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Quartzite is a nonfoliated metamorphic rock composed almost entirely of quartz (SiO2). It forms from the metamorphism of quartz sandstone. The defining characteristic of quartzite is its exceptional hardness and resistance to weathering, resulting from the complete recrystallization of quartz grains and silica cement into an interlocking mosaic of quartz crystals. This process typically obliterates the original clastic texture of the sandstone, making it difficult to distinguish individual sand grains. When fractured, quartzite typically breaks across the grains, creating a smooth, conchoidal, or splintery fracture surface, unlike sandstone which tends to break around the grains. Pure quartzite is typically white or light gray, but impurities can impart various colors, including pink, red, purple, yellow, green, or blue.
How to Identify
- Color
- Typically white to light gray, but can be pink, red, purple, yellow, green, or blue due to impurities (e.g., iron oxides for reds/pinks, chlorite for greens).
- Luster
- Vitreous (glassy) to greasy on fresh surfaces.
- Texture
- Granoblastic (interlocking, equigranular crystals), nonfoliated. Original clastic texture of sandstone is usually obliterated. Grain size is typically medium to coarse-grained.
- Crystal Form
- Anhedral (lacking well-formed crystal faces) interlocking quartz grains.
- Cleavage
- No true cleavage; exhibits conchoidal to splintery fracture. The original quartz grains themselves have poor to indistinct cleavage, but the interlocking nature of quartzite prevents macroscopic cleavage.
- Geological Environment
- Formed in regions of regional metamorphism associated with convergent plate boundaries (mountain belts) or by contact metamorphism adjacent to large igneous intrusions. Often found in ancient cratonic areas and uplifted metamorphic terrains.
Key Facts
- Hardness: 7 (Mohs scale)
- Specific Gravity: 2.65 g/cm³
- Crystal System: Trigonal (for quartz mineral)
- Color: White, gray, pink, red, purple, 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% quartz. Minor accessory minerals may include feldspar, mica, garnet, zircon, rutile, magnetite, and iron oxides.
Quick Check
- Color: White, gray, pink, red, purple, yellow, green, blue
- Luster: Vitreous (glassy) to greasy
- Streak: White
Physical Characteristics
- Crystal Habit: Granoblastic, interlocking mosaic of anhedral quartz crystals.
- Cleavage Type: Absent (no true cleavage in the rock; quartz mineral has poor to indistinct cleavage)
- Fracture Type: Conchoidal to splintery
- Tenacity: Brittle
- Luster Type: Vitreous (glassy) to greasy
Formation
Quartzite forms from the metamorphism of quartz 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 original quartz grains and the silica cement that binds them recrystallize, forming a mosaic of interlocking quartz crystals. This recrystallization eliminates the original pore spaces and produces a rock that is much harder and more durable than its sandstone protolith. The degree of metamorphism determines the extent of recrystallization; high-grade quartzite shows complete obliteration of original sedimentary textures.
Usage
Quartzite is highly valued for its exceptional hardness, durability, and resistance to weathering and chemical erosion. It is extensively used in construction as dimension stone (e.g., countertops, flooring, wall cladding), crushed stone for road construction and railway ballast, and as a raw material for manufacturing silica-rich products. High-purity quartzite is used in the production of ferrosilicon, silicon metal, silicon carbide, and as a flux in metallurgical processes. Its aesthetic appeal, particularly in various colors, makes it popular for decorative purposes. Due to its high silica content, it is also used in the ceramics industry.
Age Distribution
Found in rocks of all ages, from Precambrian to Cenozoic, wherever quartz-rich sandstones have been subjected to sufficient 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 the Baraboo Quartzite, a Precambrian formation known for its distinctive purple-red color and high purity.
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 and industrial applications.
India
Widespread quartzite formations across various states, utilized for construction and industrial purposes.
Canada
Found in various Precambrian shields, such as the Canadian Shield, and in younger mountain belts.
Finding Tips
Look for Hardness
Quartzite is extremely hard (Mohs 7). It will scratch steel and cannot be scratched by a knife or common glass. This is a primary distinguishing feature from softer rocks.
Examine Fracture
Unlike sandstone, which breaks around individual sand grains, quartzite breaks through the grains, resulting in a smooth, often conchoidal or splintery fracture surface. This gives it a 'sugary' appearance on fresh breaks, but the grains are fused.
Check for Grain Fusion
Use a hand lens to observe the texture. In quartzite, the quartz grains are tightly interlocked and recrystallized, making it difficult to discern individual clastic grains. In sandstone, individual grains are typically visible and can be rubbed off.
Assess Metamorphic Context
Quartzite is a metamorphic rock, so it is typically found in regions that have undergone significant tectonic activity, such as mountain ranges, ancient cratons, or areas near large igneous intrusions.
Observe Color and Purity
Pure quartzite is white or light gray. Variations in color often indicate the presence of minor impurities, which can be a clue to its origin or specific type.
Similar Rocks
Sandstone
Sandstone
Also known as: Arenite
Chert
Chert
Also known as: Flint
Gneiss
Gneiss
Also known as: Banded Gneiss
Scientific Classification
- Mineral Class
- Silicate (specifically Tectosilicate)
- Group
- Quartz Group
- Crystal System
- Trigonal (for the constituent quartz mineral)
- Chemical Formula
- SiO2 (Silicon Dioxide)
- Composition
- Composed almost entirely of quartz (SiO2). Minor impurities can include iron oxides, feldspar, mica, garnet, and other silicates.
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