metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 (Mohs scale); Color: White, gray, yellow, or reddish-brown; Luster: Vitreous to dull; Crystal structure: Trigonal (as quartz grains); Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.7
- Hardness
- 7 (Mohs scale)
- Color
- White, gray, yellow, or reddish-brown
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: White, gray, yellow, or reddish-brown; Luster: Vitreous to dull; Crystal structure: Trigonal (as quartz grains); Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.7
Formation & geological history
Formed through the regional or contact metamorphism of quartz-rich sandstone. High heat and pressure cause the original quartz grains to recrystallize and fuse together, creating an extremely hard and durable interlocking structure.
Uses & applications
Used primarily in construction for road ballast, roofing tiles, and flooring. Due to its durability and resistance to chemical weathering, it is also used for countertops, decorative stone, and as a source of silica in glass manufacturing.
Geological facts
Quartzite is so tough that it often survives several cycles of erosion, forming prominent ridges or peaks because it erodes much more slowly than surrounding softer rocks like shale or limestone.
Field identification & locations
Identify in the field by its hardness (it easily scratches glass) and its granular, sugary appearance. Unlike sandstone, quartzite fractures through the grains rather than around them. Found globally in mountainous regions and ancient shields like the Appalachians or the Alps.
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