metamorphic
Quartzite (or Milky Quartz pebble)
Quartzite (primarily Silicon Dioxide, SiO2)
Hardness: 7 on Mohs scale; Color: white to light gray; Luster: vitreous to dull; Crystal structure: hexagonal/trigonal (microcrystalline); Cleavage: none (conchoidal fracture); Specific gravity: 2.6 – 2.7.
- Hardness
- 7 on Mohs scale
- Color
- white to light gray
- Luster
- vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: white to light gray; Luster: vitreous to dull; Crystal structure: hexagonal/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. This process causes the individual quartz grains to recrystallize into a locked interlocking mosaic of crystals. It is found in mountain belts globally, often dating back hundreds of millions of years.
Uses & applications
Used widely in construction as crushed stone for roadbeds and railway ballast. High-purity quartzite is used to produce ferrosilicon, industrial silica sand, and silicon metal. It is also used as a decorative stone in landscaping.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rock layers, forming prominent ridges and peaks. Unlike sandstone, which breaks around sand grains, quartzite breaks through the grains due to its high degree of cementation and recrystallization.
Field identification & locations
Identify in the field by its extreme hardness (it will easily scratch glass and stainless steel) and its crystalline appearance. It does not fizz in acid, which distinguishes it from white marble or limestone. This specimen appears to be a water-worn or weathered pebble typically found in riverbeds or landscaping gravel.
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