metamorphic
Quartzite (Ferruginous)
Metamorphosed Quartz Sandstone (SiO2 with Fe2O3 impurities)
Hardness: 7 on Mohs scale; Color: Reddish-brown to orange due to iron oxide staining; Luster: Vitreous to dull; Crystal structure: Non-foliated, granular; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.
- Hardness
- 7 on Mohs scale
- Color
- Reddish-brown to orange due to iron oxide staining
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Reddish-brown to orange due to iron oxide staining; Luster: Vitreous to dull; Crystal structure: Non-foliated, granular; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.
Formation & geological history
Formed when quartz-rich sandstone is subjected to high heat and pressure (metamorphism), causing quartz grains to recrystallize and fuse together. The reddish tint comes from iron oxide (hematite/limonite) present during formation.
Uses & applications
Used in construction as road ballast, decorative stones in landscaping, and occasionally as a source of silica in glassmaking if pure enough. Small pieces are common for tumbling.
Geological facts
Quartzite is so tough that it often breaks through the constituent quartz grains rather than around them, distinguishing it from sandstone. Some varieties are billions of years old, dating back to the Proterozoic eon.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and grainy texture that resembles sugar. It is commonly found in mountain ranges and ancient metamorphic shields globally.
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