metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: typically white, gray, or tan (appearing brown due to iron staining); Luster: vitreous to dull; Crystal structure: hexagonal (cryptocrystalline in aggregate); Cleavage: none, breaks with a conchoidal or irregular fracture; Specific gravity: approx. 2.6
- Hardness
- 7 on Mohs scale
- Luster
- vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically white, gray, or tan (appearing brown due to iron staining); Luster: vitreous to dull; Crystal structure: hexagonal (cryptocrystalline in aggregate); Cleavage: none, breaks with a conchoidal or irregular fracture; Specific gravity: approx. 2.6
Formation & geological history
Quartzite forms when quartz-rich sandstone is subjected to intense heat and pressure (metamorphism), causing the sand grains to recrystallize and fuse together. Specimens like this one, showing rounded edges, have been further shaped by water erosion in rivers or coastal environments.
Uses & applications
Used widely in construction as decorative stone, road ballast, and railroad beds. High-purity quartzite is used to produce silica sand for glassmaking and silicon for electronics. Smooth river-worn cobbles are popular in landscaping.
Geological facts
Quartzite is so tough that it often outlasts the mountain ranges it was formed in. Unlike sandstone, which breaks around grains, quartzite is so well-fused that it breaks through the individual quartz grains themselves.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. It often feels 'gritty' but looks sugary or crystalline on a fresh break. Commonly found in glaciated regions, riverbeds, and ancient mountain belts like the Appalachians or the Alps.
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