sedimentary
Ferruginous Sandstone (Ironstone)
Clastic Sedimentary Rock (composed of SiO2 with Fe2O3/FeO(OH) cement)
Hardness: 6-7 (Mohs scale), Color: rusty brown, tan, to yellow-orange; Luster: earthy to dull; Structure: granular; Cleavage: none; Specific Gravity: 2.3 - 2.8
- Hardness
- 6-7 (Mohs scale), Color: rusty brown, tan, to yellow-orange
- Color
- rusty brown, tan, to yellow-orange
- Luster
- earthy to dull
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Physical properties
Hardness: 6-7 (Mohs scale), Color: rusty brown, tan, to yellow-orange; Luster: earthy to dull; Structure: granular; Cleavage: none; Specific Gravity: 2.3 - 2.8
Formation & geological history
Formed through the accumulation of sand-sized grains (mostly quartz) cemented together by iron oxides like hematite or limonite. This process often occurs in riverbeds, deltas, or shallow marine environments through groundwater precipitation.
Uses & applications
Used primarily in construction as a building stone (brownstone), road aggregate, and historically as a low-grade iron ore in early smelting processes.
Geological facts
Many famous historic buildings in the Northeastern US (Brownstones) are made of ferruginous sandstone. The 'rust' color is the result of chemical weathering where iron-rich minerals oxidize in the presence of water and oxygen.
Field identification & locations
Identify by its gritty, sand-like texture (feels like sandpaper) and its distinctive rust-red or brownish stain. It is extremely common in sedimentary basins worldwide and can be found by looking for layered rock outcrops or river pebbles with iron staining.
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