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Quartzite is a non-foliated metamorphic rock composed almost entirely of quartz (SiO2). It is characterized by its exceptional hardness, resistance to weathering, and a typically granular, sugary texture resulting from the recrystallization of quartz grains. Unlike its protolith, sandstone, quartzite breaks across the quartz grains rather than around them, creating a conchoidal or splintery fracture. Its color can vary widely, from pure white or light gray (if composed of nearly pure quartz) to shades of pink, red, yellow, green, or blue due to impurities like iron oxides, micas, or other minerals.
How to Identify
- Color
- Typically white or light gray, but can be pink, red, yellow, green, or blue due to impurities. Pure quartzite is white.
- Luster
- Vitreous (glassy) to greasy on freshly broken surfaces.
- Texture
- Granular, sugary, interlocking crystalline texture. Original clastic grains are obliterated.
- Crystal Form
- Anhedral, interlocking quartz grains, typically equigranular.
- Cleavage
- No true cleavage. Exhibits conchoidal to splintery fracture, breaking across the quartz grains.
- Geological Environment
- Formed in regions of intense heat and pressure, such as convergent plate boundaries (orogenic belts) or areas affected by deep burial and contact metamorphism. Often found in ancient mountain ranges and cratonic shields.
Key Facts
- Hardness: 7 on the Mohs scale
- Specific Gravity: 2.65 g/cm³ (for pure quartz)
- Crystal System: Trigonal (for quartz mineral)
- Color: White, gray, pink, red, yellow, green, blue
- Luster: Vitreous to greasy
- Transparency: Translucent to opaque
- Fracture: Conchoidal to splintery
- Cleavage: None (breaks across grains)
- Composition: Primarily quartz (SiO2), typically >90-95%.
Quick Check
- Color: White, gray, pink, red, yellow, green, blue
- Luster: Vitreous to greasy
- Streak: White (as it is composed primarily of quartz)
Physical Characteristics
- Crystal Habit: Massive, granular, interlocking anhedral grains.
- Cleavage Type: None. Quartz itself has no cleavage.
- Fracture Type: Conchoidal to splintery, breaking through the quartz grains.
- Tenacity: Brittle
- Luster Type: Vitreous (glassy) to greasy
Formation
Quartzite forms from the metamorphism of quartz-rich sandstone. This process involves intense heat and pressure, typically associated with regional metamorphism during orogenic events (mountain building) or contact metamorphism near igneous intrusions. During metamorphism, the quartz grains in the sandstone recrystallize, and the original clastic texture is obliterated. Silica cement, if present, also recrystallizes, leading to a rock where the quartz grains are interlocked, forming a dense, durable, and non-porous rock.
Usage
Quartzite is highly valued in construction for its durability, hardness, and resistance to weathering. It is used as dimension stone for flooring, countertops, wall cladding, and roofing tiles. Crushed quartzite is used as railroad ballast, road aggregate, and in concrete. High-purity quartzite is a source of silica for glass manufacturing, ceramics, ferrosilicon production, and as a flux in metallurgy. It is also used in refractory materials due to its high melting point.
Age Distribution
Found in geological formations ranging from Precambrian to Cenozoic, depending on the age of the protolith and the timing of metamorphism.
Where to Find
Appalachian Mountains, USA
Extensive quartzite formations, particularly in the Blue Ridge and Valley and Ridge provinces, such as the Erwin Quartzite and Weverton Formation.
Baraboo Range, Wisconsin, USA
Famous for its Precambrian Baraboo Quartzite, known for its distinctive purple-red color.
Scotland
Significant quartzite occurrences in the Scottish Highlands, including the Cambrian-age Eriboll Quartzite.
Brazil
Large deposits of high-quality quartzite, particularly in Minas Gerais, used extensively for dimension stone.
India
Widespread quartzite deposits across various states, utilized for construction and industrial purposes.
Canada
Found in various Precambrian shield areas, such as the Canadian Shield, often associated with ancient orogenic belts.
Finding Tips
Look for outcrops in ancient mountain belts
Quartzite is a product of regional metamorphism, so it is commonly found in areas that have undergone significant tectonic activity and mountain building.
Test for hardness
Quartzite is very hard (Mohs 7). It will scratch glass and steel. This helps distinguish it from softer rocks.
Examine fracture pattern
Unlike sandstone, which tends to break around individual sand grains, quartzite breaks through the grains, resulting in a smooth, conchoidal, or splintery fracture surface.
Check for sugary texture and interlocking grains
Under magnification, the recrystallized, interlocking quartz grains give quartzite a characteristic sugary appearance, without the distinct clastic grains of sandstone.
Observe resistance to weathering
Quartzite is highly resistant to chemical and physical weathering, often forming prominent ridges and peaks in landscapes where surrounding softer rocks have eroded away.
Similar Rocks
Sandstone
Sandstone
Also known as: Arenite
Chert
Chert
Also known as: Flint, Jasper, Agate
Granite
Granite
Also known as: Granitic rock
Scientific Classification
- Mineral Class
- Silicate (specifically Tectosilicate)
- Group
- Quartz Group
- Crystal System
- Trigonal (for the constituent mineral quartz)
- Chemical Formula
- SiO2 (for the primary constituent mineral quartz)
- Composition
- Composed predominantly of quartz (silicon dioxide), typically >90-95%. Minor accessory minerals may include micas (muscovite, sericite), feldspars, garnets, zircon, rutile, and iron oxides, which can impart color.
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