mineral
Limonite (after Pyrite Pseudomorph)
Limonite (Hydrated Iron Oxide Mixture, FeO(OH)·nH2O)
Hardness: 4.0-5.5 (Mohs scale); Color: Yellowish-brown to dark rusty brown; Luster: Dull, earthy, or sub-metallic; Crystal structure: Amorphous or microcrystalline (retains the cubic or pyritohedron shape of the original pyrite); Cleavage: None; Specific gravity: 2.7-4.3.
- Hardness
- 4
- Color
- Yellowish-brown to dark rusty brown
- Luster
- Dull, earthy, or sub-metallic
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Physical properties
Hardness: 4.0-5.5 (Mohs scale); Color: Yellowish-brown to dark rusty brown; Luster: Dull, earthy, or sub-metallic; Crystal structure: Amorphous or microcrystalline (retains the cubic or pyritohedron shape of the original pyrite); Cleavage: None; Specific gravity: 2.7-4.3.
Formation & geological history
Formed through the chemical weathering (oxidation and hydration) of iron-rich minerals such as pyrite or marcasite. This specific specimen is a 'pseudomorph,' meaning the original pyrite crystal was replaced by limonite while preserving its external cube-like geometric shape.
Uses & applications
Historically used as an important iron ore and as a source of yellow ochre pigment. Today, it is primarily of interest to mineral collectors and as a geological indicator of oxidation zones in ore deposits.
Geological facts
Limonite is not a single mineral but a mixture of hydrated iron oxides. These cube-shaped specimens are often nicknamed 'Propeller Stones' or 'Devil's Dice' in certain regional folklore due to their unusual geometric appearance in nature.
Field identification & locations
Identify in the field by its distinct cubic or dodecahedral 'crystal' shape that appears rusty and earthy rather than metallic. Often found in weathered sedimentary rock layers or gossans (oxidized rock capping ore deposits). Common locations include Utah, Arizona, and Germany.
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