sedimentary
Ferruginous Sandstone (Ironstone)
Ferruginous Sandstone / Limonite-cemented Quartz Sandstone
Hardness: 5-6 (Mohs); Color: Rusty brown, reddish-orange, or dark ochre; Luster: Dull/Earthy; Crystal structure: Granular/Clastic; Cleavage: None; Specific gravity: 2.5-3.0
- Hardness
- 5-6 (Mohs)
- Color
- Rusty brown, reddish-orange, or dark ochre
- Luster
- Dull/Earthy
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Physical properties
Hardness: 5-6 (Mohs); Color: Rusty brown, reddish-orange, or dark ochre; Luster: Dull/Earthy; Crystal structure: Granular/Clastic; Cleavage: None; Specific gravity: 2.5-3.0
Formation & geological history
Formed when iron-rich groundwater precipitates iron oxides (like hematite or goethite) within the pores of sandy sediment, acting as a cement. These can form in riverbeds or marine environments and range from the Paleozoic to Cenozoic eras.
Uses & applications
Historically used as a low-grade iron ore; commonly used today in riprap for erosion control, crushed stone for road base, and occasionally in rustic building masonry.
Geological facts
Ironstone concretions were once so abundant in certain regions that they fueled early industrial iron smelting. The characteristic rusty color is essentially the result of the rock 'rusting' due to contact with oxygen and water.
Field identification & locations
Identify by its gritty texture and distinct reddish-brown staining that may rub off slightly on hands. It is often found in areas with exposed sedimentary outcrop or along riverbanks. Collectors look for unique nodular shapes or fossil inclusions.
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