Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Quartzite is a non-foliated metamorphic rock composed almost entirely of quartz (SiO). It forms from the metamorphism of quartz sandstone. The intense heat and pressure during metamorphism cause the individual quartz grains to recrystallize and interlock, forming a dense, hard, and durable rock. This interlocking texture is a key characteristic, distinguishing it from its protolith, sandstone. Pure quartzite is typically white or light gray, but impurities can impart various colors, such as pink, red (from iron oxides), yellow, green, or blue. It is characterized by its extreme hardness (7 on the Mohs scale), conchoidal fracture, and vitreous luster. Unlike sandstone, quartzite typically breaks across the original grain boundaries, not around them.
How to Identify
- Color
- Typically white to light gray, but can be pink, red, yellow, green, or blue due to impurities (e.g., iron oxides, chlorite, mica).
- Luster
- Vitreous (glassy) to greasy on freshly broken surfaces.
- Texture
- Granoblastic (non-foliated, interlocking granular texture). The original sand grains are no longer visible as distinct clasts; instead, they have recrystallized into a mosaic of intergrown quartz crystals. The rock breaks across grain boundaries, not around them.
- Crystal Form
- Individual quartz crystals are anhedral (lacking well-formed crystal faces) due to mutual interference during recrystallization, forming an interlocking mosaic.
- Cleavage
- No true cleavage due to the interlocking nature of the quartz grains. It exhibits conchoidal fracture.
- Geological Environment
- Found in regions that have undergone significant regional metamorphism, such as ancient mountain belts (orogenic zones), cratonic shields, and deeply buried sedimentary basins. It is often associated with other metamorphic rocks like schists and gneisses, and sometimes with igneous intrusions that provided the heat for metamorphism.
Key Facts
- Hardness: 7 on the Mohs scale
- Specific Gravity: 2.65 g/cm (same as quartz)
- Crystal System: Trigonal (for the constituent quartz crystals)
- Color: Typically white or light gray; can be pink, red, yellow, green, or blue due to impurities.
- Luster: Vitreous (glassy) to greasy.
- Transparency: Translucent to opaque.
- Fracture: Conchoidal to subconchoidal, breaking across grain boundaries.
- Cleavage: None (though individual quartz crystals may show poor rhombohedral cleavage, it is not observed in the rock mass).
- Composition: Predominantly quartz (SiO), typically >90-95%. Minor accessory minerals may include mica, feldspar, garnet, magnetite, rutile, zircon, and tourmaline.
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: Granoblastic, interlocking mosaic of anhedral quartz grains.
- Cleavage Type: Absent in the rock mass; individual quartz crystals have poor rhombohedral cleavage.
- Fracture Type: Conchoidal to subconchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to greasy.
Formation
Quartzite forms from the metamorphism of quartz-rich sandstone. This process typically occurs under conditions of regional metamorphism associated with continental collision zones (orogenic belts) or burial metamorphism. During metamorphism, the original quartz grains in the sandstone recrystallize, along with the silica cement, to form a mosaic of interlocking quartz crystals. This interlocking texture makes quartzite extremely durable and resistant to weathering. The temperature and pressure conditions required for quartzite formation are generally in the range of 200-700 C and 2-10 kilobars, respectively, though these can vary.
Usage
Quartzite is highly valued for its exceptional hardness, durability, and resistance to weathering and chemical attack. It is extensively used in construction as dimension stone (e.g., flooring, countertops, wall cladding), crushed stone for road construction and railway ballast, and as an aggregate in concrete. Its high silica content makes it a valuable raw material in the production of ferrosilicon, silicon metal, and silica refractories. Some varieties are used as decorative stones due to their attractive colors and textures. Historically, its toughness made it suitable for primitive tools.
Age Distribution
Found in geological formations ranging from Precambrian to Cenozoic, depending on the age of the protolith and the metamorphic event.
Where to Find
Appalachian Mountains, USA
Extensive quartzite formations are found throughout the Appalachian Orogen, particularly in states like Pennsylvania, Maryland, and Virginia, where they form prominent ridges due to their resistance to erosion.
Baraboo Range, Wisconsin, USA
Known for its distinctive Baraboo Quartzite, a Precambrian formation that forms a prominent topographic feature.
Scotland
Significant quartzite occurrences are found in the Scottish Highlands, forming part of the Caledonian Orogen.
Brazil
Large deposits of high-quality quartzite are found, particularly in Minas Gerais, used extensively for dimension stone.
India
Various regions, including Rajasthan and parts of the Himalayas, contain significant quartzite deposits.
South Africa
The Witwatersrand Basin contains extensive quartzite layers, which are economically significant due to their association with gold deposits.
Finding Tips
Look for resistant ridges and outcrops
Quartzite is highly resistant to erosion, so it often forms prominent ridges, hills, and cliffs in metamorphic terrains. Look for areas with exposed bedrock that stands out topographically.
Examine the texture
Distinguish quartzite from sandstone by examining the fracture. Quartzite breaks across the quartz grains, creating a smooth, conchoidal fracture surface that often cuts through the original sand grains. Sandstone, conversely, tends to break around the individual sand grains, leaving a gritty texture.
Test hardness
Quartzite is very hard (Mohs 7). It will scratch steel and glass. This can help differentiate it from softer rocks.
Check for metamorphic associations
Quartzite is typically found in regions that have undergone regional metamorphism. Look for it in association with other metamorphic rocks like schists, gneisses, and slates.
Safety Precautions
Quartzite contains a high percentage of crystalline silica. When cutting, grinding, or crushing quartzite, fine silica dust (respirable crystalline silica) can be generated. Inhalation of this dust can lead to silicosis, a serious and potentially fatal lung disease, as well as other respiratory illnesses and increased risk of lung cancer. Always use appropriate personal protective equipment (PPE), including NIOSH-approved respirators (e.g., N95 or P100), and employ dust control measures (e.g., wet cutting, local exhaust ventilation) when working with quartzite. Avoid prolonged exposure to airborne silica dust. Always wear eye protection and sturdy gloves when handling or breaking rock samples.
Similar Rocks
Sandstone
Sandstone
Also known as: Arenite
Chert
Chert
Also known as: Flint
Gneiss
Gneiss
Also known as: Banded Gneiss
Marble
Marble
Also known as: Crystalline Limestone
Scientific Classification
- Mineral Class
- Silicate (specifically Tectosilicate, for quartz)
- Group
- Metamorphic Rock
- Crystal System
- Trigonal (for quartz)
- Chemical Formula
- SiO (for the dominant mineral, quartz)
- Composition
- Primarily silicon dioxide (SiO), with minor amounts of other minerals depending on the protolith and metamorphic conditions.
Explore Quartzite
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.