metamorphic
Quartzite
Quartzite (primarily SiO2)
Hardness: 7 (Mohs scale), Color: tan, grey, or reddish-brown (from iron oxidation), Luster: vitreous to dull, Crystal structure: trigonal (microcrystalline), Cleavage: none (conchoidal fracture), Specific gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, grey, or reddish-brown (from iron oxidation), Luster: vitreous to dull, Crystal structure: trigonal (microcrystalline), Cleavage: none (conchoidal fracture), Specific gravity: 2.6-2.7
Formation & geological history
Formed through the regional or contact metamorphism of quartz-rich sandstone. Under intense heat and pressure, original quartz grains recrystallize and fuse into a solid mass of interlocking quartz crystals.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and flooring. Highly durable for use as decorative landscaping stones and, in high purity grades, as a source for silica in glassmaking.
Geological facts
Quartzite is so hard and chemically resistant that it often forms the crests of mountain ridges because it withstands erosion better than surrounding rocks. It's often mistaken for marble but can be distinguished because quartzite will not react to acid.
Field identification & locations
Identify in the field by its sugary texture on fresh surfaces and its ability to scratch glass easily. Commonly found in folded mountain belts and as water-worn cobbles (like this specimen) in riverbeds and glacial deposits.
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