metamorphic
Quartzite
Quartzite (primarily SiO2)
Hardness: 7 on Mohs scale; Color: typically white, gray, or tan, but can be red or purple due to iron oxides; Luster: vitreous to dull; Crystal structure: hexagonal (macro-crystalline transition); Cleavage: none (fractures across grains); Specific gravity: 2.6-2.7.
- Hardness
- 7 on Mohs scale
- Luster
- vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically white, gray, or tan, but can be red or purple due to iron oxides; Luster: vitreous to dull; Crystal structure: hexagonal (macro-crystalline transition); Cleavage: none (fractures across grains); Specific gravity: 2.6-2.7.
Formation & geological history
Formed through the metamorphism of quartz sandstone under high heat and pressure, causing quartz grains to recrystallize and fuse into a dense, interlocking matrix. Ages vary from Precambrian to younger orogenic cycles.
Uses & applications
Used in construction as road ballast, railway ballast, and building stone. Highly crushed quartzite can be used as a source of silica for glassmaking. Used in landscaping and occasionally as ornamental stones.
Geological facts
Quartzite is so tough that it often outlasts surrounding rocks; when broken, it fractures through the quartz grains rather than along the boundaries between them, unlike sandstone.
Field identification & locations
Identify in the field by its extreme hardness (it will easily scratch glass and steel) and its grainy, sugary appearance despite being a solid mass. Commonly found in folded mountain belts and ancient shield areas.
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