metamorphic
Quartzite
Quartzite (SiO2 principal component)
Hardness: 7 on Mohs scale; Color: typically white to gray, often with tan/yellow tinting from iron oxides; Luster: vitreous to dull; Crystal structure: trigonal (microcrystalline); Cleavage: none (conchoidal or irregular fracture); Specific gravity: approx. 2.65.
- Hardness
- 7 on Mohs scale
- Luster
- vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically white to gray, often with tan/yellow tinting from iron oxides; Luster: vitreous to dull; Crystal structure: trigonal (microcrystalline); Cleavage: none (conchoidal or irregular fracture); Specific gravity: approx. 2.65.
Formation & geological history
Formed through the metamorphism of quartz-rich sandstone. High heat and pressure cause the original sand grains to recrystallize and fuse together into a dense, hard interlocking network of crystals.
Uses & applications
Used as crushed stone for road construction and railway ballast; frequently used for kitchen countertops (as 'natural quartz'), floor tiles, and as a source of silica for glass manufacturing.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rocks in a landscape, forming prominent ridges and hills. Unlike sandstone, when quartzite is hit with a hammer, it breaks through the quartz grains rather than around them.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its grainy, crystalline appearance. It is commonly found in mountain folds or ancient core regions of continents (cratons).
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