metamorphic
Limonite-stained Quartzite
Quartzite with Limonite (FeO(OH)·nH2O) staining
Hardness: 7 (Mohs scale); Color: Yellow-orange to rusty brown exterior with a white/grey interior; Luster: Dull to vitreous; Crystal Structure: Granular/Massive; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7 (high for the quartz component).
- Hardness
- 7 (Mohs scale)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Yellow-orange to rusty brown exterior with a white/grey interior; Luster: Dull to vitreous; Crystal Structure: Granular/Massive; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7 (high for the quartz component).
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone under high heat and pressure, typically in orogenic (mountain-building) belts. The yellow coloration is a secondary feature caused by the chemical weathering of iron-bearing minerals, depositing iron oxide (limonite) into the cracks and surface.
Uses & applications
Used primarily in construction as crushed stone for road ballast or railway bedding. High-purity quartzite is used in glass making and the production of silicon. Stained pieces are sometimes used as decorative garden stones.
Geological facts
Quartzite is so tough that it often survives intense weathering better than the mountains it forms. Limonite, the yellow stain, was once a primary source of iron ore known as 'bog iron' and was used as a pigment in prehistoric cave paintings.
Field identification & locations
Identify in the field by its extreme hardness (will easily scratch glass) and its granular, sugary texture that looks like compressed sand. Common in Precambrian shields and fold-and-thrust mountain ranges. Look for the distinct yellow/brown staining along fracture planes.
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