metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 (Mohs scale); Color: Tan, brown, white, and reddish-brown due to iron oxide staining; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None, exhibits conchoidal or irregular fracture.
- Hardness
- 7 (Mohs scale)
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, brown, white, and reddish-brown due to iron oxide staining; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None, exhibits conchoidal or irregular fracture.
Formation & geological history
Quartzite forms when quartz-rich sandstone is subjected to high heat and pressure during regional metamorphism, causing the quartz grains to recrystallize and fuse together. This specimen appears water-worn, likely from a river or creek bed.
Uses & applications
Used extensively in road construction as aggregate, as railway ballast, and in ornamental stone for floors, walls, and countertops. High-purity quartzite is used to produce silica sand for glassmaking and silicon metal.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rocks during erosion, forming ridges and mountain peaks. Unlike sandstone, which breaks around grains, quartzite breaks through the grains due to its incredibly tight interlocking structure.
Field identification & locations
Identify in the field by its extreme hardness (it will easily scratch glass) and lack of reaction to acid. It often feels slightly sugary to the touch in raw form but becomes very smooth when river-tumbled. Commonly found in metamorphic belts worldwide.
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