mineral

Limonite after Pyrite (Pseudomorph)

FeO(OH)·nH2O following FeS2

Hardness: 4-5.5 (but brittle); Color: dark brown to blackish-brown; Luster: dull to sub-metallic; Crystal Structure: Isometric (cubic habit); Streak: yellow-brown; Cleavage: none.

Hardness: 4-5
Color: dark brown to blackish-brown
Luster: dull to sub-metallic

Identified April 12, 2026More mineral →

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Physical properties

Hardness: 4-5.5 (but brittle); Color: dark brown to blackish-brown; Luster: dull to sub-metallic; Crystal Structure: Isometric (cubic habit); Streak: yellow-brown; Cleavage: none.

Formation & geological history

Formed through the chemical weathering and oxidation of pyrite (iron disulfide), where groundwater replaces the sulfur/iron atoms with hydrated iron oxides while maintaining the original crystal shape. Common in oxidized zones of ore deposits.

Uses & applications

Primarily of value to mineral collectors and hobbyists. Historically used as a minor iron ore and as a source of ochre pigments.

Geological facts

This specimen is a 'pseudomorph' (meaning 'false form'). It looks exactly like a cube of pyrite but has been chemically transformed into limonite over geological time. They are often colloquially called 'glimmer cubes' or 'devil's dice'.

Field identification & locations

Identify in the field by looking for perfect cubes or pentagonal dodecahedrons that are brown and earthy rather than metallic gold. They are often found in weathered shale or schist. Use a streak test on unglazed porcelain; it will leave a yellowish-brown mark.

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