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 (SiO2). It is characterized by its exceptional hardness, vitreous luster, and conchoidal fracture. Unlike its protolith, sandstone, quartzite breaks across the quartz grains rather than around them, indicating the complete recrystallization and interlocking of the constituent 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 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
- Granoblastic, non-foliated. Individual quartz grains are tightly interlocked and recrystallized, making it difficult to distinguish original clastic texture. The rock breaks through the grains, not around them.
- Crystal Form
- Microcrystalline to macrocrystalline interlocking quartz grains, typically anhedral (lacking well-formed crystal faces).
- Cleavage
- None, as quartz itself lacks cleavage. However, a conchoidal fracture is characteristic.
- Geological Environment
- Formed in regions subjected to regional metamorphism (e.g., mountain-building events) or contact metamorphism (e.g., adjacent to igneous intrusions) where quartz-rich sandstones are present. Often found in ancient cratonic areas, mountain belts, and stable continental platforms.
Key Facts
- Hardness: 7 on the Mohs scale (due to its primary composition of quartz).
- Specific Gravity: 2.65 g/cm³ (typical for quartz).
- Crystal System: Trigonal (for the constituent quartz crystals). The rock itself is an aggregate of these crystals.
- Color: Pure quartzite is white or light gray; impurities can lead to pink, red, yellow, green, or blue hues.
- Luster: Vitreous (glassy) to greasy.
- Transparency: Translucent to opaque, depending on grain size and purity.
- Fracture: Conchoidal to subconchoidal, splintery in some cases.
- Cleavage: None (quartz lacks cleavage).
- Composition: Primarily quartz (SiO2), typically >90-95%. Minor accessory minerals may include mica, feldspar, garnet, zircon, rutile, and iron oxides.
Quick Check
- Color: White, gray, or various colors due to impurities.
- Luster: Vitreous to greasy.
- Streak: White (as quartz is colorless in powder form).
Physical Characteristics
- Crystal Habit: Granoblastic, anhedral interlocking grains of quartz.
- Cleavage Type: Absent.
- Fracture Type: Conchoidal to subconchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous 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 within the sandstone to recrystallize. This process obliterates the original clastic texture of the sandstone, fusing the individual quartz grains together with newly formed quartz cement. The interlocking crystalline structure makes quartzite extremely durable and resistant to weathering. The degree of metamorphism determines the extent of recrystallization; high-grade metamorphism results in a very dense, vitreous rock where individual sand grains are no longer discernible.
Usage
Quartzite is highly valued for its exceptional hardness, durability, and resistance to weathering and chemical attack. It is extensively used as a dimension stone for building facades, flooring, countertops, and paving. Its resistance to abrasion makes it suitable for railway ballast and road construction aggregate. High-purity quartzite is a critical raw material for the production of ferrosilicon, silicon metal, silicon carbide, and as a flux in metallurgical processes. It is also used in the manufacture of silica brick for refractory applications and in glassmaking.
Age Distribution
Found in geological formations ranging from Precambrian to Cenozoic, depending on the age of the protolith sandstone and the timing of metamorphism.
Where to Find
Appalachian Mountains, USA
Extensive quartzite formations, particularly in the Blue Ridge and Valley and Ridge provinces, such as the Erwin Quartzite and Weverton Quartzite.
Baraboo Range, Wisconsin, USA
Famous for the Baraboo Quartzite, a Precambrian formation known for its distinctive purple-red color.
Scotland
Significant quartzite occurrences in the Scottish Highlands, including the Cambrian-age Eriboll Quartzite.
Brazil
Large deposits of high-purity quartzite, particularly in Minas Gerais, used for industrial applications and dimension stone.
India
Widespread quartzite formations across various states, utilized for construction and industrial purposes.
Finding Tips
Look for outcrops in metamorphic terrains
Quartzite is a common rock in areas that have undergone regional metamorphism, such as ancient mountain belts or shield areas. Look for resistant ridges and hills, as quartzite is very erosion-resistant.
Test for hardness
Quartzite is extremely hard (Mohs 7). It will scratch steel and glass. This is a key differentiator from softer rocks.
Examine fracture patterns
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 individual quartz grains in quartzite appear tightly interlocked and recrystallized, with no visible pore space or original clastic texture.
Consider the protolith
Quartzite forms from sandstone. If you are in an area with extensive sandstone formations that have been subjected to metamorphism, there's a good chance of finding quartzite.
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 (for the primary mineral quartz)
- Composition
- Predominantly silicon dioxide (SiO2), with minor amounts of other minerals as impurities.
Explore Quartzite
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.