metamorphic
Quartzite
Quartzite (primarily SiO2)
Hardness: 7 on Mohs scale; Color: typically white to gray, but iron impurities often cause tan, yellow, or reddish-brown staining as seen here; Luster: vitreous to dull; Crystal structure: non-clastic, interlocking grains; Cleavage: none (breaks with conchoidal fracture).
- 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 iron impurities often cause tan, yellow, or reddish-brown staining as seen here; Luster: vitreous to dull; Crystal structure: non-clastic, interlocking grains; Cleavage: none (breaks with conchoidal fracture).
Formation & geological history
Formed through the regional or contact metamorphism of quartz-rich sandstone. High heat and pressure cause the original sand grains to recrystallize and fuse together, creating an extremely durable rock. Geological age can range from Precambrian to Cenozoic.
Uses & applications
Used extensively in construction as road ballast, railroad track ballast, and as a decorative stone in landscaping. High-purity quartzite is used in the glass industry and to produce silicon metal for electronics.
Geological facts
Quartzite is so hard and dense that when it breaks, it fractures through the quartz grains rather than along the boundaries between them, unlike its parent rock, sandstone.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. It often forms prominent, resistant ridges in mountain ranges. It is ubiquitous in riverbeds as rounded cobbles due to its resistance to erosion.
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