sedimentary
Limonitic Iron Ore / Goethite
Hydrated Iron(III) oxide-hydroxide (primarily FeO(OH)·nH2O)
Hardness: 4.0-5.5 (Mohs); Color: Dark brown to yellow-ochre; Luster: Submetallic, dull, or earthy; Crystal Structure: Orthorhombic (if crystalline); Cleavage: Perfect (if individual crystals); Specific Gravity: 2.7-4.3
- Hardness
- 4
- Color
- Dark brown to yellow-ochre
- Luster
- Submetallic, dull, or earthy
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Physical properties
Hardness: 4.0-5.5 (Mohs); Color: Dark brown to yellow-ochre; Luster: Submetallic, dull, or earthy; Crystal Structure: Orthorhombic (if crystalline); Cleavage: Perfect (if individual crystals); Specific Gravity: 2.7-4.3
Formation & geological history
Formed through the chemical weathering of iron-rich minerals such as pyrite, magnetite, and siderite in oxidation zones. It often occurs as a secondary mineral in sedimentary bog iron deposits or gossans over sulfide ore bodies.
Uses & applications
Historically a major source of iron ore. Today, it is used as a natural earth pigment (yellow ochre) in paints, an iron supplement in fertilizers, and occasionally as a low-value collector specimen.
Geological facts
Limonite is not a true mineral but a 'mineraloid' mixture of hydrated iron oxides. It was a primary source of iron for ancient civilizations and is the substance responsible for the yellow and brown colors in many soils and sedimentary rocks.
Field identification & locations
Identify in the field by its distinct yellow-brown 'ochre' streak when rubbed against unglazed porcelain. It is commonly found in weathered rock outcrops, swamps (bog iron), and as a coating on other stones in mining districts.
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