metamorphic
Quartzite
Metamorphosed Quartz Sandstone (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, or grayish; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None; Conchoidal fracture common.
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, or grayish
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, or grayish; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None; Conchoidal fracture common.
Formation & geological history
Formed through the regional or contact metamorphism of quartz-rich sandstone. High heat and pressure cause quartz grains to recrystallize and fuse together, obliterating the original grain boundaries.
Uses & applications
Used in construction as crushed stone for road gravel and railroad ballast, in glass manufacturing if high purity, and as decorative stone or countertops.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rock in weathering processes, frequently forming the tops of ridges and hills. It is harder than a steel knife and will scratch glass easily.
Field identification & locations
Identify in the field by its extreme hardness and the way it breaks through the grains rather than around them (unlike sandstone). Commonly found in folded mountain belts and ancient shield regions.
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