metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: typically white, gray, or pink/red (due to iron oxides); Luster: vitreous (glassy) to dull; Crystal structure: hexagonal (as quartz grain residues); Cleavage: none, breaks with conchoidal or splintery fracture.
- Hardness
- 7 on Mohs scale
- Color
- typically white, gray, or pink/red (due to iron oxides)
- Luster
- vitreous (glassy) to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically white, gray, or pink/red (due to iron oxides); Luster: vitreous (glassy) to dull; Crystal structure: hexagonal (as quartz grain residues); Cleavage: none, breaks with conchoidal or splintery fracture.
Formation & geological history
Formed through the regional or contact metamorphism of quartz-rich sandstone. Under intense heat and pressure, quartz grains of the parent sandstone recrystallize and fuse into a dense, crystalline rock. This process can happen over millions of years.
Uses & applications
Used as crushed stone in road construction, as railroad ballast, and for architectural stone (countertops, wall coverings, flooring). High-purity quartzite is used to produce silica sand for glassmaking and industrial silicon.
Geological facts
Quartzite is so tough that it often forms the caprock or ridges of mountain ranges because it resists weathering more effectively than surrounding soft rocks. It is harder than steel and will easily scratch a glass plate.
Field identification & locations
Identify in the field by its extreme hardness and its sugary, granular surface that looks like it has fused grains. It is commonly found in fold mountain belts worldwide, such as the Appalachians or the Alps. Unlike limestone, it will not fizz with acid.
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