metamorphic
Quartzite
Quartzite (primarily SiO2)
Hardness: 7 (Mohs scale), Color: Gray to reddish-purple, Luster: Vitreous to dull, Crystal Structure: Hexagonal (trigonal) in component grains, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: Gray to reddish-purple, Luster: Vitreous to dull, Crystal Structure: Hexagonal (trigonal) in component grains, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the metamorphism of quartz-rich sandstone. High heat and pressure cause the quartz grains to recrystallize and fuse into a dense rock. Commonly associated with mountain-building events (orogenies).
Uses & applications
Used in construction as crushed stone for road ballast, as a decorative stone in architecture (countertops/tiles), and as a source of silica in glass manufacturing.
Geological facts
Quartzite is so tough that it often fractures across the original grain boundaries rather than around them, unlike sandstone. It is extremely resistant to chemical weathering.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. It often appears sugary or sparkly. Commonly found in folded mountain belts like the Appalachians or the Rockies.
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