metamorphic
Quartzite
Quartzite (primarily SiO2)
Hardness: 7 on Mohs scale; Color: typically white to gray, but iron staining can turn it yellow, orange, or reddish-brown; Luster: vitreous to waxy (when wet); Crystal structure: non-clastic, interlocking quartz grains; Cleavage: none (conchoidal fracture).
- Hardness
- 7 on Mohs scale
- Luster
- vitreous to waxy (when wet)
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Physical properties
Hardness: 7 on Mohs scale; Color: typically white to gray, but iron staining can turn it yellow, orange, or reddish-brown; Luster: vitreous to waxy (when wet); Crystal structure: non-clastic, interlocking quartz grains; Cleavage: none (conchoidal fracture).
Formation & geological history
Formed through the metamorphism of quartz-rich sandstone. High heat and pressure cause the sand grains to recrystallize and fuse together, creating a very dense and durable rock. This process typically occurs during mountain-building events (orogeny).
Uses & applications
Used extensively as crushed stone for road construction and railway ballast. Polished varieties are used for countertops, floor tiles, and decorative landscaping. High-purity quartzite is also a source of silica for glassmaking.
Geological facts
Quartzite is so tough that it often survives intense weathering better than most other rocks, frequently forming the tops of ridges and hills. When struck with a hammer, it typically breaks through the sand grains rather than around them, which distinguishes it from sandstone.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. It often appears sugary or grainy in texture but feels smooth when polished. Common in old mountain belts and riverbeds as rounded cobbles.
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