metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 (Mohs scale), Color: White, grey, or tan with iron oxide staining, Luster: Vitreous to dull, Crystal structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.63 - 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: White, grey, or tan with iron oxide staining, Luster: Vitreous to dull, Crystal structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.63 - 2.65
Formation & geological history
Formed through the metamorphism of quartz-rich sandstone under intense heat and pressure, typically during regional mountain-building events (orogeny). The original sand grains recrystallize into an interlocking mosaic.
Uses & applications
Used in construction as crushed stone for road ballast, as a dimension stone for flooring and walls, and in glassmaking if it has a very high silica purity.
Geological facts
Quartzite is so tough that it often breaks through the constituent sand grains rather than around them, which distinguishes it from sandstone. Some of the oldest rock formations on Earth contain quartzite layers.
Field identification & locations
Identify by its extreme hardness (it will easily scratch glass) and its granular but fused texture. It is commonly found in mountainous regions or ancient shield areas where uplift has exposed old metamorphic sequences.
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