sedimentary
Ironstone (Limonite/Hematite concretion)
Ferruginous Siltstone / Goethite (FeO(OH))
Hardness: 4.0-5.5 Mohs; Color: dark brown to rusty orange-brown; Luster: earthy to sub-metallic; Crystal Structure: Amorphous or Cryptocrystalline; Specific Gravity: 3.3-4.3
- Hardness
- 4
- Color
- dark brown to rusty orange-brown
- Luster
- earthy to sub-metallic
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Physical properties
Hardness: 4.0-5.5 Mohs; Color: dark brown to rusty orange-brown; Luster: earthy to sub-metallic; Crystal Structure: Amorphous or Cryptocrystalline; Specific Gravity: 3.3-4.3
Formation & geological history
Formed by the precipitation of iron oxides from groundwater, often concentrating around a nucleus in sedimentary layers. These typically form in oxygen-rich environments where iron moves in solution and deposits as cement between mineral grains.
Uses & applications
Historically used as a low-grade iron ore; commonly used today in landscaping, as a decorative mineral specimen, or as a natural pigment (ochre).
Geological facts
Ironstone concretions are frequently mistaken for meteorites because of their high density and dark, weathered rinds, earning them the nickname 'meteor-wrongs'. Some ironstones contain hollow centers or fossil imprints.
Field identification & locations
Identify in the field by its high density relative to other sedimentary rocks and its characteristic rusty streak test (giving a yellow or reddish-brown streak on unglazed porcelain). Found globally in sedimentary basins.
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