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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. The interlocking quartz grains give it a distinctive fracture pattern that often cuts across the original sand grains, unlike sandstone where the fracture tends to go around the grains. Pure quartzite is 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 freshly broken surfaces.
- Texture
- Granular, sugary, and often interlocking. The grains are typically equigranular. Unlike sandstone, individual sand grains are not easily discernible, and the rock breaks across the grains.
- Crystal Form
- Microcrystalline to macrocrystalline interlocking quartz grains, often anhedral (lacking well-formed crystal faces).
- Cleavage
- None. Quartz itself lacks cleavage, and the interlocking nature of quartzite prevents macroscopic cleavage planes. It exhibits conchoidal to subconchoidal fracture.
- Geological Environment
- Formed in regions of regional metamorphism (e.g., mountain belts, cratonic shields) or contact metamorphism (adjacent to igneous intrusions) where quartz-rich sandstones are subjected to high temperatures and pressures.
Key Facts
- Hardness: 7 on the Mohs scale
- Specific Gravity: 2.65 g/cm³ (for pure quartz)
- Crystal System: Trigonal (for quartz component)
- Color: White, gray, pink, red, purple, yellow, green, blue
- Luster: Vitreous to greasy
- Transparency: Translucent to opaque
- Fracture: Conchoidal to subconchoidal, irregular
- Cleavage: None
- 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, or various colors due to impurities
- Luster: Vitreous to greasy
- Streak: White
Physical Characteristics
- Crystal Habit: Anhedral, interlocking granular mosaic of quartz crystals.
- Cleavage Type: Absent.
- Fracture Type: Conchoidal to subconchoidal, irregular.
- Tenacity: Brittle.
- Luster Type: Vitreous (glassy) to greasy.
Formation
Quartzite forms from the metamorphism of quartz-rich sandstone. During regional or contact metamorphism, intense heat and pressure cause the quartz grains to recrystallize and interlock, forming a dense, durable rock. The original pore spaces in the sandstone are eliminated, and the silica cement that bound the sand grains together is replaced by new quartz crystals that grow into and around the original grains, creating a mosaic of interlocking quartz crystals. This process typically occurs at temperatures above 200-400 °C and pressures of several kilobars.
Usage
Due to its extreme hardness, durability, and resistance to weathering, quartzite is widely used in construction as dimension stone (e.g., flooring, countertops, wall cladding), crushed stone for road construction and railway ballast, and as a refractory material in furnaces. High-purity quartzite is also a source of silica for glass manufacturing, ceramics, and metallurgical fluxes. Some varieties are used as decorative aggregates and in landscaping.
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, such as the Erwin Quartzite and Weverton Formation, are found throughout the Appalachian orogen, particularly in the Blue Ridge and Valley and Ridge provinces.
Baraboo Range, Wisconsin, USA
Famous for its Precambrian Baraboo Quartzite, known for its distinctive purple-red color and excellent preservation of sedimentary structures.
Scotland
The Cambrian-Ordovician Quartzite of the Northwest Highlands Geopark is a prominent geological feature.
Brazil
Significant deposits, particularly in Minas Gerais, known for high-quality quartzite used in construction and as a source of industrial silica.
India
Widespread occurrences in various mountain ranges and cratonic regions, used extensively as building material.
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.
Examine Fracture
Observe the fracture pattern. Quartzite typically breaks across the original sand grains, creating a smooth, conchoidal, or subconchoidal fracture surface, often with a glassy luster. Sandstone, in contrast, tends to break around the grains, resulting in a more granular, friable surface.
Check for Grain Interlocking
Under a hand lens, look for tightly intergrown, interlocking quartz grains. The original clastic texture of sandstone is largely obliterated in quartzite.
Consider Geological Context
Quartzite is found in metamorphic terrains, often associated with other metamorphic rocks like schist, gneiss, and marble, or in areas that have undergone significant tectonic deformation.
Acid Test (Caution)
While not definitive for quartzite, a drop of dilute hydrochloric acid will not react with quartzite, distinguishing it from carbonate rocks like marble or dolostone which would effervesce.
Similar Rocks
Sandstone
Sandstone
Also known as: Arenite
Chert
Chert
Also known as: Flint, Jasper
Novaculite
Novaculite
Also known as: Arkansas Stone
Scientific Classification
- Mineral Class
- Silicate (specifically Tectosilicate for quartz)
- Group
- Metamorphic Rock
- Crystal System
- Trigonal (for quartz)
- Chemical Formula
- SiO2 (primarily)
- Composition
- Monomineralic, composed almost entirely of quartz. Impurities like iron oxides, micas, feldspars, and other silicates can be present in minor amounts.
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