metamorphic
Quartzite
Quartzite (Primary mineral: Quartz, SiO2)
Hardness: 7 on Mohs scale (scratches glass). Color: White, gray, or pinkish/tan. Luster: Vitreous to dull. Crystal structure: Granoblastic (interlocking crystals). Cleavage: None (breaks across grains). Specific gravity: 2.6 to 2.7.
- Hardness
- 7 on Mohs scale (scratches glass)
- Color
- White, gray, or pinkish/tan
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale (scratches glass). Color: White, gray, or pinkish/tan. Luster: Vitreous to dull. Crystal structure: Granoblastic (interlocking crystals). Cleavage: None (breaks across grains). Specific gravity: 2.6 to 2.7.
Formation & geological history
Formed through the regional or contact metamorphism of quartz-rich sandstone. High heat and pressure cause the original quartz grains to recrystallize and fuse into a solid, dense crystalline rock. This process can occur over various geological ages, often in mountain-building belts.
Uses & applications
Used extensively in construction as road ballast, gravel, and dimension stone for flooring or countertops. High-purity quartzite is used to produce ferrosilicon, industrial silica sand, and silicon metal. It is also a popular lapidary material for garden stones or decorative carvings.
Geological facts
Quartzite is so tough and durable that it often survives weathering better than surrounding rocks, forming prominent ridges or hilltops. Unlike sandstone, which breaks around its individual grains, quartzite breaks right through the grains due to the intense fusion during metamorphism.
Field identification & locations
Identify in the field by its extreme hardness (it will not be scratched by a steel knife) and its grainy, sugary appearance despite being a solid mass. It lacks the 'fizz' reaction to acid that marble has. Common in ancient mountain ranges like the Appalachians or the Canadian Shield.
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