metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: typically white to gray, often with pinkish or yellowish hues; Luster: vitreous to dull; Crystal structure: hexagonal (trigonal); Cleavage: none, breaks with conchoidal fracture
- 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 pinkish or yellowish hues; Luster: vitreous to dull; Crystal structure: hexagonal (trigonal); Cleavage: none, breaks with conchoidal fracture
Formation & geological history
Quartzite is formed when quartz-rich sandstone is subjected to high heat and pressure during regional metamorphism. The individual quartz grains recrystallize into a dense, interlocking mosaic of crystals.
Uses & applications
Used extensively in road construction and railway ballast due to its durability. High-purity quartzite is used to produce glass, silicon metal, and silicon carbide. It is also used as a decorative building stone and for countertops.
Geological facts
Quartzite is so hard and chemically resistant that it often remains as high ridges or peaks on mountainsides while surrounding softer rocks erode away. It is often confused with marble but can be distinguished because quartzite will scratch glass and does not fizz with acid.
Field identification & locations
Identify by its sugary appearance and extreme hardness (it will easily scratch a steel knife or glass). It is commonly found in mountain belts and areas of ancient stable continental crust. For collectors, look for samples with interesting colors or visible crystal inclusions.
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