metamorphic
Quartzite
Quartzite (Primary mineral: Silicon Dioxide, SiO2)
Hardness: 7 on Mohs scale; Color: typically white to gray, often stained tan/yellow/brown by iron oxides; Luster: vitreous to dull; Crystal structure: Trigonal (granular texture); 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, often stained tan/yellow/brown by iron oxides; Luster: vitreous to dull; Crystal structure: Trigonal (granular texture); Cleavage: none (conchoidal fracture); Specific gravity: 2.6-2.7.
Formation & geological history
Formed through the regional metamorphism of quartz sandstone. Intense heat and pressure cause quartz grains to recrystallize and fuse together into a dense, hard mass. Most deposits originate from the Precambrian or Paleozoic eras.
Uses & applications
Used extensively in construction for road ballast, railroad beds, and roofing granules. High-purity quartzite is used to produce ferrosilicon, industrial silica sand, and glass. High-quality varieties are used as ornamental stone or countertops.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rock, forming prominent ridges and hills. Unlike sandstone, which breaks around grains, quartzite breaks through the quartz grains because the silica cement is as strong as the grains themselves.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel nail) and granular, 'sugary' texture. It is commonly found in mountain folds (e.g., Appalachians) and riverbeds as rounded cobbles. Collectors look for pieces with interesting iron staining or visible crystal faces.
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