mineral
Limonite pseudomorph after pyrite
Goethite / Limonite pseudomorph after Pyrite (Fe3+O(OH) replacing FeS2)
Hardness: 5.0-5.5 on Mohs scale. Color: Brownish-red to dark reddish-brown or rusty orange. Luster: Sub-metallic to dull, earthy. Crystal structure: Cubic or pyritohedral form inherited from the original pyrite crystal. Cleavage: None.
- Hardness
- 5
- Color
- Brownish-red to dark reddish-brown or rusty orange
- Luster
- Sub-metallic to dull, earthy
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Physical properties
Hardness: 5.0-5.5 on Mohs scale. Color: Brownish-red to dark reddish-brown or rusty orange. Luster: Sub-metallic to dull, earthy. Crystal structure: Cubic or pyritohedral form inherited from the original pyrite crystal. Cleavage: None.
Formation & geological history
Formed via chemical weathering and oxidation of iron sulfide minerals (pyrite) in aqueous conditions, where iron oxide/hydroxide minerals like limonite or goethite slowly replace the original pyrite structure atom-by-atom while preserving the cubic shape.
Uses & applications
Popular as mineral collection specimens, educational examples of pseudomorphism, and minor iron ore.
Geological facts
These specimens are often called 'devil's dice' or 'iron stone cubes' due to their distinctive cubic shape and dark, rusty appearance. Pseudomorph literally means 'false form'.
Field identification & locations
Identified by its sharp cubic or sub-cubic geometry paired with a rusty, brownish-red color and streak, devoid of metallic brassy pyrite luster inside. Commonly found in weathered sedimentary and metamorphic rock layers, particularly in regions like Spain, Morocco, and the southwestern USA.
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