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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