metamorphic
Quartzite
Quartzite (primarily SiO2)
Hardness: 7 on Mohs scale; Color: Gray, white, with orange/red iron staining; Luster: Vitreous to dull; Crystal structure: Trigonal (granular); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Gray, white, with orange/red iron staining
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, white, with orange/red iron staining; Luster: Vitreous to dull; Crystal structure: Trigonal (granular); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the metamorphism of quartz-rich sandstone under intense heat and pressure, typically during mountain-building events (orogeny). The original quartz grains recrystallize into a locked mosaic of crystals.
Uses & applications
Used in construction as crushed stone for road grit or railway ballast, as decorative building stone (countertops/tiles), and in the manufacture of glass and ceramics due to high silica content.
Geological facts
Quartzite is so hard and chemically resistant that it often forms the caps of ridges and mountains because it resists weathering better than the surrounding rocks.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. In the field, look for a sugary or glassy texture where individual sand grains are no longer visible because they have fused. Found globally in ancient mountain belts.
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