metamorphic
Quartzite
Metamorphosed Quartz Sandstone (SiO2)
Hardness: 7 on the Mohs scale; Color: Typically white to gray, but can be pink, red, or yellow due to iron impurities; Luster: Vitreous to greasy; Crystal structure: Trigonal (microcrystalline quartz grains fused together); Cleavage: None (fractures conchoidally through grains, not around them);…
- Hardness
- 7 on the Mohs scale
- Luster
- Vitreous to greasy
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Physical properties
Hardness: 7 on the Mohs scale; Color: Typically white to gray, but can be pink, red, or yellow due to iron impurities; Luster: Vitreous to greasy; Crystal structure: Trigonal (microcrystalline quartz grains fused together); Cleavage: None (fractures conchoidally through grains, not around them); Specific Gravity: 2.65 - 2.70
Formation & geological history
Formed when quartz-rich sandstone is subjected to high heat and pressure, typically through tectonic compression in orogenic belts. The original quartz grains recrystallize along with the cementing material, forming an interlocking mosaic of quartz crystals.
Uses & applications
Used in construction, as crushed stone for road bases, railroad ballast, manufacturing of silica brick, and sometimes as a decorative dimension stone or countertop material.
Geological facts
Because quartzite is extremely hard and resistant to chemical weathering, ridges formed by quartzite layers often form prominent hills or mountain ridges (monadnocks) in eroded landscapes. It does not fizz with hydrochloric acid, distinguishing it from marble.
Field identification & locations
Identified in the field by its extreme hardness (scratches glass easily), inability to be split along bedding planes (unlike sandstone, it breaks straight through the quartz grains), and vitreous sheen on freshly broken surfaces. Found worldwide wherever ancient sandstones have undergone regional metamorphism.
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