metamorphic
Quartzite
Quartzite (chiefly SiO2)
Hardness: 7 (Mohs scale); Color: white, gray, or tan; Luster: vitreous (glassy); Crystal structure: trigonal (usually granular); Breakage: conchoidal or irregular; Very resistant to weathering.
- Hardness
- 7 (Mohs scale)
- Color
- white, gray, or tan
- Luster
- vitreous (glassy)
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Physical properties
Hardness: 7 (Mohs scale); Color: white, gray, or tan; Luster: vitreous (glassy); Crystal structure: trigonal (usually granular); Breakage: conchoidal or irregular; Very resistant to weathering.
Formation & geological history
Formed through the metamorphism of quartz sandstone. Intense heat and pressure cause the quartz grains to recrystallize and fuse together, completely filling the void spaces between the original sand grains.
Uses & applications
Used in construction as crushed stone for road beds or railway ballast; slabs are used for countertops, flooring, and decorative wall cladding; high-purity quartzite is a source of silica for industrial glass making.
Geological facts
Quartzite is so tough that it usually breaks through the quartz grains rather than along the boundaries between them, which distinguishes it from sandstone. Many mountain ranges, such as parts of the Appalachians, are capped with quartzite because it is so resistant to erosion.
Field identification & locations
Identify in the field by its extreme hardness (it will easily scratch glass) and lack of reaction to acid. It is commonly found in ancient mountain belts and glaciated regions as erratic boulders or cobbles.
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