metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 (Mohs scale), Color: White, often with grey or brownish tints from iron oxidation, Luster: Vitreous to dull, Crystal Structure: Trigonal (in the constituent quartz grains), Cleavage: None (breaks with conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: White, often with grey or brownish tints from iron oxidation, Luster: Vitreous to dull, Crystal Structure: Trigonal (in the constituent quartz grains), Cleavage: None (breaks with conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Quartzite is formed when quartz-rich sandstone is subjected to intense heat and pressure (regional or contact metamorphism). This process causes the individual sand grains to recrystallize and fuse into a solid, interlocking network of quartz crystals.
Uses & applications
Used extensively as crushed stone for road construction, railroad ballast, and building stone. High-purity quartzite is used in the manufacture of glass, silicon metal, and abrasive materials. It is also popular for decorative paving and countertops.
Geological facts
Unlike sandstone, which breaks around the individual grains, quartzite is so well-fused that it breaks directly through the sand grains. It is one of the most durable and weather-resistant rocks found on the Earth's surface.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its grainy, crystalline texture. It often appears sugary or 'sparkly' the way light reflects off the internal crystal faces. Commonly found in mountainous regions or ancient cratonic shields.
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