metamorphic
Quartzite
Quartzite (primarily SiO2)
Hardness: 7 on the Mohs scale; Color: typically white, gray, or tan (may show greens/pinks due to mineral impurities); Luster: vitreous to dull; Structure: non-foliated, granular; Cleavage: none; Specific Gravity: 2.6-2.7
- Hardness
- 7 on the Mohs scale
- Luster
- vitreous to dull
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Physical properties
Hardness: 7 on the Mohs scale; Color: typically white, gray, or tan (may show greens/pinks due to mineral impurities); Luster: vitreous to dull; Structure: non-foliated, granular; Cleavage: none; 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 together. Formation can range from the Archean to the Cenozoic era, occurring in orogenic (mountain-building) zones.
Uses & applications
Used widely in construction as road ballast, railway ballast, and crushed stone. High-purity quartzite is used to produce ferrosilicon, industrial silica sand, and glass. In jewelry, it is often polished as cabochons or used as 'river stones' for landscaping.
Geological facts
Quartzite is so tough that it often survives intense weathering better than other rocks, frequently forming the caps of ridges and mountains. Unlike sandstone, which breaks around sand grains, quartzite breaks through the sand grains due to its crystalline fusion.
Field identification & locations
Identify in the field by its extreme hardness (it will easily scratch glass) and its sugary, crystalline texture under a hand lens. It lacks the 'fizz' reaction to HCl acid that marble has. Common in areas with ancient mountain ranges like the Appalachians or the Scottish Highlands.
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