metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, white, or reddish; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None, exhibits conchoidal fracture; Specific gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, white, or reddish
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, white, or reddish; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None, exhibits conchoidal fracture; Specific gravity: 2.6-2.7
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. These often date back to the Proterozoic or Paleozoic eras.
Uses & applications
Used in construction as crushed stone, road ballast, and decorative stone. High-purity quartzite is used to produce ferrosilicon, industrial silica sand, and silicon metal.
Geological facts
Quartzite is so tough that it often survives intense weathering and glacial transport, which is why it is frequently found as rounded river or beach pebbles like the one in the image.
Field identification & locations
Identify in the field by its extreme hardness (it will scratch glass and steel) and its granular, sugary texture. Unlike sandstone, it breaks across the grains rather than around them. Common in mountainous regions and glacial till.
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