sedimentary
Ironstone Concretion / Limonite Node
Limonite (Hydrated Iron Oxide, FeO(OH)·nH2O) with Goethite/Hematite
Hardness: 4.0-5.5 on Mohs scale. Color: Dull brown, ochre-yellow, or rusty-orange. Luster: Earthy to sub-metallic. Crystal structure: Amorphous to microcrystalline. Cleavage: None. Specific gravity: 2.7-4.3.
- Hardness
- 4
- Color
- Dull brown, ochre-yellow, or rusty-orange
- Luster
- Earthy to sub-metallic
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Physical properties
Hardness: 4.0-5.5 on Mohs scale. Color: Dull brown, ochre-yellow, or rusty-orange. Luster: Earthy to sub-metallic. Crystal structure: Amorphous to microcrystalline. Cleavage: None. Specific gravity: 2.7-4.3.
Formation & geological history
Formed through the precipitation of iron-rich minerals in the spaces between grains of sedimentary rocks (like sandstone or shale). Usually forms in oxygen-rich groundwater environments or bog deposits. Age varies widely from millions to hundreds of millions of years old.
Uses & applications
Historically used as a low-grade iron ore. Today, it is primarily used as a pigment (ochre) for paints and dyes. In landscaping, it is used as decorative stone. Collectors value unusual shapes.
Geological facts
Ironstone concretions can sometimes form 'rattle stones' (Klapperstein) if the inner core shrinks and becomes loose within the harder outer shell. They are often mistaken for meteorites due to their heavy weight and rusty exterior.
Field identification & locations
Identify by its high density relative to other rocks, a 'rusty' streak (yellow-brown) when rubbed on unglazed porcelain, and lack of magnetism (unless it contains magnetite). Common in riverbeds, clay pits, and eroding cliffs.
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