mineral

Limonite pseudomorph after Pyrite

Goethite/Limonite FeO(OH)·nH2O

Hardness: 4-5.5 (Mohs), Color: Dark brown to yellowish-brown, Luster: Dull or earthy, Crystal structure: Isometric (original cube shape), Specific gravity: 2.7-4.3

Hardness: 4-5

Identified June 19, 2026More mineral →

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Physical properties

Hardness: 4-5.5 (Mohs), Color: Dark brown to yellowish-brown, Luster: Dull or earthy, Crystal structure: Isometric (original cube shape), Specific gravity: 2.7-4.3

Formation & geological history

Formed through the chemical weathering of pyrite cubes where oxygen and water replace the iron sulfide, leaving a hydrated iron oxide structure that retains the original cubic shape. Common in oxidized zones of ore deposits.

Uses & applications

Used minorly as an iron ore; primarily collected as mineral specimens and for educational purposes regarding chemical pseudomorphism.

Geological facts

This is a 'pseudomorph', which literally means 'false form'. The mineral has changed chemically into limonite, but it keeps the external cubic crystal shape of the pyrite it used to be.

Field identification & locations

Identify by the distinct cubic shape paired with a dull, rusty brown color and lack of metallic brassy luster associated with fresh pyrite. Commonly found in areas with heavy soil weathering like the Piedmont region of the USA.

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