metamorphic
Quartzite (Milky)
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: milky white to light grey; Luster: vitreous to waxy when wet; Crystal structure: Trigonal (microcrystalline); Cleavage: none (conchoidal fracture); Specific gravity: 2.6-2.7.
- Hardness
- 7 on Mohs scale
- Color
- milky white to light grey
- Luster
- vitreous to waxy when wet
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Physical properties
Hardness: 7 on Mohs scale; Color: milky white to light grey; Luster: vitreous to waxy when wet; 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. This process causes the individual quartz grains to recrystallize into a locked mosaic of quartz crystals.
Uses & applications
Used in construction as road ballast, roofing granules, and decorative stone. Industrially, high-purity quartzite is used to produce ferrosilicon, industrial silica sand, and silicon metal.
Geological facts
Quartzite is so tough that it usually breaks through the quartz grains rather than around them, unlike its parent stone, sandstone. It is often more resistant to weathering than the surrounding rock, frequently forming ridges or hilltops.
Field identification & locations
Identify in the field by its extreme hardness (it will easily scratch glass) and lack of visible grains typical of sandstone. Commonly found in mountainous regions or ancient river beds as water-worn cobbles.
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