metamorphic
Quartzite
Quartzite (primarily Silicon Dioxide, SiO2)
Hardness: 7 on Mohs scale; Color: typically white to gray, with yellowish or reddish staining from iron oxides; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 - 2.7.
- Hardness
- 7 on Mohs scale
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically white to gray, with yellowish or reddish staining from iron oxides; 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 intense heat and pressure. This process causes the original quartz grains to recrystallize and fuse together into a dense, hard rock. Quartzite occurs in many geological ages, often in Precambrian continental shields.
Uses & applications
Used extensively in construction as crushed stone for road beds, railroad ballast, and concrete aggregate. High-purity quartzite is used to produce glass, ferrosilicon, and silicon metal. It is also used in decorative stone applications and landscaping.
Geological facts
Quartzite is so hard and chemically resistant that it often forms the caps of ridges and mountains, as it resists weathering more effectively than surrounding rocks. It is often confused with marble, but can be distinguished because quartzite will scratch glass and does not fizz in contact with acid.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel nail) and its granular, sugary texture that resembles sandpaper but feels smooth due to the fused crystals. Commonly found in mountainous regions or as rounded river cobbles. For collectors, look for translucent pieces or those with unique mineral inclusions.
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