Granite (as a common rock type containing these minerals)

Igneous (Plutonic), Metamorphic (Gneiss, Schist), Sedimentary (Arkose, Greywacke)

Quartz (SiO2) with Feldspar and Mica group minerals

Also known as: Granitic rock, Gneissic rock, Schistose rock, Arkosic sandstone

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Description

Quartz, feldspar, and mica are three of the most abundant rock-forming minerals in the Earth's continental crust. Their co-occurrence is fundamental to the composition of many common igneous, metamorphic, and sedimentary rocks. Quartz (SiO2) is a tectosilicate, typically clear to milky white, with a vitreous luster and conchoidal fracture. Feldspar, a group of tectosilicate minerals (e.g., orthoclase KAlSi3O8, plagioclase NaAlSi3O8 - CaAl2Si2O8), is generally white, pink, or gray, with two distinct cleavages at or near 90 degrees. Mica, a group of phyllosilicate minerals (e.g., muscovite KAl2(AlSi3O10)(OH)2, biotite K(Mg,Fe)3AlSi3O10(OH)2), is characterized by its perfect basal cleavage, allowing it to split into thin, flexible sheets. The relative proportions and textures of these minerals define many rock types.

How to Identify

Color: Variable. Quartz is typically colorless, white, or gray. Feldspar can be white, pink, gray, or green. Mica can be silvery (muscovite), black/dark brown (biotite), or green (chlorite, a mica-group mineral). The overall rock color depends on the proportions and colors of these minerals.
Luster: Quartz: Vitreous. Feldspar: Vitreous to pearly. Mica: Pearly to vitreous, sometimes submetallic.
Texture: Variable. In igneous rocks (e.g., granite), interlocking granular texture. In metamorphic rocks (e.g., gneiss), foliated (banded) or schistose (platy minerals aligned). In sedimentary rocks (e.g., arkose), clastic (detrital grains).
Crystal Form: Quartz: Anhedral to euhedral (hexagonal prisms with pyramidal terminations). Feldspar: Anhedral to subhedral (blocky crystals). Mica: Platy, tabular, or flaky crystals.
Cleavage: Quartz: No cleavage (conchoidal fracture). Feldspar: Two distinct cleavages at or near 90 degrees. Mica: Perfect basal cleavage (splits into thin sheets).
Geological Environment: Continental crust, particularly in orogenic belts, stable cratons, and sedimentary basins. Found in igneous intrusions (plutons), metamorphic terrains (regional and contact metamorphism), and clastic sedimentary deposits.

Key Facts

Hardness: Quartz: 7 (Mohs). Feldspar: 6-6.5 (Mohs). Mica: 2-4 (Mohs).
Specific Gravity: Quartz: 2.65. Feldspar: 2.5-2.8. Mica: 2.7-3.3.
Crystal System: Quartz: Trigonal. Feldspar: Monoclinic (orthoclase) or Triclinic (plagioclase). Mica: Monoclinic.
Color: Quartz: Colorless, white, gray, various. Feldspar: White, pink, gray, green. Mica: Silvery, black, brown, green.
Luster: Quartz: Vitreous. Feldspar: Vitreous to pearly. Mica: Pearly to vitreous.
Transparency: Quartz: Transparent to translucent. Feldspar: Translucent to opaque. Mica: Transparent to translucent.
Fracture: Quartz: Conchoidal. Feldspar: Uneven to conchoidal. Mica: Uneven.
Cleavage: Quartz: None. Feldspar: Two distinct cleavages at or near 90 degrees. Mica: Perfect basal cleavage.
Composition: Quartz: SiO2. Feldspar: KAlSi3O8 (orthoclase), NaAlSi3O8 - CaAl2Si2O8 (plagioclase). Mica: KAl2(AlSi3O10)(OH)2 (muscovite), K(Mg,Fe)3AlSi3O10(OH)2 (biotite).

Quick Check

Color: Variable (white, gray, pink, black, silvery)
Luster: Vitreous to pearly
Streak: White (for quartz and feldspar), colorless (for muscovite), brownish (for biotite)

Physical Characteristics

Crystal Habit: Quartz: Prismatic, massive, granular. Feldspar: Blocky, tabular, massive. Mica: Platy, flaky, tabular.
Cleavage Type: Quartz: Absent. Feldspar: Good to perfect in two directions. Mica: Perfect in one direction (basal).
Fracture Type: Quartz: Conchoidal. Feldspar: Uneven to conchoidal. Mica: Uneven.
Tenacity: Quartz: Brittle. Feldspar: Brittle. Mica: Flexible and elastic (muscovite), flexible but inelastic (biotite).
Luster Type: Vitreous (quartz), Vitreous to pearly (feldspar), Pearly to vitreous (mica)

Formation

This mineral assemblage is characteristic of many common crustal rocks. Quartz, feldspar, and mica form under a wide range of geological conditions. In igneous rocks like granite, they crystallize from cooling magma. In metamorphic rocks like gneiss and schist, they recrystallize under high temperature and pressure. In sedimentary rocks like arkose, they are detrital grains derived from the weathering of pre-existing rocks.

Usage

The rocks containing this assemblage (e.g., granite) are widely used as building materials, dimension stone, aggregates, and in various industrial applications. Individual minerals have specific uses: quartz (electronics, glass), feldspar (ceramics, glass), mica (insulators, paints).

Age Distribution

Precambrian to Cenozoic, depending on the specific rock type and geological setting.

Where to Find

Sierra Nevada Batholith, USA

Extensive granitic and granodioritic intrusions, rich in quartz, feldspar, and mica, forming the core of the Sierra Nevada mountains.

Appalachian Mountains, USA

Numerous exposures of granites, gneisses, and schists containing these minerals, particularly in the Blue Ridge and Piedmont provinces.

Fennoscandian Shield, Europe

Vast areas of Precambrian granites, gneisses, and schists, showcasing diverse assemblages of quartz, feldspar, and mica.

Canadian Shield, Canada

One of the largest areas of exposed Precambrian igneous and metamorphic rocks, abundant in quartz, feldspar, and mica.

Himalayan Orogen, Asia

High-grade metamorphic rocks (gneisses, schists) and granitic intrusions formed during continental collision, rich in these minerals.

Finding Tips

Look for Granitic Outcrops

In areas with exposed bedrock, look for light-colored, coarse-grained igneous rocks. Granites are common and easily identifiable by their interlocking grains of quartz (glassy), feldspar (blocky, often pink or white), and mica (shiny, flaky).

Examine Metamorphic Terrains

In regions known for metamorphism, search for foliated rocks like gneiss (banded appearance) or schist (shiny, platy minerals aligned). The presence of quartz, feldspar, and mica is key to their identification.

Check Sedimentary Rocks

For arkose, look for sandstones with a significant proportion of feldspar grains (often pinkish or whitish) in addition to quartz. These are typically found near source areas of granitic rocks.

Use a Hand Lens

A hand lens (10x magnification) is invaluable for distinguishing the individual mineral grains and their characteristic features (cleavage, luster, crystal form) within the rock.

Observe Weathering Patterns

Feldspar weathers more readily than quartz, often altering to clay minerals. Mica is relatively resistant but can be eroded into flakes. Observing differential weathering can help identify the mineral components.

Similar Rocks

Granite

Also known as: Granitic rock

Gneiss

Also known as: Gneissic rock

Schist

Also known as: Schistose rock

Arkose

Also known as: Arkosic sandstone

Granodiorite

Also known as: Granodioritic rock

Scientific Classification

Mineral Class: Quartz: Tectosilicate. Feldspar: Tectosilicate. Mica: Phyllosilicate.
Group: Quartz Group, Feldspar Group, Mica Group
Crystal System: Quartz: Trigonal. Feldspar: Monoclinic or Triclinic. Mica: Monoclinic.
Chemical Formula: Quartz: SiO2. Feldspar: KAlSi3O8 (orthoclase), NaAlSi3O8 - CaAl2Si2O8 (plagioclase). Mica: KAl2(AlSi3O10)(OH)2 (muscovite), K(Mg,Fe)3AlSi3O10(OH)2 (biotite).
Composition: Quartz: Silicon dioxide. Feldspar: Aluminosilicates of potassium, sodium, and calcium. Mica: Hydrous aluminosilicates of potassium, magnesium, and iron.

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