sedimentary
Ironstone Concretion
Ironstone (primarily Siderite or Limonite / Goethite, FeO(OH)·nH2O)
Hardness: 5-5.5 on Mohs scale. Color: Deep reddish-brown to dark brown/ochre. Luster: Dull, earthy, or sub-metallic. Crystal Structure: Often amorphous or microcrystalline mass. Specific Gravity: 3.3-4.3.
- Hardness
- 5-5
- Color
- Deep reddish-brown to dark brown/ochre
- Luster
- Dull, earthy, or sub-metallic
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Physical properties
Hardness: 5-5.5 on Mohs scale. Color: Deep reddish-brown to dark brown/ochre. Luster: Dull, earthy, or sub-metallic. Crystal Structure: Often amorphous or microcrystalline mass. Specific Gravity: 3.3-4.3.
Formation & geological history
Formed through the precipitation of iron minerals within sedimentary layers, often around a nucleus like a leaf, shell, or fossil. These form in oxygen-poor environments like swamps or shallow marine basins over millions of years.
Uses & applications
Historically used as a low-grade iron ore. Today, it is primarily of interest to geological collectors and sometimes used as decorative garden stone or crushed for road base.
Geological facts
Ironstone concretions can sometimes contain perfectly preserved fossils inside if cracked open. They are often much harder than the surrounding sandstone or shale that they formed within.
Field identification & locations
Identify by its high weight (density) compared to other rocks and its characteristic rusty 'skin.' Commonly found in clay or sandstone beds. Collectors should look for rounded or nodular shapes that feel unusually heavy.
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