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Quartzite is a non-foliated 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 the way it fractures: unlike sandstone, which breaks around the individual sand grains, quartzite breaks through the grains, creating a smooth, conchoidal fracture surface. This indicates that the original quartz grains and the silica cement have recrystallized and intergrown to form a dense, interlocking mosaic of quartz crystals. 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 has a vitreous (glassy) luster and a granular to sugary texture, though the grains are so tightly interlocked that they are often not easily discernible without magnification. It is one of the most durable and chemically resistant rocks found on Earth's surface.
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.
- Texture
- Granular, sugary, or massive. The grains are tightly interlocked and often not visible to the naked eye. Non-foliated, though relict bedding or cross-bedding from the protolith sandstone may occasionally be preserved.
- Crystal Form
- Anhedral, interlocking mosaic of quartz crystals. Individual crystals are typically microscopic to macroscopic, forming a dense, compact mass.
- Cleavage
- None. Quartzite breaks with a conchoidal fracture, cutting across the quartz grains, unlike sandstone which breaks around the grains.
- Geological Environment
- Formed in regional metamorphic settings associated with mountain-building events (orogenesis) or deep burial. Found in ancient cratonic shields, fold-and-thrust belts, and areas that have undergone significant tectonic compression and heating.
Key Facts
- Hardness: 7 on the Mohs scale
- Specific Gravity: 2.65 g/cm³
- Crystal System: Trigonal (for quartz, the primary mineral)
- 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 through the grains.
- Cleavage: None (quartz itself has no cleavage, and the interlocking texture prevents rock cleavage).
- Composition: Predominantly quartz (SiO₂), typically >90% quartz. Minor accessory minerals may include mica, feldspar, garnet, zircon, rutile, and iron oxides.
Quick Check
- Color: White, gray, pink, red, yellow, green, blue
- Luster: Vitreous (glassy) to greasy
- Streak: White (as it is harder than the streak plate)
Physical Characteristics
- Crystal Habit: Anhedral, interlocking granular mosaic.
- Cleavage Type: Absent.
- Fracture Type: Conchoidal to subconchoidal.
- Tenacity: Brittle, extremely tough and durable.
- 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 is what gives quartzite its characteristic hardness and durability, making it much tougher than its protolith sandstone. The transformation involves elevated temperatures (typically 200-700 °C) and pressures (2-10 kbar), which cause the quartz grains to dissolve at their boundaries and reprecipitate, effectively welding them together.
Usage
Quartzite is highly valued for its extreme hardness, durability, and resistance to weathering and chemical erosion. It is extensively used as a construction material, including dimension stone for flooring, wall cladding, countertops, and paving. Its high silica content makes it suitable for manufacturing refractory bricks and as an aggregate in concrete. Crushed quartzite is used in road construction and railway ballast. High-purity quartzite is a source of silicon for the electronics industry (e.g., silicon chips) and for producing ferrosilicon alloys. Due to its aesthetic appeal, some varieties are used in decorative applications and as ornamental stone.
Age Distribution
Ranges from Precambrian to Cenozoic, depending on the age of the protolith sandstone and the timing of metamorphism. Many significant quartzite formations are Proterozoic or Paleozoic.
Where to Find
Appalachian Mountains, USA
Extensive quartzite formations, such as the Erwin Quartzite and Weverton Formation, are found throughout the Appalachian Mountains, particularly in the Blue Ridge and Valley and Ridge provinces. These are often Proterozoic to early Paleozoic in age.
Baraboo Range, Wisconsin, USA
Famous for the Baraboo Quartzite, a Precambrian (Paleoproterozoic) formation known for its distinctive purple-red color and excellent preservation of sedimentary structures.
Scotland
The Cambrian-age Eriboll Quartzite in the Northwest Highlands is a classic example, forming prominent peaks and ridges.
Brazil
Significant deposits, particularly in Minas Gerais, known for high-quality quartzite used in construction and as a source of silicon.
India
Various regions, including Rajasthan and parts of the Himalayas, contain extensive quartzite formations of different ages.
Canada
Found in the Canadian Shield and other orogenic belts, such as the Rocky Mountains.
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 key differentiator from softer rocks like marble or even some sandstones.
Examine Fracture Pattern
Unlike sandstone, which tends to break around individual sand grains, quartzite breaks through the grains, resulting in a smooth, often conchoidal fracture surface. This 'through-grain' fracture is diagnostic.
Check for Grain Interlocking
The grains in quartzite are tightly interlocked and recrystallized, making it difficult to dislodge individual grains with your fingers or a tool. Sandstone, in contrast, often feels gritty and individual grains can be rubbed off.
Observe Luster
Quartzite typically exhibits a vitreous (glassy) to greasy luster on freshly broken surfaces due to the quartz composition.
Consider Geological Context
Quartzite is found in metamorphic terrains, often associated with mountain ranges or ancient shield areas. Look for it in areas where regional metamorphism has occurred.
Acid Test (for differentiation from marble)
Quartzite will not react with dilute hydrochloric acid, as it is composed of silica. Marble, which is composed of calcite, will effervesce vigorously. This is a crucial test to distinguish white quartzite from white marble.
Similar Rocks
Sandstone
Sandstone
Also known as: Arenite
Chert
Chert
Also known as: Flint, Jasper
Marble
Marble
Also known as: Crystalline Limestone
Gneiss
Gneiss
Also known as: Banded Gneiss
Scientific Classification
- Mineral Class
- Silicate (specifically Tectosilicate, as quartz is the dominant mineral)
- Group
- Metamorphic Rock
- Crystal System
- Trigonal (referring to the quartz crystals within the rock)
- Chemical Formula
- SiO₂ (for the dominant mineral, quartz)
- Composition
- Primarily silicon dioxide (SiO₂), with minor amounts of other minerals depending on the impurities in the protolith sandstone.
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