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Quartzite is a non-foliated metamorphic rock composed almost entirely of quartz (SiO2). It is typically white or light-colored, though impurities can impart shades of pink, red, yellow, green, or blue. It is characterized by its exceptional hardness, resistance to weathering, and a sugary to vitreous luster on freshly broken surfaces. Unlike sandstone, quartzite breaks across the quartz grains rather than around them, resulting in a conchoidal fracture. The interlocking texture of its quartz crystals makes it extremely tough and durable.
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 for red/pink, chlorite for green).
- Luster
- Vitreous (glassy) to sugary on freshly broken surfaces.
- Texture
- Granoblastic, interlocking mosaic of quartz crystals. Original clastic texture of sandstone is usually obliterated. Can be fine-grained to coarse-grained.
- Crystal Form
- Anhedral, interlocking grains of quartz.
- Cleavage
- No true cleavage; exhibits conchoidal fracture.
- Geological Environment
- Forms in regions of regional metamorphism (e.g., mountain belts) or contact metamorphism where quartz-rich sandstones are subjected to high temperatures and pressures. Often found in association with other metamorphic rocks like schist, gneiss, and marble.
Key Facts
- Hardness: 7 on the Mohs scale
- Specific Gravity: 2.65 g/cm³
- Crystal System: Trigonal (for quartz component)
- Color: White, gray, pink, red, yellow, green, blue
- Luster: Vitreous (glassy) to sugary
- Transparency: Translucent to opaque
- Fracture: Conchoidal to subconchoidal
- Cleavage: None (poor parting may occur along relict bedding planes)
- Composition: Primarily quartz (SiO2), typically >90%.
Quick Check
- Color: White to light gray (can be various colors due to impurities)
- Luster: Vitreous to sugary
- Streak: White
Physical Characteristics
- Crystal Habit: Granoblastic, interlocking mosaic of anhedral quartz grains.
- Cleavage Type: None; breaks by fracture.
- Fracture Type: Conchoidal to subconchoidal, producing sharp edges.
- Tenacity: Brittle, but extremely tough due to interlocking grains.
- Luster Type: Vitreous (glassy) on fresh surfaces, can appear sugary.
Formation
Quartzite forms from the metamorphism of quartz-rich sandstone. During regional or contact metamorphism, the original quartz grains and the silica cement that binds them recrystallize, forming a mosaic of interlocking quartz crystals. This process obliterates the original clastic texture of the sandstone, resulting in a rock that is much harder and more durable than its protolith. The intense pressure and temperature cause the quartz grains to fuse together, reducing porosity and increasing density.
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 metallurgical flux, ferrosilicon production, and in the manufacture of glass, ceramics, and abrasives. Its aesthetic appeal also makes it popular for decorative landscaping.
Age Distribution
Found in rocks of all ages, from Precambrian to Cenozoic, wherever quartz-rich sandstones have undergone sufficient metamorphism.
Where to Find
Appalachian Mountains, USA
Extensive quartzite formations are found throughout the Appalachian chain, particularly in the eastern United States, often forming 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 Orogeny.
Brazil
Brazil is a major producer of quartzite, particularly for dimension stone, with various colored varieties.
India
Extensive quartzite deposits are found across India, used for construction and industrial purposes.
Finding Tips
Look for Hardness
Quartzite is extremely hard (Mohs 7). It will scratch glass and cannot be scratched by a steel knife. This is a key differentiator from softer sandstones.
Examine Fracture
Unlike sandstone, which tends to break around individual sand grains, quartzite breaks through the grains, often exhibiting a conchoidal (shell-like) fracture surface.
Check for Grain Interlocking
Under a hand lens, the quartz grains in quartzite appear tightly interlocked, forming a dense, crystalline mass, rather than distinct, rounded grains cemented together as in sandstone.
Assess Metamorphic Context
Quartzite is a metamorphic rock, so it is typically found in areas that have undergone significant tectonic activity, such as mountain ranges or ancient cratons, often alongside other metamorphic rocks.
Consider Color and Purity
While often white, variations in color can indicate impurities. High-purity quartzite is typically very light-colored. The presence of relict sedimentary structures (like bedding) can sometimes be observed, but they are usually highly distorted.
Similar Rocks
Sandstone
Sandstone
Also known as: Arenite
Chert
Chert
Also known as: Flint, Jasper
Marble
Marble
Also known as: Crystalline Limestone
Scientific Classification
- Mineral Class
- Silicate (specifically, a rock composed of the mineral quartz)
- Group
- Metamorphic Rock
- Crystal System
- Trigonal (referring to the constituent quartz crystals)
- Chemical Formula
- SiO2 (for the primary mineral quartz)
- Composition
- Composed almost entirely of quartz (SiO2), with minor amounts of other minerals like feldspar, mica, iron oxides, and zircon as impurities.
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