Gneiss with Pyrite

Metamorphic Rock (Gneiss) with Sulfide Mineral (Pyrite)

Gneiss (metamorphic rock) with Pyrite (FeS2)

Also known as: Pyritic Gneiss, Sulfidic Gneiss

Got a photo? Identify rocks instantly with the Rockby app

Description

Gneiss with Pyrite is a high-grade metamorphic rock characterized by distinct compositional banding (gneissic foliation) of alternating light-colored (felsic, e.g., quartz, feldspar) and dark-colored (mafic, e.g., biotite, hornblende) mineral layers. Embedded within this banded matrix, or sometimes concentrated in specific layers, are crystals or aggregates of pyrite. Pyrite typically appears as brassy yellow, metallic, cubic, or irregular grains. The overall appearance can vary greatly depending on the protolith of the gneiss and the abundance and distribution of pyrite.

How to Identify

Color: Gneiss matrix: Variable, often light to dark gray, pink, or reddish due to feldspar and quartz, with darker bands of biotite, hornblende, or garnet. Pyrite: Brassy yellow, metallic.
Luster: Gneiss matrix: Variable, typically vitreous to dull for silicates. Pyrite: Metallic.
Texture: Gneiss: Coarse-grained, foliated with distinct compositional banding (gneissic texture). Pyrite: Crystalline, often euhedral (cubic, pyritohedral) or anhedral grains.
Crystal Form: Gneiss minerals: Anhedral to subhedral, elongated or platy minerals aligned to form foliation. Pyrite: Typically cubic, pyritohedral, or octahedral crystals; also massive, granular, or reniform aggregates.
Cleavage: Gneiss minerals: Varies (e.g., perfect in micas, good in hornblende, poor in feldspar, none in quartz). Pyrite: Indistinct to poor, conchoidal to uneven fracture.
Geological Environment: High-grade regional metamorphic terrains, often associated with ancient continental shields, orogenic belts, and deep crustal sections. Pyrite can be syngenetic with metamorphism or introduced by hydrothermal fluids.

Key Facts

Hardness: Gneiss: Variable, typically 6-7 (Mohs) for quartz and feldspar. Pyrite: 6-6.5 (Mohs).
Specific Gravity: Gneiss: 2.6-3.0 g/cm³. Pyrite: 4.95-5.03 g/cm³.
Crystal System: Gneiss: Constituent minerals have various systems (e.g., quartz-trigonal, feldspar-triclinic/monoclinic, micas-monoclinic). Pyrite: Isometric (cubic).
Color: Gneiss: Light to dark gray, pink, red, black, banded. Pyrite: Pale brass-yellow.
Luster: Gneiss: Vitreous, silky, dull. Pyrite: Metallic.
Transparency: Gneiss: Opaque to translucent (for individual minerals). Pyrite: Opaque.
Fracture: Gneiss: Irregular to conchoidal (for quartz). Pyrite: Uneven to conchoidal.
Cleavage: Gneiss: Variable, depending on constituent minerals (e.g., perfect in micas, poor in feldspar). Pyrite: Indistinct to poor.
Composition: Gneiss: Predominantly quartz, feldspar (orthoclase, plagioclase), micas (biotite, muscovite), amphiboles (hornblende), pyroxenes, garnet. Pyrite: Iron disulfide (FeS2).

Quick Check

Color: Gneiss: Banded light and dark (gray, pink, black). Pyrite: Brassy yellow.
Luster: Gneiss: Vitreous to dull. Pyrite: Metallic.
Streak: Gneiss: White to light gray (for felsic minerals), dark (for mafic minerals). Pyrite: Greenish-black to brownish-black.

Physical Characteristics

Crystal Habit: Gneiss: Granoblastic to lepidoblastic/nematoblastic texture with aligned minerals. Pyrite: Euhedral cubes, pyritohedra, octahedra; also massive, granular, reniform.
Cleavage Type: Gneiss: Varies by mineral (e.g., perfect basal in micas, two directions at ~90° in feldspar). Pyrite: Indistinct to poor.
Fracture Type: Gneiss: Irregular to conchoidal. Pyrite: Uneven to conchoidal.
Tenacity: Gneiss: Brittle. Pyrite: Brittle.
Luster Type: Gneiss: Vitreous, silky, dull. Pyrite: Metallic.

Formation

Gneiss forms from the high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (e.g., granite, shale, sandstone). This involves intense heat (typically >320°C) and pressure (>4 kbar), leading to recrystallization and the development of characteristic banding (gneissic foliation). Pyrite (FeS2) can be present in the protolith (e.g., as detrital pyrite in sediments or magmatic pyrite in igneous rocks) and recrystallize during metamorphism, or it can form metasomatically during or after metamorphism through hydrothermal fluid activity, introducing sulfur and iron into the rock.

Usage

Gneiss is widely used as a building material, dimension stone, and crushed stone for road construction. When pyrite is present, especially in significant quantities, its use can be limited due to the potential for acid generation (acid rock drainage) when exposed to weathering, which can stain and degrade the stone. However, pyrite itself is a minor ore of sulfur and iron, and historically, it was used for sulfuric acid production. In some cases, pyritic gneisses are associated with economic sulfide ore deposits (e.g., gold, copper, zinc) where pyrite acts as a host or indicator mineral.

Age Distribution

Gneiss can range from Archean to Cenozoic, depending on the protolith and metamorphic events. Pyrite can form synchronously with metamorphism or later.

Where to Find

Canadian Shield

Extensive areas of Archean and Proterozoic gneisses, often containing pyrite, particularly in metamorphosed volcanic and sedimentary sequences.

Fennoscandian Shield (Baltic Shield)

Similar to the Canadian Shield, with vast exposures of ancient gneisses, some of which are pyritic.

Appalachian Mountains, USA

Metamorphic belts within the Appalachians contain various gneisses, including those with pyrite, especially in areas of former volcanic or sedimentary protoliths.

Himalayan Orogen

High-grade metamorphic rocks, including gneisses, are abundant, and pyrite can be found in association with various lithologies.

Brazil (e.g., Minas Gerais)

Precambrian shields in Brazil host extensive gneissic complexes, some with pyrite occurrences.

Finding Tips

Look for Banding

Identify the characteristic alternating light and dark bands of gneiss. Pyrite will typically be embedded within these bands or concentrated in specific layers.

Metallic Luster

Search for brassy yellow, metallic specks or crystals within the gneiss matrix. This is the most distinctive feature of pyrite.

Crystal Shape

If visible, look for cubic or pyritohedral crystal forms of pyrite, which are characteristic.

Associated Minerals

Pyrite in gneiss can be associated with other sulfide minerals, graphite, or certain metamorphic minerals like garnet or staurolite, depending on the protolith and metamorphic conditions.

Weathering Effects

In weathered outcrops, pyrite may be oxidized, forming rusty brown iron oxides (limonite, goethite) and potentially creating acid rock drainage, which can stain the surrounding rock. This can be an indicator of pyrite presence.

Similar Rocks

Schist with Pyrite

Schist (metamorphic rock) with Pyrite (FeS2)

Also known as: Pyritic Schist

Granite with Pyrite

Granite (igneous rock) with Pyrite (FeS2)

Also known as: Pyritic Granite

Sulfide Ore

Various sulfide minerals (e.g., chalcopyrite, sphalerite, galena) in a host rock

Also known as: Massive Sulfide

Scientific Classification

Mineral Class: Sulfide (for Pyrite)
Group: Pyrite Group (for Pyrite)
Crystal System: Isometric (for Pyrite)
Chemical Formula: FeS2 (for Pyrite)
Composition: Iron disulfide (for Pyrite). Gneiss is a rock composed of various silicate minerals.

Explore Gneiss with Pyrite

Identification Localities & Collecting Look-alikes

Identify rocks anywhere

Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.

Download on theApp Store Get it onGoogle Play Continue onWeb

Open in app