metamorphic
Quartzite
Quartzite (primarily SiO2)
Hardness: 7 on Mohs scale (scratches glass). Color: Gray, white, or pinkish. Luster: Vitreous to dull. Crystal structure: Hexagonal (microcrystalline). Cleavage: None, conchoidal fracture. Specific gravity: 2.6-2.7.
- Hardness
- 7 on Mohs scale (scratches glass)
- Color
- Gray, white, or pinkish
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale (scratches glass). Color: Gray, white, or pinkish. Luster: Vitreous to dull. Crystal structure: Hexagonal (microcrystalline). Cleavage: None, conchoidal fracture. Specific gravity: 2.6-2.7.
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone subjected to high heat and pressure. This recrystallization fuses quartz grains together into a dense, hard rock. It can range from Precambrian to Cenozoic in age.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and as a decorative stone in architecture. High-purity quartzite is used to produce silica sand for glassmaking and silicon metal.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rocks during erosion, frequently forming the peaks of hills and mountain ranges. It is often mistaken for marble but is much harder and does not react with acid.
Field identification & locations
Identify in the field by its extreme hardness and sugary, crystalline appearance on fresh breaks. It is commonly found in folded mountain belts globally. Collectors look for unique colors or associated minerals like hematite.
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