sedimentary

Limonite/Goethite on Shale

Hydrous ferric oxides (FeO(OH)·nH2O) often associated with argillaceous rock

AI-generated identification · may be incorrect

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

Identified April 6, 2026More sedimentary →

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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.