sedimentary
Ferruginous Sandstone (Ironstone)
Sandstone with Hematite/Goethite cement (SiO2 + FeO(OH))
Hardness: 6-7 (Mohs); Color: Reddish-brown, tan, or rusty orange; Luster: Dull/Earthy; Crystal Structure: Clastic (grainy); Cleavage: None; Specific Gravity: 2.6-3.0.
- Hardness
- 6-7 (Mohs)
- Color
- Reddish-brown, tan, or rusty orange
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs); Color: Reddish-brown, tan, or rusty orange; Luster: Dull/Earthy; Crystal Structure: Clastic (grainy); Cleavage: None; Specific Gravity: 2.6-3.0.
Formation & geological history
Formed through the lithification of sand grains where iron-rich groundwater precipitates iron oxides (hematite or goethite) that act as a binding cement between quartz grains. These are common in Paleozoic to Mesozoic sedimentary basins.
Uses & applications
Commonly used as a building stone (brownstone), in road construction as aggregate, or as a low-grade iron ore in historical contexts. Primarily a collector or geological specimen in this size.
Geological facts
Iron-rich sandstones are often responsible for the spectacular red color of rock formations in places like the Grand Canyon and Sedona, Arizona. The iron oxide acts as a natural rust that colors the entire rock mass.
Field identification & locations
Identify in the field by its gritty sandpaper-like texture and its characteristic rusty staining that doesn't wash off. It is commonly found in riverbeds, desert outcrops, and areas with heavy erosion of sedimentary layers.
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