metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 (Mohs scale), Color: White to light gray (with reddish-brown iron staining), Luster: Vitreous to dull, Crystal structure: Hexagonal/Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.6 - 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: White to light gray (with reddish-brown iron staining), Luster: Vitreous to dull, Crystal structure: Hexagonal/Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.6 - 2.65
Formation & geological history
Formed through the regional or contact metamorphism of quartz-rich sandstone. High heat and pressure cause the sand grains to recrystallize and fuse together. It can be found in various geological eras, notably the Precambrian and Paleozoic.
Uses & applications
Used in construction as road ballast, railway ballast, and building stone. Highly crushed quartzite is used in the glass and ceramic industries. Polished slabs are used for countertops and flooring.
Geological facts
Quartzite is so hard and dense that when it breaks, it fractures through the individual sand grains rather than around them, which is a key way to distinguish it from its parent rock, 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 but feels very smooth where fractured. Commonly found in folded mountain belts globally.
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