metamorphic
Quartzite
Metamorphosed Quartz Sandstone (SiO2)
Hardness: 7 on Mohs scale; Color: typically white, gray, or tan; Luster: vitreous to grainy; Crystal structure: Trigonal (granular); Cleavage: none (conchoidal fracture); Specific gravity: 2.6–2.7
- Hardness
- 7 on Mohs scale
- Color
- typically white, gray, or tan
- Luster
- vitreous to grainy
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Physical properties
Hardness: 7 on Mohs scale; Color: typically white, gray, or tan; Luster: vitreous to grainy; Crystal structure: Trigonal (granular); Cleavage: none (conchoidal fracture); Specific gravity: 2.6–2.7
Formation & geological history
Formed through the regional metamorphism of quartz-rich sandstone. Specifically, heat and pressure cause the individual sand grains to recrystallize and fuse together into a dense, interlocking matrix. It is found in mountain belts worldwide and can date from the Precambrian to more recent eras.
Uses & applications
Used extensively in construction as crushed stone for road beds, railroad ballast, and concrete aggregate. High-quality slabs are used for countertops, flooring, and decorative stone. In the past, it was used for stone tools due to its hardness and sharp edges when fractured.
Geological facts
Quartzite is so tough that it usually breaks straight through the quartz grains rather than around them, which is a key distinction from its parent rock, sandstone. It is often more resistant to weathering than the surrounding rock, often forming rugged ridges and hilltops.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its sugary, granular appearance. It will not fizz in contact with acid (unlike marble). Common locations include the Appalachian Mountains (USA), the Alps, and the Brazilian Highlands.
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