metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 (Mohs scale); Color: Yellow-brown, tan, or olive-toned (due to irony impurities); Luster: Vitreous to waxy when polished; Crystal structure: Trigonal/Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 (Mohs scale)
- Color
- Yellow-brown, tan, or olive-toned (due to irony impurities)
- Luster
- Vitreous to waxy when polished
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Physical properties
Hardness: 7 (Mohs scale); Color: Yellow-brown, tan, or olive-toned (due to irony impurities); Luster: Vitreous to waxy when polished; Crystal structure: Trigonal/Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the regional or contact metamorphism of quartz-rich sandstone. High heat and pressure cause the original quartz grains and silica cement to recrystallize into an interlocking mosaic of quartz crystals. Most specimens date back to the Proterozoic or Paleozoic eras.
Uses & applications
Used widely in construction for railway ballast, as a decorative stone in landscaping, for kitchen countertops (in slab form), and as a semi-precious stone for lapidary work and tumbled stones.
Geological facts
Quartzite is so tough that it often outlasts the mountain ranges in which it forms. In a thin section under a microscope, the recrystallized quartz grains show interlocking boundaries like a jigsaw puzzle, making it much stronger than its parent sandstone.
Field identification & locations
Identify in the field by its extreme hardness (will easily scratch glass) and lack of reaction to acid. Unlike sandstone, quartzite breaks through the quartz grains rather than around them. Commonly found in eroded riverbeds or mountainous regions with ancient tectonic history.
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