metamorphic
Quartzite
Quartzite (primarily SiO2)
Hardness: 7 on Mohs scale; Color: typically white to gray, but can be tan or yellow due to iron inclusions; Luster: vitreous to dull; Crystal structure: hexagonal (cryptocrystalline grains); Cleavage: none (conchoidal fracture); 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 to gray, but can be tan or yellow due to iron inclusions; Luster: vitreous to dull; Crystal structure: hexagonal (cryptocrystalline grains); Cleavage: none (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the metamorphism of quartz sandstone. Under intense heat and pressure, original quartz grains recrystallize and fuse together, creating a hard, interlocking network. It can date from almost any geological era where sandstone was buried deeply.
Uses & applications
Used as crushed stone for road construction, railway ballast, and as a decorative stone in landscaping. High-purity quartzite is used to produce silica sand for glassmaking and silicon metal.
Geological facts
Quartzite is so tough that it often survives erosion better than the surrounding rocks, frequently forming the crests of ridges and mountains. It is distinct from sandstone because it will break through the quartz grains rather than around them.
Field identification & locations
Identify in the field by its hardness (it will easily scratch glass and steel) and its crystalline, sugary appearance. It lacks the 'gritty' feel of sandstone because the grains are fused. Commonly found in folded mountain belts and ancient shield areas.
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