metamorphic
Quartzite
Quartzite (chiefly SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, or grayish (iron staining often causes yellow/orange tints); 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: Tan, brown, or grayish (iron staining often causes yellow/orange tints); 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-rich sandstone. Under intense heat and pressure, quartz grains recrystallize and fuse together. These are extremely durable and can survive hundreds of millions of years of erosion.
Uses & applications
Used in construction as road ballast, railway track bedding, and building stone. Because of its hardness, it is also used as an abrasive and in the production of glass and silicon.
Geological facts
Quartzite is so tough that it often survives as large, rounded cobbles in riverbeds while surrounding softer rocks erode away. It is often mistake for chert or flint, but usually has a slightly 'sugary' texture under magnification.
Field identification & locations
Identify in the field by its inability to be scratched by steel and its tendency to break across quartz grains rather than around them. Commonly found in mountain ranges and riverbeds globally.
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