Rock Identifier
Limonite and Goethite encrusted Quartz (Iron-stained Gossan) (FeO(OH)·nH2O (Limonite) / FeO(OH) (Goethite) on SiO2 (Quartz)) — mineral
mineral

Limonite and Goethite encrusted Quartz (Iron-stained Gossan)

FeO(OH)·nH2O (Limonite) / FeO(OH) (Goethite) on SiO2 (Quartz)

Hardness: 5-5.5 (limonite) to 7 (quartz). Color: Rusty brown, yellowish-orange, dark brown, and black. Luster: Dull, earthy, or sub-metallic. Crystal Structure: Often amorphous or microcrystalline crusts; trigonal (quartz core). Cleavage: None. Specific Gravity: 2.7 to 4.3.

Hardness
5-5
Color
Rusty brown, yellowish-orange, dark brown, and black
Luster
Dull, earthy, or sub-metallic
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Physical properties

Hardness: 5-5.5 (limonite) to 7 (quartz). Color: Rusty brown, yellowish-orange, dark brown, and black. Luster: Dull, earthy, or sub-metallic. Crystal Structure: Often amorphous or microcrystalline crusts; trigonal (quartz core). Cleavage: None. Specific Gravity: 2.7 to 4.3.

Formation & geological history

Formed through the chemical weathering and oxidation of iron-rich sulfide minerals (like pyrite) in the 'gossan' or oxidized zone of an ore deposit. Most specimens are found in weathered outcrops and can date from recent times back to millions of years depending on the exposure history.

Uses & applications

Historically used as a minor iron ore. Today, it is primarily used as a source of ochre for pigments and as decorative specimens for mineral collectors interested in oxidation zones.

Geological facts

Commonly known as 'ironstone' by laypeople, these crusts are often a sign of valuable metal deposits (like gold or copper) deeper underground, serving as a 'cap' or 'hat' for ore veins. Limonite is not a single mineral but a mixture of hydrous iron oxides.

Field identification & locations

Identify in the field by its distinct yellow-brown streak, rusty appearance, and tendency to stain fingers. It is commonly found in mining districts and areas with heavy groundwater mineral leaching. Collectors should look for unique botryoidal (grape-like) textures within the rust.

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