metamorphic
Quartzite
Quartzite (mostly SiO2)
Hardness: 7 on Mohs scale; Color: tan to light grey with white speckling; Luster: dull to vitreous; Crystal structure: granoblastic (interlocking quartz grains); Cleavage: none; Specific gravity: 2.6-2.7. Highly resistant to physical weathering.
- Hardness
- 7 on Mohs scale
- Color
- tan to light grey with white speckling
- Luster
- dull to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: tan to light grey with white speckling; Luster: dull to vitreous; Crystal structure: granoblastic (interlocking quartz grains); Cleavage: none; Specific gravity: 2.6-2.7. Highly resistant to physical weathering.
Formation & geological history
Formed through the regional metamorphism of quartz-rich sandstone. High heat and pressure cause the individual sand grains to recrystallize and fuse together, forming an extremely tough, interlocking rock fabric. This specific specimen has been further shaped by fluvial or glacial erosion into a rounded cobble.
Uses & applications
Used extensively in the construction industry as crushed stone for road ballast, sub-base, and concrete aggregate. High-purity quartzite is used to produce ferrosilicon, industrial silica sand, and silicon metal. It is also used as decorative stone in landscaping.
Geological facts
Quartzite is so tough that when it breaks, it fractures through the quartz grains rather than along the boundaries between them, unlike its parent rock sandstone. It is one of the most durable rocks found on the Earth's surface.
Field identification & locations
Identify in the field by its extreme hardness (it will easily scratch glass and stainless steel) and its crystalline, sugary appearance on fresh breaks. It is commonly found in mountain belts where tectonic activity has occurred and as rounded 'river rocks' in stream beds far from its source.
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