sedimentary
Shale with Limonite/Hematite Staining
Argillaceous sedimentary rock with Iron Oxides (FeO(OH)·nH2O / Fe2O3)
Hardness: 3-4 (excluding coatings); Color: Reddish-brown, yellowish-orange, and dark brown; Luster: Dull to sub-metallic when wet; Crystal structure: Amorphous or microcrystalline coatings; Cleavage: Fissile (splits into thin layers).
- Hardness
- 3-4 (excluding coatings)
- Color
- Reddish-brown, yellowish-orange, and dark brown
- Luster
- Dull to sub-metallic when wet
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Physical properties
Hardness: 3-4 (excluding coatings); Color: Reddish-brown, yellowish-orange, and dark brown; Luster: Dull to sub-metallic when wet; Crystal structure: Amorphous or microcrystalline coatings; Cleavage: Fissile (splits into thin layers).
Formation & geological history
Formed through the lithification of mud and clay particles in low-energy environments like river deltas or lake beds. The vibrant colors are caused by the oxidation (rusting) of iron-rich minerals within the rock, often occurring in various geological eras from Paleozoic to Cenozoic.
Uses & applications
Common shale is used as a source for natural gas (fracking), in the production of bricks, cement, and pottery. Iron-stained specimens are primarily of interest to amateur collectors or used as decorative garden stones.
Geological facts
The reddish-brown 'stains' are essentially geological rust. When iron-bearing minerals are exposed to water and oxygen, they transform into limonite or hematite, often preserving the layered structure of the host rock.
Field identification & locations
Identify in the field by its thin, plate-like bedding and the way it easily crumbles or flakes. It is commonly found in riverbeds, canyons, and road cuts through sedimentary basins. Collectors look for unique banding patterns caused by the iron oxidation.
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