metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: typically white, gray, or tan, but can be brownish/rusty due to iron oxides; Luster: Vitreous to dull; Crystal structure: Hexagonal (microcrystalline/granoblastic texture); Cleavage: None (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, gray, or tan, but can be brownish/rusty due to iron oxides; Luster: Vitreous to dull; Crystal structure: Hexagonal (microcrystalline/granoblastic texture); Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the metamorphism of quartz sandstone. Intense heat and pressure from tectonic compression cause the sand grains to recrystallize and fuse together. Can range from Precambrian to much younger geological ages depending on the region.
Uses & applications
Used in construction as crushed stone for road gravel and railway ballast. High-purity quartzite is used to produce silica sand for glassmaking, ceramics, and silicon metal production. Some attractive varieties are used as ornamental stone or kitchen countertops.
Geological facts
Quartzite is so hard that it can scratch steel and glass easily. Unlike sandstone, which breaks around the individual sand grains, quartzite is so well-cemented that it breaks across (through) the grains when fractured.
Field identification & locations
Identify it by its extreme hardness, sugary texture, and the way it fractures across grains rather than around them. It is commonly found in mountain belts and areas where ancient continental crust has been compressed. Collectors look for pieces with interesting mineral inclusions.
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