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Quartzite with a quartz vein is a metamorphic rock characterized by its dominant mineral, quartz, which has undergone recrystallization to form a tough, interlocking mosaic of quartz grains. The distinguishing feature is the presence of one or more distinct veins composed almost entirely of quartz, cutting across the quartzite host rock. These veins can range in thickness from hairline fractures to several meters wide. The vein quartz may exhibit different textures (e.g., massive, crystalline, drusy) and colors compared to the host quartzite, depending on the purity of the hydrothermal fluids and the conditions of deposition.
How to Identify
- Color
- Quartzite: Typically white, gray, or light brown, but can be pink, red, yellow, or green due to impurities (e.g., iron oxides, chlorite). Quartz Vein: Usually milky white or clear, but can be stained by impurities.
- Luster
- Vitreous (glassy) to greasy on fresh surfaces for both quartzite and vein quartz.
- Texture
- Quartzite: Granoblastic, interlocking mosaic of quartz grains, often sugary or gritty to the touch. Vein Quartz: Can be massive, crystalline (euhedral to anhedral crystals), or drusy (small crystals lining a cavity). The vein texture will be distinct from the host rock.
- Crystal Form
- Quartzite: Anhedral, interlocking quartz grains. Vein Quartz: Can be anhedral (massive) or euhedral (well-formed hexagonal prisms with pyramidal terminations) if space allowed for crystal growth.
- Cleavage
- None. Quartz exhibits conchoidal fracture.
- Geological Environment
- Quartzite forms in regions of regional metamorphism (e.g., mountain belts, cratonic shields) or contact metamorphism (near igneous intrusions). Quartz veins form in brittle fracture zones within these metamorphic terrains, where hydrothermal fluids circulate.
Key Facts
- Hardness: 7 (Mohs scale) for both quartzite and vein quartz.
- Specific Gravity: 2.65 g/cm³ for both quartzite and vein quartz.
- Crystal System: Trigonal (for individual quartz crystals within the vein, and the constituent grains of quartzite).
- Color: Quartzite: White, gray, pink, red, yellow, green. Vein Quartz: Milky white, clear, sometimes stained.
- Luster: Vitreous to greasy.
- Transparency: Quartzite: Opaque to translucent. Vein Quartz: Translucent to transparent.
- Fracture: Conchoidal to subconchoidal.
- Cleavage: None.
- Composition: Primarily Silicon Dioxide (SiO2) for both the quartzite and the quartz vein.
Quick Check
- Color: Quartzite: Variable (white, gray, pink, etc.). Vein: Milky white to clear.
- Luster: Vitreous to greasy.
- Streak: White.
Physical Characteristics
- Crystal Habit: Quartzite: Granular, interlocking anhedral grains. Vein Quartz: Massive, crystalline (prismatic, often terminated), or drusy.
- Cleavage Type: None.
- Fracture Type: Conchoidal to subconchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to greasy.
Formation
Quartzite forms from the metamorphism of quartz-rich sandstone (quartz arenite). During regional or contact metamorphism, the original quartz grains recrystallize and interlock, often obliterating the original sedimentary textures. Quartz veins typically form later, when silica-rich hydrothermal fluids circulate through fractures and fissures within the already formed quartzite. These fluids precipitate quartz (SiO2) as they cool or undergo pressure changes, filling the open spaces.
Usage
Quartzite itself is highly valued as a durable building material, dimension stone, and aggregate. The presence of quartz veins generally does not detract from these uses, and in some cases, can add aesthetic appeal. Pure quartz from veins can be used in electronics (piezoelectric applications), optics, and as an abrasive, though veins within quartzite are usually not pure enough or large enough for these specialized uses. Historically, quartz veins were often prospected for associated economic mineral deposits (e.g., gold, silver, base metals) that might have been transported and deposited by the same hydrothermal fluids.
Age Distribution
Varies widely depending on the protolith (sandstone) age and the timing of metamorphism and vein formation. Can range from Precambrian to Cenozoic.
Where to Find
Appalachian Mountains, USA
Extensive quartzite formations (e.g., Erwin Quartzite, Weverton Formation) with numerous quartz veins are common throughout the Appalachian orogen.
Rocky Mountains, USA/Canada
Many Proterozoic and Paleozoic quartzite units (e.g., Belt Supergroup) are present, often cut by quartz veins.
Brazilian Shield
Large areas of Precambrian quartzite with associated quartz veins are found in Brazil.
Fennoscandian Shield, Scandinavia
Ancient quartzite formations with abundant quartz veining are characteristic of this cratonic region.
Western Australia
Precambrian quartzites are widespread, often hosting significant quartz veins, sometimes associated with gold mineralization.
Finding Tips
Look for Outcrops in Metamorphic Terrains
Focus on areas known for regional or contact metamorphism, particularly ancient mountain belts or cratonic shields. Road cuts, stream beds, and quarry exposures are good places to start.
Identify the Host Rock First
Confirm the presence of quartzite by its hardness, sugary texture, and lack of cleavage. Then, look for distinct linear or irregular features cutting across the quartzite.
Observe Color and Texture Differences
Quartz veins will often appear lighter (milky white) and may have a different texture (more crystalline or massive) than the surrounding quartzite. The contact between the vein and host rock is usually sharp.
Check for Associated Mineralization
While not always present, quartz veins can be indicators of past hydrothermal activity that might have deposited other minerals. Look for sulfides (pyrite, chalcopyrite), iron oxides, or other alteration minerals within or adjacent to the vein.
Use a Hardness Test
Both quartzite and vein quartz are very hard (Mohs 7) and will scratch glass and steel. This helps distinguish them from softer minerals or rocks.
Similar Rocks
Chert
Cryptocrystalline Quartz
Also known as: Flint, Jasper, Agate
Sandstone
Clastic Sedimentary Rock
Also known as: Quartz Arenite
Gneiss
High-grade Metamorphic Rock
Also known as: Banded Gneiss
Scientific Classification
- Mineral Class
- Silicate (Tectosilicate)
- Group
- Quartz Group
- Crystal System
- Trigonal
- Chemical Formula
- SiO2
- Composition
- Silicon Dioxide
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