sedimentary
Limonite/Goethite on Shale
Hydrous ferric oxides (FeO(OH)·nH2O) often associated with argillaceous rock
Hardness: 4.0-5.5 (limonite coating), 3.0 (shale base); Color: Rusty yellow to dark brown; Luster: Dull, earthy; Crystal structure: Amorphous or cryptocrystalline; Cleavage: None visible in coating.
- Hardness
- 4
- Color
- Rusty yellow to dark brown
- Luster
- Dull, earthy
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Physical properties
Hardness: 4.0-5.5 (limonite coating), 3.0 (shale base); Color: Rusty yellow to dark brown; Luster: Dull, earthy; Crystal structure: Amorphous or cryptocrystalline; Cleavage: None visible in coating.
Formation & geological history
Formed through the chemical weathering of iron-rich minerals in a sedimentary environment. The yellow-orange coating is a secondary oxidation product of iron minerals like pyrite or siderite commonly found in dark shales.
Uses & applications
Historical source of iron ore (pigment/ochre); primarily used today for geological study and as a low-value collector specimen.
Geological facts
Limonite is not a true mineral but a 'mineraloid' composed of various iron hydroxides. It was the original source of the yellow pigment used in cave paintings over 15,000 years ago.
Field identification & locations
Identify by its characteristic 'rust' color and earthy texture. It will leave a yellow-brown streak on a porcelain plate and is commonly found in road cuts through sedimentary basins.
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