metamorphic
Quartzite (River Pebble)
Quartzite (composed primarily of Silica, SiO2)
Hardness: 7 (Mohs scale); Color: Tan, beige, or brownish-grey due to iron impurities; Luster: Dull to vitreous; Crystal Structure: Trigonal/Hexagonal microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.
- Hardness
- 7 (Mohs scale)
- Color
- Tan, beige, or brownish-grey due to iron impurities
- Luster
- Dull to vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, beige, or brownish-grey due to iron impurities; Luster: Dull to vitreous; Crystal Structure: Trigonal/Hexagonal microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.
Formation & geological history
Formed through the regional metamorphism of quartz-rich sandstone. Under intense heat and pressure, quartz grains recrystallize and fuse together. This specific specimen is a water-worn river pebble, shaped by fluvial transport over thousands of years.
Uses & applications
Commonly used as construction aggregate, road ballast, and decorative landscaping stone. In a high-purity state, it is used to produce silica sand for glassmaking.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rock layers during erosion, forming resistant ridges or 'monadnocks.' Unlike sandstone, which breaks around grains, quartzite breaks through the grains because the cement is as strong as the quartz itself.
Field identification & locations
Identify in the field by its extreme hardness (it will easily scratch glass or a steel knife) and its grainy, sugary texture on fresh breaks. It is ubiquitous in riverbeds and glacial till throughout North America and Europe.
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