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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 resulting from the recrystallization of quartz grains. Unlike sandstone, quartzite fractures across the original grain boundaries, creating a smooth, conchoidal fracture surface, rather than around them. Its color is most commonly white or light gray, but impurities can impart shades of pink, red, yellow, green, or blue.
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 greasy on freshly broken surfaces.
- Texture
- Granular, sugary, interlocking mosaic of quartz crystals. Non-foliated, though relict bedding or cross-bedding from the protolith may occasionally be preserved.
- Crystal Form
- Anhedral, interlocking quartz grains, typically equigranular.
- Cleavage
- No true cleavage. Exhibits conchoidal to subconchoidal fracture, breaking across the quartz grains rather than along grain boundaries.
- Geological Environment
- Formed in regions subjected to high-grade regional metamorphism (e.g., mountain belts, cratonic shields) or contact metamorphism around igneous intrusions. Often found in association with other metamorphic rocks like schist, gneiss, and marble.
Key Facts
- Hardness: 7 on the Mohs scale (due to quartz)
- Specific Gravity: 2.65 g/cm³ (typical for quartz)
- Crystal System: Trigonal (for individual quartz crystals, but the rock is an aggregate)
- Color: White, gray, pink, red, yellow, green, blue
- Luster: Vitreous to greasy
- Transparency: Opaque to translucent (individual grains can be transparent)
- Fracture: Conchoidal to subconchoidal, breaking across grains
- Cleavage: None (though individual quartz crystals may show imperfect rhombohedral parting)
- Composition: Predominantly quartz (SiO2), typically >90-95%, with minor accessory minerals like mica, feldspar, zircon, rutile, magnetite, and hematite.
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 (no true cleavage in the rock mass).
- Fracture Type: Conchoidal to subconchoidal, breaking through the quartz grains.
- Tenacity: Brittle.
- Luster Type: Vitreous (glassy) to greasy.
Formation
Quartzite forms from the metamorphism of quartz-rich sandstone. This process involves intense heat and pressure, typically associated with regional metamorphism during mountain-building events (orogeny) or contact metamorphism adjacent to igneous intrusions. During metamorphism, the original quartz grains recrystallize, and the silica cement that bound them together also recrystallizes, forming an interlocking mosaic of quartz crystals. This recrystallization obliterates the original clastic texture of the sandstone, resulting in a rock that is much harder and more cohesive than its protolith.
Usage
Quartzite is highly valued for its durability, hardness, and resistance to weathering. It is extensively used as a construction material (dimension stone, paving, roofing tiles), in road construction (aggregate), and as a decorative stone for countertops and flooring. High-purity quartzite is a source of silica for industrial applications, including the manufacture of glass, ceramics, ferrosilicon, and silicon metal. It is also used as a refractory material due to its high melting point.
Age Distribution
Found in geological formations ranging from Precambrian to Cenozoic, with significant occurrences in ancient cratons and orogenic belts.
Where to Find
Appalachian Mountains, USA
Extensive quartzite formations are found throughout the Appalachian Orogen, particularly in states like Pennsylvania, Maryland, Virginia, and Tennessee, often forming prominent ridges due to its resistance to erosion.
Baraboo Range, Wisconsin, USA
Famous for its Precambrian Baraboo Quartzite, known for its distinctive purple-red color and excellent preservation of sedimentary structures.
Brazil
Significant deposits of high-quality quartzite, including exotic varieties used for decorative purposes, are found in various states, particularly Minas Gerais.
India
Widespread occurrences in the Aravalli Range and other ancient shield areas, used extensively as building material.
Scotland
The Cambrian-age Eriboll Quartzite in the Northwest Highlands is a classic example, forming distinctive white peaks.
Finding Tips
Look for Resistant Ridges
Quartzite is highly resistant to erosion, so it often forms prominent ridges, hogbacks, and mountain peaks in metamorphic terrains. Look for these topographic features on geological maps.
Examine Fracture Surfaces
A key diagnostic feature is its fracture. Unlike sandstone, which tends to break around individual sand grains, quartzite breaks through the grains, producing a smooth, often conchoidal fracture surface that feels gritty but not sandy.
Perform a Hardness Test
Quartzite is very hard (Mohs 7). It will scratch steel and glass. This distinguishes it from softer rocks.
Check for Relict Sedimentary Structures
While typically obliterated, sometimes relict bedding, cross-bedding, or ripple marks from the original sandstone protolith can be observed, providing clues to its origin.
Consider the Geological Context
Quartzite is a metamorphic rock. It will be found in areas that have undergone significant regional or contact metamorphism, often alongside other metamorphic rocks like slate, schist, and gneiss.
Similar Rocks
Sandstone
Sandstone
Also known as: Arenite
Chert
Chert
Also known as: Flint, Jasper, Agate
Novaculite
Novaculite
Also known as: Arkansas Stone
Scientific Classification
- Mineral Class
- Silicate (specifically Tectosilicate, as quartz is the primary component)
- Group
- Metamorphic Rock
- Crystal System
- Trigonal (for quartz, the dominant mineral)
- Chemical Formula
- SiO2 (primarily, with trace impurities)
- Composition
- Monomineralic, composed almost entirely of quartz (SiO2). Minor accessory minerals may include muscovite, biotite, feldspar, garnet, zircon, rutile, and iron oxides.
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