sedimentary
Ironstone Concretion / Goethite Nodules
Ironstone (composed primarily of Goethite FeO(OH) and Limonite)
Hardness: 5.0 - 5.5 Mohs scale; Color: dark brown, rusty black, or ochre; Luster: earthy to sub-metallic; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 3.3 - 4.3.
- Hardness
- 5
- Color
- dark brown, rusty black, or ochre
- Luster
- earthy to sub-metallic
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Physical properties
Hardness: 5.0 - 5.5 Mohs scale; Color: dark brown, rusty black, or ochre; Luster: earthy to sub-metallic; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 3.3 - 4.3.
Formation & geological history
Formed through the chemical precipitation of iron-rich minerals around a nucleus (like a leaf or shell) within sedimentary rock layers. These commonly date from the Carboniferous to the Cretaceous periods and form in swampy, marine, or marshy anaerobic environments.
Uses & applications
Historically used as a low-grade iron ore. Today, they are primarily used as landscape accents, for educational geological collections, and occasionally for carving or as natural 'curiosities'.
Geological facts
These are often mistaken for meteorites due to their dark, heavy appearance and 'fusion crust' look, earning them the nickname 'pseudo-meteorites'. Some internal cavities may contain 'rattle stones' (aetites) where a loose piece of mineral vibrates inside.
Field identification & locations
Identify in the field by its high density relative to other sedimentary rocks and its 'rusty' streak when rubbed on unglazed porcelain. Commonly found in eroded creek beds or shale outcrops in regions like the Appalachian Basin or the UK.
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