sedimentary
Siderite Concretion in Shale
Iron carbonate (FeCO3) in Mudstone/Shale matrix
Hardness: 3.5-4.5 (concretion), 2-3 (matrix); Color: Dark grey with brownish-orange core; Luster: Dull/Earthy; Structure: Massive to rhombohedral crystals inside; Specific Gravity: 3.8
- Hardness
- 3
- Color
- Dark grey with brownish-orange core
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3.5-4.5 (concretion), 2-3 (matrix); Color: Dark grey with brownish-orange core; Luster: Dull/Earthy; Structure: Massive to rhombohedral crystals inside; Specific Gravity: 3.8
Formation & geological history
Formed via chemical precipitation of iron carbonate around an organic nucleus in oxygen-poor sedimentary basins, typically during the Carboniferous period.
Uses & applications
Historically used as a low-grade iron ore; primarily of interest today as geological specimens and indicators of paleoenvironments.
Geological facts
These nodules often contain fossils at their center, as the chemical environment that forms the siderite is often triggered by the decay of organic matter.
Field identification & locations
Identify by the distinct heavy weight (due to iron) and the rusty 'oxidized' outer layer. Commonly found in coal-bearing shale formations.
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