metamorphic
Quartzite
Quartzite (primarily SiO2)
Hardness: 7 (Mohs scale), Color: White, gray, tan, or yellowish due to impurities, Luster: Vitreous to dull, Crystal structure: Hexagonal (microcrystalline/interlocking), Cleavage: None (conchoidal fracture), Specific gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: White, gray, tan, or yellowish due to impurities, Luster: Vitreous to dull, Crystal structure: Hexagonal (microcrystalline/interlocking), Cleavage: None (conchoidal fracture), Specific gravity: 2.6-2.7
Formation & geological history
Formed through the regional or contact metamorphism of quartzose sandstone. Under intense heat and pressure, the individual quartz grains recrystallize and fuse together into a dense, hard rock. Typically associated with mountain-building events.
Uses & applications
Used as crushed stone for road construction and railroad ballast, in the manufacture of glass and ceramics, as a decorative stone in architecture (countertops, tiles), and as an abrasive in sandblasting.
Geological facts
Quartzite is so durable that it often forms the crests of mountain ridges because it resists weathering better than surrounding rocks. It is often confused with marble, but can be distinguished because quartzite is harder than glass and will not react with acid.
Field identification & locations
Identify in the field by its hardness (will easily scratch glass) and its sugary, crystalline texture. It usually breaks across the sand grains rather than around them. Commonly found in folded mountain belts globally.
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