metamorphic
Quartzite (or Quartz Rich Conglomerate)
Quartzite (SiO2 silicon dioxide)
Hardness: 7 on the Mohs scale (will scratch glass). Color: Off-white, grey, and tan crystal grains with darker matrix. Luster: Vitreous/glassy on crystal faces, dull on weathered surfaces. Crystal structure: Hexagonal (trigonal) in grains. Cleavage: Indistinct/conchoidal fracture.
- Hardness
- 7 on the Mohs scale (will scratch glass)
- Color
- Off-white, grey, and tan crystal grains with darker matrix
- Luster
- Vitreous/glassy on crystal faces, dull on weathered surfaces
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Physical properties
Hardness: 7 on the Mohs scale (will scratch glass). Color: Off-white, grey, and tan crystal grains with darker matrix. Luster: Vitreous/glassy on crystal faces, dull on weathered surfaces. Crystal structure: Hexagonal (trigonal) in grains. Cleavage: Indistinct/conchoidal fracture.
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone. Under intense heat and pressure, original quartz grains recrystallize and fuse together into a dense, hard rock. This specimen appears to have been tumbled in a river or glacial environment, rounding its edges.
Uses & applications
Quartzite is used heavily in construction as crushed stone for road gravel and railway ballast. High-purity quartzite is used to produce glass, silicon metal, and ferrosilicon for industrial electronics.
Geological facts
Quartzite is so durable that it often remains as ridges on mountains long after softer rocks have eroded away. It is often much harder and more resistant than the sandstone from which it originated.
Field identification & locations
Identify in the field by its extreme hardness and the fact that it breaks across grain boundaries rather than around them (unlike sandstone). Commonly found in ancient mountain belts and glacial till. This specimen shows 'sugary' textures typical of recrystallized quartz.
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