metamorphic
Quartzite (Milky Quartzite)
Quartzite (SiO2 silicon dioxide)
Hardness: 7 on Mohs scale; Color: typically white, gray, or tan with iron staining (shown as reddish-brown); Luster: vitreous or dull; Crystal structure: Trigonal/Hexagonal; Cleavage: none (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Luster
- vitreous or dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically white, gray, or tan with iron staining (shown as reddish-brown); Luster: vitreous or dull; Crystal structure: Trigonal/Hexagonal; Cleavage: none (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the metamorphism of quartz-rich sandstone. Under high heat and pressure, the individual quartz grains recrystallize into a locked interlocking mosaic of quartz crystals. Most quartzite is from the Proterozoic or Paleozoic eras.
Uses & applications
Used as aggregate in construction, as a decorative stone in landscaping, for glass making (if high purity), and in the production of silicon or ferrosilicon.
Geological facts
Quartzite is one of the most durable and weather-resistant rocks on Earth. This specific specimen shows iron oxide staining, which is common when rainwater leaches metallic minerals into the rock's micro-fractures.
Field identification & locations
Identify in the field by its inability to be scratched by a steel knife and its grainy, sandpaper-like texture that doesn't rub off. Commonly found in mountain belts and river beds as rounded cobbles like the one pictured.
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