sedimentary
Limonite and Goethite encrusted Chert/Ironstone
Hydrated Iron Oxide (FeO(OH)·nH2O) with cryptocrystalline Silica (SiO2)
Hardness: 4 to 7 (depending on iron vs. silica content); Color: Yellow-ochre, dark brown, and charcoal grey; Luster: Dull, earthy, to sub-metallic; Crystal Structure: Amorphous or orthorhombic (microscopic); Cleavage: Conchoidal in silica parts, none in iron parts
- Hardness
- 4 to 7 (depending on iron vs
- Color
- Yellow-ochre, dark brown, and charcoal grey
- Luster
- Dull, earthy, to sub-metallic
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Physical properties
Hardness: 4 to 7 (depending on iron vs. silica content); Color: Yellow-ochre, dark brown, and charcoal grey; Luster: Dull, earthy, to sub-metallic; Crystal Structure: Amorphous or orthorhombic (microscopic); Cleavage: Conchoidal in silica parts, none in iron parts
Formation & geological history
Formed through the chemical precipitation of iron-rich fluids in sedimentary environments or as a secondary weathering product ('gossan') of iron sulfides. Often found in bogs, lake beds, or as oxidation zones in ore deposits spanning various geological ages.
Uses & applications
Historically used as a low-grade iron ore and as a pigment for ochre paints. In modern times, it is primarily a geological specimen or used in landscape construction.
Geological facts
Limonite is not a true mineral but a mixture of hydrated iron oxides. It was the primary source of 'yellow ochre' used by prehistoric humans in cave paintings and is often responsible for the yellow color of many desert soils.
Field identification & locations
Identify by its distinctive yellow-brown 'rust' streak and its heavy weight relative to its size (high specific gravity). It is commonly found in areas with high limestone or sedimentary iron presence and should be cleaned with water but no harsh chemicals to preserve the ochre patina.
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