Calcite with Pyrite

Mineral Association

CaCO3 (Calcite) with FeS2 (Pyrite)

Also known as: Pyritic Calcite

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Description

Calcite with Pyrite refers to a mineral specimen where crystals of calcite (calcium carbonate) are found in close association with, or intergrown with, crystals of pyrite (iron disulfide). This combination is common and visually striking due to the contrasting colors and crystal habits of the two minerals. Calcite typically forms transparent to translucent crystals, often rhombohedral or scalenohedral, while pyrite forms opaque, metallic, brassy-yellow crystals, commonly cubic, pyritohedral, or octahedral.

How to Identify

Color: Calcite: Typically colorless, white, or light shades (yellow, pink, blue, green, brown, black). Pyrite: Opaque, metallic brass-yellow.
Luster: Calcite: Vitreous to sub-vitreous. Pyrite: Metallic.
Texture: Calcite: Smooth to slightly rough, depending on crystal habit. Pyrite: Smooth, often with striations on crystal faces.
Crystal Form: Calcite: Highly variable, commonly rhombohedral, scalenohedral, prismatic, or tabular. Pyrite: Commonly cubic, pyritohedral, or octahedral; also massive, granular, or radiating.
Cleavage: Calcite: Perfect rhombohedral cleavage in three directions (1011). Pyrite: Indistinct to poor cleavage, conchoidal to uneven fracture.
Geological Environment: Hydrothermal veins, sedimentary rocks (especially shales, limestones, and coal seams), metamorphic rocks, and as a gangue mineral in various ore deposits.

Key Facts

Hardness: Calcite: 3 (Mohs). Pyrite: 6-6.5 (Mohs).
Specific Gravity: Calcite: 2.71. Pyrite: 4.95-5.10.
Crystal System: Calcite: Trigonal. Pyrite: Isometric.
Color: Calcite: Colorless, white, various tints. Pyrite: Brassy yellow.
Luster: Calcite: Vitreous to sub-vitreous. Pyrite: Metallic.
Transparency: Calcite: Transparent to translucent. Pyrite: Opaque.
Fracture: Calcite: Conchoidal to uneven. Pyrite: Conchoidal to uneven.
Cleavage: Calcite: Perfect rhombohedral (3 directions at 74°55'). Pyrite: Indistinct to poor.
Composition: Calcite: Calcium Carbonate (CaCO3). Pyrite: Iron Disulfide (FeS2).

Quick Check

Color: Calcite: Variable, often white/colorless. Pyrite: Brassy yellow.
Luster: Calcite: Vitreous. Pyrite: Metallic.
Streak: Calcite: White. Pyrite: Greenish-black to brownish-black.

Physical Characteristics

Crystal Habit: Calcite: Rhombohedral, scalenohedral, prismatic, tabular, massive, granular. Pyrite: Cubic, pyritohedral, octahedral, massive, granular, radiating.
Cleavage Type: Calcite: Perfect rhombohedral. Pyrite: Indistinct/poor.
Fracture Type: Calcite: Conchoidal to uneven. Pyrite: Conchoidal to uneven.
Tenacity: Calcite: Brittle. Pyrite: Brittle.
Luster Type: Calcite: Vitreous. Pyrite: Metallic.

Formation

Calcite and Pyrite often form together in hydrothermal veins, sedimentary environments (especially in anoxic conditions), and metamorphic rocks. Calcite precipitates from calcium-rich solutions, while pyrite forms in reducing environments where iron and sulfur are present. Their co-occurrence indicates specific geochemical conditions during mineralization.

Usage

Primarily of interest to mineral collectors and researchers. Pyrite can be a minor ore of sulfur, and historically, it was mistaken for gold ('fool's gold'). Calcite is a major component of cement and agricultural lime. The combination itself has no specific industrial use beyond the individual minerals.

Age Distribution

Found in geological formations ranging from Precambrian to Cenozoic, depending on the specific depositional environment.

Where to Find

Elmwood Mine, Carthage, Tennessee, USA

Known for world-class calcite crystals, often associated with sphalerite, fluorite, and sometimes pyrite.

Naica Mine, Chihuahua, Mexico

Famous for giant selenite crystals, but also produces calcite and pyrite in hydrothermal veins.

Various localities in Peru

Many Peruvian ore deposits yield excellent specimens of calcite and pyrite, often with other sulfides.

Dalnegorsk, Primorsky Krai, Russia

Produces large, well-formed calcite crystals, sometimes with associated pyrite.

Certain coal seams and shales worldwide

Pyrite concretions and crystals are common in anoxic sedimentary environments, often with calcite as a cementing agent or later infill.

Finding Tips

Hydrothermal Veins

Look for these minerals in quartz or carbonate veins cutting through various rock types, especially in mining districts.

Sedimentary Rocks

Examine shales, limestones, and coal deposits for concretions or disseminated crystals. Pyrite often forms in reducing conditions within these rocks.

Acid Test for Calcite

Calcite will effervesce vigorously when a drop of dilute hydrochloric acid (HCl) is applied. Pyrite will not react.

Hardness Test

Calcite (Mohs 3) can be scratched by a copper coin, while pyrite (Mohs 6-6.5) cannot and will scratch glass.

Streak Test for Pyrite

Pyrite leaves a greenish-black to brownish-black streak on an unglazed porcelain plate, distinguishing it from gold (yellow streak).

Similar Rocks

Dolomite with Pyrite

CaMg(CO3)2 (Dolomite) with FeS2 (Pyrite)

Also known as: Pyritic Dolomite

Quartz with Pyrite

SiO2 (Quartz) with FeS2 (Pyrite)

Also known as: Pyritic Quartz

Ankerite with Pyrite

Ca(Fe,Mg,Mn)(CO3)2 (Ankerite) with FeS2 (Pyrite)

Also known as: Pyritic Ankerite

Scientific Classification

Mineral Class: Calcite: Carbonates. Pyrite: Sulfides.
Group: Calcite: Calcite Group. Pyrite: Pyrite Group.
Crystal System: Calcite: Trigonal. Pyrite: Isometric.
Chemical Formula: Calcite: CaCO3. Pyrite: FeS2.
Composition: Calcite: Calcium Carbonate. Pyrite: Iron Disulfide.

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