metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: typically white, gray, or tan, but often yellow, green, or blue due to impurities; Luster: Vitreous (glassy) to dull; Crystal structure: Trigonal (as quartz); Cleavage: None, exhibits conchoidal fracture
- Hardness
- 7 on Mohs scale
- Luster
- Vitreous (glassy) to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically white, gray, or tan, but often yellow, green, or blue due to impurities; Luster: Vitreous (glassy) to dull; Crystal structure: Trigonal (as quartz); Cleavage: None, exhibits conchoidal fracture
Formation & geological history
Formed through the regional metamorphism of quartz-rich sandstone. High heat and pressure cause the quartz grains to recrystallize and fuse together, eliminating the original pore spaces. This process can occur over millions of years in orogenic (mountain-building) belts.
Uses & applications
Used widely in construction as decorative stone, crushed stone for road bases, and roofing granules. Some varieties are used in lapidary work for jewelry or sculptures. High-purity quartzite is a source of silica for glass manufacturing.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rocks in the landscape, forming prominent ridges and peaks. Unlike sandstone, when quartzite is broken, it fractures through the quartz grains rather than around them.
Field identification & locations
Identify in the field by its extreme hardness (it will easily scratch glass and steel) and its crystalline, sugary appearance on fresh surfaces. It is found globally in ancient mountain ranges such as the Appalachians or the Alps.
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