sedimentary
Ironstone with Pyrite
Ferruginous Sandstone (Fe2O3) with minor Sulfide minerals (FeS2)
Hardness: 5-6.5 (Mohs); Color: Rusty brown, dark grey, with metallic brassy flecks; Luster: Earthy to metallic; Crystal structure: Isometric (pyrite inclusions) in an amorphous/granular matrix; Specific Gravity: 3.0-4.8 (variable due to iron content).
- Hardness
- 5-6
- Color
- Rusty brown, dark grey, with metallic brassy flecks
- Luster
- Earthy to metallic
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Physical properties
Hardness: 5-6.5 (Mohs); Color: Rusty brown, dark grey, with metallic brassy flecks; Luster: Earthy to metallic; Crystal structure: Isometric (pyrite inclusions) in an amorphous/granular matrix; Specific Gravity: 3.0-4.8 (variable due to iron content).
Formation & geological history
Formed through the chemical precipitation of iron-rich sediments in maritime or swamp environments, often during the Proterozoic or Paleozoic eras. Oxidation of iron-rich minerals creates the brown 'rust' patina seen on the surface.
Uses & applications
Historical source of low-grade iron ore. Primarily used today as aggregate material in construction, as a pigment (ochre), or as landscape rock. Rare, well-crystallized specimens are collectors' items.
Geological facts
Ironstone nodules often contain fossils or minerals like pyrite (fool's gold). Some ironstones are so dense with magnetite that they can affect compass readings in the field.
Field identification & locations
Identify by its high density (heavy for its size), rusty appearance, and faint metallic glint. Commonly found in coal-bearing sedimentary basins or weathered coastal cliffs. Collectors should look for internal crystallization during cracking.
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