Pyrite with Magnetite

Mineral Association

FeS2 (Pyrite) with Fe3O4 (Magnetite)

Also known as: Iron Sulfide and Iron Oxide Association

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Description

Pyrite with Magnetite refers to the physical association of the mineral pyrite (iron disulfide) and the mineral magnetite (iron oxide). This association is common in many ore deposits and metamorphic rocks. Pyrite typically presents as brassy yellow, metallic crystals, often cubic, pyritohedral, or octahedral. Magnetite is black to dark gray, opaque, and strongly magnetic, often occurring as granular masses or octahedral crystals. The presence of both minerals together provides insights into the geochemical conditions of their formation, particularly concerning oxygen fugacity and sulfur activity. For example, in some iron ore deposits, magnetite is the primary ore mineral, and pyrite may be present as an accessory, indicating a sulfur-rich environment during or after magnetite formation. Conversely, in some sulfide deposits, pyrite is dominant, and magnetite may be present due to local oxidation or specific fluid compositions.

How to Identify

Color: Pyrite: Brassy yellow, metallic. Magnetite: Black to dark gray.
Luster: Pyrite: Metallic. Magnetite: Metallic to submetallic.
Texture: Pyrite: Crystalline, often euhedral to subhedral crystals. Magnetite: Granular, massive, or octahedral crystals.
Crystal Form: Pyrite: Cubes, pyritohedrons, octahedrons, often striated. Magnetite: Octahedrons, dodecahedrons, granular masses.
Cleavage: Pyrite: Poor to indistinct on {001}. Magnetite: None (parting on {111} sometimes observed).
Geological Environment: Hydrothermal veins, skarns, banded iron formations, sedimentary rocks (e.g., black shales), metamorphic rocks, igneous intrusions.

Key Facts

Hardness: Pyrite: 6-6.5 (Mohs). Magnetite: 5.5-6.5 (Mohs).
Specific Gravity: Pyrite: 4.95-5.02. Magnetite: 5.17-5.18.
Crystal System: Pyrite: Isometric. Magnetite: Isometric.
Color: Pyrite: Brassy yellow. Magnetite: Black to dark gray.
Luster: Pyrite: Metallic. Magnetite: Metallic to submetallic.
Transparency: Opaque for both.
Fracture: Pyrite: Uneven to conchoidal. Magnetite: Uneven to subconchoidal.
Cleavage: Pyrite: Poor to indistinct on {001}. Magnetite: None (parting on {111} sometimes observed).
Composition: Pyrite: Iron disulfide (FeS2). Magnetite: Iron oxide (Fe3O4).

Quick Check

Color: Brassy yellow (Pyrite) and Black (Magnetite)
Luster: Metallic (Pyrite) and Metallic to Submetallic (Magnetite)
Streak: Greenish-black (Pyrite) and Black (Magnetite)

Physical Characteristics

Crystal Habit: Pyrite: Cubes, pyritohedrons, octahedrons, massive, granular. Magnetite: Octahedrons, dodecahedrons, granular, massive.
Cleavage Type: Pyrite: Poor/Indistinct. Magnetite: None (parting).
Fracture Type: Pyrite: Uneven to conchoidal. Magnetite: Uneven to subconchoidal.
Tenacity: Pyrite: Brittle. Magnetite: Brittle.
Luster Type: Pyrite: Metallic. Magnetite: Metallic to submetallic.

Formation

Pyrite and Magnetite can form together in a variety of geological environments. Pyrite (FeS2) typically forms under reducing conditions, often in hydrothermal veins, sedimentary rocks (e.g., black shales), and metamorphic rocks. Magnetite (Fe3O4) forms under a wider range of conditions, including igneous (e.g., mafic and ultramafic intrusions), metamorphic (e.g., skarns, banded iron formations), and hydrothermal settings. Their co-occurrence often indicates specific redox conditions and iron availability during mineralization. For instance, in some skarn deposits, both minerals can be abundant. In banded iron formations (BIFs), magnetite is a primary component, and pyrite can form diagenetically or later during metamorphism. In hydrothermal systems, their paragenesis depends on fluid chemistry, temperature, and pressure.

Usage

While not a 'rock' in itself, the association of pyrite and magnetite is significant. Pyrite is a major source of sulfur and can be a minor source of gold (auriferous pyrite). Magnetite is a primary ore of iron. Their co-occurrence can indicate potential for economic deposits of iron, base metals (if associated with other sulfides), or gold. Pyrite is also used in the production of sulfuric acid. Magnetite is used in heavy media separation, as a pigment, and in some magnetic applications.

Age Distribution

Found in geological formations ranging from Precambrian to Cenozoic, depending on the specific deposit type and host rock age.

Where to Find

Kola Peninsula, Russia

Known for various iron and sulfide deposits where both minerals can be found.

Kiruna, Sweden

Famous for its large iron ore deposits, primarily magnetite, with associated sulfides including pyrite.

Broken Hill, Australia

A world-class lead-zinc-silver deposit with significant pyrite and magnetite in the metamorphic host rocks.

El Indio, Chile

High-sulfidation epithermal gold deposit where pyrite is abundant, and magnetite can be present in certain alteration zones.

Banded Iron Formations (various global locations)

Ancient sedimentary rocks rich in iron oxides (magnetite, hematite) where pyrite can form diagenetically or during metamorphism.

Finding Tips

Look for Metallic Luster

Pyrite's distinctive brassy metallic luster is often visible, even in small grains. Magnetite will have a darker, metallic to submetallic luster.

Test for Magnetism

Magnetite is strongly magnetic and will attract a common magnet. This is a key diagnostic feature to distinguish it from other black minerals.

Observe Crystal Habits

Pyrite often forms well-developed cubes, pyritohedrons, or octahedrons. Magnetite can also form octahedrons or appear as granular masses.

Check Streak Color

Pyrite has a greenish-black streak. Magnetite has a black streak. This can help differentiate them if they are finely intergrown.

Consider Geological Context

Their co-occurrence is common in ore deposits (e.g., skarns, hydrothermal veins, BIFs) and metamorphic terrains. Understanding the local geology can guide your search.

Similar Rocks

Pyrrhotite with Magnetite

Fe(1-x)S (Pyrrhotite) with Fe3O4 (Magnetite)

Also known as: Magnetic Pyrite with Magnetite

Chalcopyrite with Magnetite

CuFeS2 (Chalcopyrite) with Fe3O4 (Magnetite)

Also known as: Copper Iron Sulfide with Iron Oxide

Hematite with Pyrite

Fe2O3 (Hematite) with FeS2 (Pyrite)

Also known as: Iron Oxide with Iron Sulfide

Scientific Classification

Mineral Class: Pyrite: Sulfide. Magnetite: Oxide.
Group: Pyrite: Pyrite Group. Magnetite: Spinel Group.
Crystal System: Isometric for both.
Chemical Formula: FeS2 (Pyrite) and Fe3O4 (Magnetite)
Composition: Pyrite: Iron (46.55%), Sulfur (53.45%). Magnetite: Iron (72.36%), Oxygen (27.64%).

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