sedimentary
Limonite Concretion
FeO(OH)qnH2O (Hydrated iron(III) oxide-hydroxide mixture)
Hardness: 4-5.5; Color: Yellow-brown, rusty orange, to dark brown; Luster: Earthy, dull, or sub-metallic; Structure: Often amorphous or microcrystalline; Streak: Yellowish-brown.
- Hardness
- 4-5
- Color
- Yellow-brown, rusty orange, to dark brown
- Luster
- Earthy, dull, or sub-metallic
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Physical properties
Hardness: 4-5.5; Color: Yellow-brown, rusty orange, to dark brown; Luster: Earthy, dull, or sub-metallic; Structure: Often amorphous or microcrystalline; Streak: Yellowish-brown.
Formation & geological history
Formed through the chemical weathering of iron-rich minerals (like pyrite or magnetite) or precipitation from iron-saturated groundwater in sedimentary environments. Common in bog deposits and weathered soil layers.
Uses & applications
Historically used as a low-grade iron ore and as a pigment for ochre paint. Primarily of interest to collectors and geologists as a secondary mineral specimen.
Geological facts
Limonite is not a true mineral but a mixture of hydrated iron oxide minerals, mostly goethite. It is responsible for the yellowish-brown coloring in many soils and desert varnishes.
Field identification & locations
Identify by its characteristic yellowish-brown streak, earthy appearance, and tendency to occur as small nodules or crusts. Common in rusty sand deposits or near oxidizing iron sources.
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