metamorphic
Quartzite
Quartzite (SiO2-rich metamorphic rock)
Hardness: 7 (Mohs scale), Color: Gray, purple, or reddish-brown due to iron impurities, Luster: Vitreous to dull, Crystal structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: Gray, purple, or reddish-brown due to iron impurities, Luster: Vitreous to dull, Crystal structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the metamorphism of quartz sandstone under high heat and pressure. This process causes the quartz grains to recrystallize and fuse together, creating a dense and durable rock. Most specimens date from the Proterozoic to Paleozoic eras.
Uses & applications
Used as crushed stone for road construction and railway ballast. Also used in architectural stone, countertops, and as a raw material for producing silicon products. Tumbled versions are common in landscaping and rock collections.
Geological facts
Quartzite is so tough that it often breaks through the individual quartz grains rather than around them, unlike its parent sandstone. It is resistant to chemical weathering and often forms prominent ridges or hills.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. Look for a sugary texture on fresh breaks. It is commonly found in mountainous regions or glacial deposits globally, such as the Appalachian Mountains or the Great Lakes region.
