Quartz in Metamorphic Rock

Description

Quartz (SiO2) is one of the most abundant minerals in the Earth's crust and is a common constituent of many metamorphic rocks, particularly those derived from siliciclastic sediments (like sandstone) or felsic igneous rocks (like granite). In metamorphic rocks such as schist and gneiss, quartz typically appears as colorless, white, or gray anhedral to subhedral grains, often exhibiting a vitreous luster. Its presence is fundamental to the rock's overall composition and physical properties. In schist, quartz grains are often flattened and elongated, contributing to the rock's foliation. In gneiss, quartz forms distinct layers or lenses, often segregated from darker, ferromagnesian minerals, creating the characteristic banding. The recrystallization of quartz during metamorphism results in a strong, interlocking texture, making these rocks very durable.

How to Identify

Color

Typically colorless, white, or gray within the metamorphic matrix. Can be stained by other minerals.

Luster

Vitreous (glassy) to greasy.

Texture

In schist, quartz grains are often elongated and intergrown with micas, contributing to a foliated texture. In gneiss, quartz forms granular layers or lenses, contributing to the banded texture. Grains are typically anhedral to subhedral, with interlocking boundaries due to recrystallization.

Crystal Form

Anhedral to subhedral grains, often flattened or elongated parallel to foliation in schist, or forming granular aggregates in gneiss. Well-formed euhedral crystals are rare in metamorphic rocks unless found in veins or vugs.

Cleavage

None. Quartz exhibits conchoidal fracture.

Geological Environment

Regional metamorphic terrains, contact metamorphic aureoles, and shear zones. Common in mountain belts and ancient cratons where continental crust has undergone significant deformation and metamorphism.

Key Facts

• Hardness: 7 (Mohs scale)
• Specific Gravity: 2.65 g/cm³
• Crystal System: Trigonal (alpha-quartz, stable at surface conditions)
• Color: Colorless, white, gray, but can be various colors due to impurities (e.g., rose quartz, amethyst). In metamorphic rocks, often appears colorless to gray.
• Luster: Vitreous to greasy
• Transparency: Transparent to translucent
• Fracture: Conchoidal
• Cleavage: None
• Composition: Silicon dioxide (SiO2)

Quick Check

• Color: Colorless, white, or gray
• Luster: Vitreous to greasy
• Streak: White

Physical Characteristics

• Crystal Habit: Anhedral to subhedral grains, often elongated or flattened in metamorphic rocks due to recrystallization and deformation. Can form granular aggregates.
• Cleavage Type: Absent
• Fracture Type: Conchoidal
• Tenacity: Brittle
• Luster Type: Vitreous (glassy) to greasy

Formation

Quartz is a primary constituent of many protoliths (e.g., sandstones, granites) that undergo metamorphism. During regional metamorphism, under conditions of elevated temperature and pressure, quartz grains recrystallize. In schist, quartz typically forms elongated grains or lenses aligned with the foliation, often intergrown with mica, feldspar, and other metamorphic minerals. In gneiss, quartz forms distinct bands or lenses, alternating with darker, mafic mineral bands, contributing to the characteristic gneissic banding. The recrystallization process can lead to an increase in grain size and a more interlocking texture.

Usage

Quartz-rich metamorphic rocks like quartzite are used as building materials, dimension stone, and in road construction. Pure quartz is used in glass manufacturing, electronics (piezoelectric properties), and as an abrasive. The presence of quartz in metamorphic rocks is crucial for understanding the metamorphic grade, deformation history, and protolith composition of the rock.

Age Distribution

Precambrian to Cenozoic, depending on the age of the metamorphic event and protolith.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicate (Tectosilicate)

Group

Quartz Group

Crystal System

Trigonal (alpha-quartz)

Chemical Formula

SiO2

Composition

100% Silicon Dioxide

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