Quartz with Feldspar

Igneous, Metamorphic, Sedimentary (as clasts)

SiO2 (Quartz) and (K,Na)AlSi3O8 (Feldspar)

Also known as: Granite (if intrusive igneous), Gneiss (if metamorphic), Sandstone (if sedimentary with significant feldspar)

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Description

Quartz and Feldspar are two of the most abundant minerals in Earth's crust, often found together in a wide variety of rock types. Quartz is a tectosilicate mineral composed of silicon and oxygen atoms in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Feldspar is a group of tectosilicate minerals that make up about 60% of the Earth's crust. They crystallize from magma and lava in both intrusive and extrusive igneous rocks and are also present in many types of metamorphic and sedimentary rocks. The general chemical formula for feldspar is (K,Na,Ca)Al(Al,Si)Si2O8. The specific formula (K,Na)AlSi3O8 refers to alkali feldspars, which include orthoclase (KAlSi3O8) and albite (NaAlSi3O8), and their solid solutions (e.g., anorthoclase, sanidine).

How to Identify

Color: Quartz: Typically colorless or white, but can be pink, purple (amethyst), yellow (citrine), brown (smoky quartz), green, or black. Feldspar: Typically white, pink, gray, or green. K-feldspar is often salmon-pink to reddish-brown; plagioclase feldspar is typically white to gray.
Luster: Quartz: Vitreous (glassy). Feldspar: Vitreous to pearly.
Texture: In rocks, quartz often appears anhedral (irregular grains) filling spaces, or as euhedral crystals in vugs. Feldspar often forms subhedral to euhedral crystals, sometimes exhibiting twinning. The overall texture of a rock containing both will depend on its origin (e.g., phaneritic in granite, foliated in gneiss, clastic in sandstone).
Crystal Form: Quartz: Hexagonal prisms terminating in hexagonal pyramids (when euhedral). Often anhedral in massive rocks. Feldspar: Prismatic, blocky crystals, often twinned (e.g., Carlsbad twinning in K-feldspar, polysynthetic twinning in plagioclase).
Cleavage: Quartz: No cleavage, conchoidal fracture. Feldspar: Two distinct cleavage planes intersecting at or near 90 degrees (orthoclase) or slightly less than 90 degrees (plagioclase).
Geological Environment: Found together in felsic igneous rocks (granite, granodiorite, rhyolite), many metamorphic rocks (gneiss, schist, granulite), and clastic sedimentary rocks (sandstone, arkose). They are indicative of continental crustal compositions.

Key Facts

Hardness: Quartz: 7 (Mohs). Feldspar: 6-6.5 (Mohs).
Specific Gravity: Quartz: 2.65 g/cm3. Feldspar: 2.55-2.76 g/cm3 (varies with composition).
Crystal System: Quartz: Trigonal. Feldspar: Monoclinic (K-feldspar) or Triclinic (Plagioclase).
Color: Quartz: Colorless, white, gray, purple, pink, yellow, brown, black. Feldspar: White, pink, gray, green, reddish-brown.
Luster: Quartz: Vitreous. Feldspar: Vitreous to pearly.
Transparency: Quartz: Transparent to translucent. Feldspar: Translucent to opaque.
Fracture: Quartz: Conchoidal. Feldspar: Uneven to conchoidal.
Cleavage: Quartz: None. Feldspar: Two distinct cleavages at or near 90 degrees.
Composition: Quartz: Silicon dioxide (SiO2). Feldspar: Aluminosilicates of potassium, sodium, and calcium ((K,Na)AlSi3O8 for alkali feldspar).

Quick Check

Color: Quartz: Colorless, white, gray, various. Feldspar: White, pink, gray, green.
Luster: Quartz: Vitreous. Feldspar: Vitreous to pearly.
Streak: Both: White (non-diagnostic for these minerals).

Physical Characteristics

Crystal Habit: Quartz: Prismatic, massive, granular. Feldspar: Prismatic, tabular, blocky, massive.
Cleavage Type: Quartz: Absent. Feldspar: Perfect to good in two directions.
Fracture Type: Quartz: Conchoidal. Feldspar: Uneven to conchoidal.
Tenacity: Quartz: Brittle. Feldspar: Brittle.
Luster Type: Quartz: Vitreous. Feldspar: Vitreous to pearly.

Formation

Quartz and Feldspar are primary rock-forming minerals. They form under a wide range of geological conditions. Quartz (SiO2) crystallizes from silica-rich magmas, precipitates from hydrothermal solutions, or forms through diagenesis in sedimentary environments. Feldspar ((K,Na)AlSi3O8) crystallizes from magmas (igneous rocks), recrystallizes under high temperature and pressure (metamorphic rocks), or forms as detrital grains in sedimentary rocks. Their co-occurrence is characteristic of felsic igneous rocks (e.g., granite, rhyolite), many metamorphic rocks (e.g., gneiss, schist), and mature clastic sedimentary rocks (e.g., arkose, quartz arenite).

Usage

These minerals, especially in combination within rocks like granite, are extensively used as building materials (countertops, flooring, dimension stone), in construction (crushed stone for aggregate), ceramics, glass manufacturing (silica from quartz, alumina and alkalis from feldspar), and as a source of industrial silica and feldspar for various chemical processes. Pure quartz is used in electronics (oscillators, filters) and optics.

Age Distribution

Ubiquitous throughout Earth's geological history, from Archean to Cenozoic.

Where to Find

Worldwide

As major rock-forming minerals, quartz and feldspar are found globally in regions with significant igneous, metamorphic, and sedimentary rock formations. Notable occurrences for large crystals or specific varieties include Brazil (quartz), Madagascar (feldspar, quartz), Russia (quartz), and the United States (various states for both, e.g., Colorado, North Carolina, California).

Continental Shields and Cratons

These ancient, stable parts of continents are rich in granitic and gneissic rocks, which are primary hosts for quartz and feldspar.

Mountain Ranges

Areas of active or past orogeny (mountain building) often expose large volumes of igneous and metamorphic rocks containing these minerals.

Sedimentary Basins

Mature sedimentary basins can contain sandstones and arkoses rich in detrital quartz and feldspar grains, eroded from older igneous and metamorphic sources.

Finding Tips

Look for Granitic Outcrops

Granite is an excellent place to observe quartz and feldspar together. Quartz will appear as glassy, often gray or colorless grains, while feldspar will be opaque, typically white, pink, or salmon-colored, with distinct cleavage planes.

Examine Metamorphic Rocks

In gneisses, quartz and feldspar often form alternating light-colored bands. Look for the characteristic glassy luster of quartz and the blocky, cleavable nature of feldspar.

Check Sedimentary Rocks

In sandstones, quartz grains are typically rounded and clear, while feldspar grains may be less rounded and exhibit a duller luster or faint cleavage. Arkose will have a noticeable pinkish hue due to abundant K-feldspar.

Hardness Test

Quartz (Mohs 7) will scratch glass and steel. Feldspar (Mohs 6-6.5) will scratch glass but may be scratched by quartz. This is a key differentiator between the two in hand samples.

Cleavage vs. Fracture

Feldspar's distinct cleavage (smooth, flat surfaces) is a critical identifying feature, contrasting with quartz's conchoidal (shell-like) fracture and lack of cleavage.

Similar Rocks

Granite

Quartz, Feldspar (Orthoclase, Plagioclase), Mica (Biotite, Muscovite), Amphibole

Also known as: Plutonic rock

Gneiss

Quartz, Feldspar, Mica, Amphibole, Garnet (variable)

Also known as: Banded metamorphic rock

Rhyolite

Quartz, Feldspar (Sanidine, Plagioclase), Biotite, Hornblende (fine-grained)

Also known as: Volcanic rock

Arkose

Quartz, Feldspar (predominantly K-feldspar), Mica, rock fragments

Also known as: Feldspathic sandstone

Scientific Classification

Mineral Class: Quartz: Tectosilicate. Feldspar: Tectosilicate.
Group: Quartz: Quartz Group. Feldspar: Feldspar Group.
Crystal System: Quartz: Trigonal. Feldspar: Monoclinic (K-feldspar), Triclinic (Plagioclase).
Chemical Formula: Quartz: SiO2. Feldspar: (K,Na)AlSi3O8 (for alkali feldspar).
Composition: Quartz: Silicon dioxide. Feldspar: Potassium, sodium, and aluminum silicates.

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