metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: white to light gray with tan staining; Luster: vitreous to dull; Crystal structure: granular, non-foliated interlocking quartz crystals; Cleavage: none (fractures across grains); Specific gravity: 2.6-2.7.
- Hardness
- 7 on Mohs scale
- Color
- white to light gray with tan staining
- Luster
- vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: white to light gray with tan staining; Luster: vitreous to dull; Crystal structure: granular, non-foliated interlocking quartz crystals; Cleavage: none (fractures across grains); 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 quartz grains to recrystallize and fuse together, eliminating pore space. Found in ancient mountain belts and cratons globally.
Uses & applications
Used extensively in road construction (aggregate), as railway ballast, in the production of glass and industrial silica, and as building stone for countertops and flooring. High-purity quartzite is used to produce silicon metal.
Geological facts
Quartzite is so hard and chemically resistant that it often forms the caprock of ridges and mountains because it resists erosion much better than surrounding rocks. It can be distinguished from its parent rock, sandstone, because quartzite will fracture THROUGH the grains rather than AROUND them.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its grainy, sugary texture. Common in Appalachian and Rocky Mountain regions. Collectors look for unique colors or banding caused by iron oxide impurities.
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