metamorphic
Quartzite
Quartzite (Metamorphosed Quartz Sandstone)
Hardness: 7 on the Mohs scale. Color: Generally white to gray, but can be pink, red, or purple due to iron impurities. Luster: Vitreous to greasy on fresh surfaces. Crystal structure: Trigonal (microcrystalline quartz grains interlocked). Cleavage: None (fractures conchoidally). Specific gravity: 2.65.
- Hardness
- 7 on the Mohs scale
- Luster
- Vitreous to greasy on fresh surfaces
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Physical properties
Hardness: 7 on the Mohs scale. Color: Generally white to gray, but can be pink, red, or purple due to iron impurities. Luster: Vitreous to greasy on fresh surfaces. Crystal structure: Trigonal (microcrystalline quartz grains interlocked). Cleavage: None (fractures conchoidally). Specific gravity: 2.65.
Formation & geological history
Formed when quartz-rich sandstone is subjected to high heat and pressure, typically through tectonic compression in orogenic belts. The original quartz grains recrystallize and weld together, creating an extremely hard and durable rock.
Uses & applications
Used in construction, as crushed stone for roadbeds, in railway ballast, for making dimension stone, and as a source of silica for manufacturing glass and ferrosilicon.
Geological facts
Because quartz is extremely resistant to weathering, quartzite ridges often form prominent, highly resistant mountain crests and hills. Unlike sandstone, which breaks around the sand grains, quartzite breaks straight through the grains.
Field identification & locations
Can be identified in the field by its extreme hardness (scratches glass and steel), lack of visible fossils, sugary texture of interlocking quartz crystals on broken surfaces, and the fact that it does not fizz with hydrochloric acid.
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