sedimentary
Ironstone Concretion (Limonite/Goethite)
Ferruginous Siltstone / Limonite Concretion (FeOO(OH)·nH2O)
Hardness: 4-5.5 (Mohs scale); Color: Rusty brown, yellowish-tan, dark brown; Luster: Dull, earthy; Crystal structure: Amorphous or microcrystalline; Cleavage: None; Specific gravity: 2.7-4.3
- Hardness
- 4-5
- Color
- Rusty brown, yellowish-tan, dark brown
- Luster
- Dull, earthy
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Physical properties
Hardness: 4-5.5 (Mohs scale); Color: Rusty brown, yellowish-tan, dark brown; Luster: Dull, earthy; Crystal structure: Amorphous or microcrystalline; Cleavage: None; Specific gravity: 2.7-4.3
Formation & geological history
Formed by the precipitation of iron oxide minerals within the pore spaces of sedimentary rock (usually sandstone or shale). The iron acts as a cement, binding particles together into a hard mass, often through the movement of iron-rich groundwater.
Uses & applications
Commonly collected as a curiosity; historically used as a low-grade iron ore; pigmented varieties used in ocher paints.
Geological facts
These are frequently mistaken for meteorites because of their high density and rusty appearance. They often form in round or irregular shapes known as 'Moqui Marbles' in certain regions.
Field identification & locations
Identify in the field by its high density relative to other stones and its yellowish-brown streak on unglazed porcelain. Commonly found in desert environments or eroded sedimentary outcrops.
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