structure: Amorphous (no visible crystals); Streak: Yellowish-brown; Specific Gravity: 2.7-4.3.

Formation & Geological History

Formed by the oxidation and hydration of iron-rich mineral deposits (like pyrite or magnetite) in weathering environments. It is a secondary mineral commonly found in gossans (iron hats) above ore deposits.

Uses & Applications

Historically used as a major source of iron ore and as a pigment for yellow ochre. Currently used in soil conditioners and for industrial coloring.

Geological Facts

Limonite is not technically a true mineral but a mixture of hydrated iron oxide minerals. It was one of the earliest pigments used by early humans in cave paintings.

Field Identification & Locations

Identify by its distinctive mustard-yellow or rusty brown streak. Often found in bog deposits or as coatings on other rocks. Look for its characteristic porous or 'clunky' earthy appearance.

Identified on: 4/28/2026

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