The mineral Pyrite

Chemical Formula

FeS₂

Composition

Iron sulfide, sometimes containing small amounts of cobalt, nickel, silver, and gold

Color

Yellowish gray to gray. Some specimens oxidize and form a yellow-brown film on the crystal faces.

Streak

Black with a slightly green tinge

Hardness

6 - 6½

Crystal Forms
and Aggregates

(Isometric) Pyrite can form very well crystallized specimens, which occur as cubes, pyritohedrons, and, less commonly, octahedrons. Crystals are usually striated. Pyrite frequently forms penetration twinned crystals, most commonly in the cubes. Pyrite also occurs massive, radiating, grainy, flaky, mammilary, encrusting, nodular, fibrous, and as groups of small crystals.

Transparency

Opaque

Specific Gravity

4.9 - 5.2

Luster

Metallic

Cleavage

None

Fracture

Conchoidal

Tenacity

Brittle

Other ID Marks

Some specimens develop a yellow-brown film on crystal faces.

Other Names

Fools Gold

Varieties

Dollar - Flat disc of radiating Pyrite

Pyrite Cube - Large, undistorted, cubic shaped Pyrite

Pyrite Sun - Synonym of Pyrite Dollar

Pyritohedron - Pyrite with with a pyritohedral shape

Cathedral Pyrite - Pyrite with etched, gothic-like markings

Polymorphs

Marcasite - Crystallizes in the Orthorhombic system

In Group

Sulfides ; Simple sulfides

All About

Pyrite is commonly called "Fools Gold" because of its similarity in color, shape, and habit to Gold. In the old mining days, Pyrite was many times confused with Gold, even though they can be easily distinguished (see distinguishing similar minerals below).

Pyrite occurs in many interesting shapes. It occurs in masses of very small Pyritohedrons or cubes, which creates a glistening effect when rotated in the light. It also occurs in large Cubes, either singular or intergrown, with striated faces. Most cubes are irregularly shaped; mostly elongated.

Pyrite has the same chemical formula as Marcasite, but crystallizes in a different crystal system, therefore being a separate mineral. Aggregates of FeS₂ where the crystal structure cannot be determined without complex analyzing material may be wrongly labeled by dealers. Some Pyrite specimens are labeled as Marcasite, and some Marcasite specimens as Pyrite.

Uses

Pyrite was once used as the main source of sulfur, but is now only a minor source for sulfur and iron. Pyrite from some localities is auriferous, and therefore is used as an ore of gold.

Pyrite was polished by the Native Americans in the early times and used as mirrors. Today, it is used as an ornamental stone, as well as a very popular stone for the amateur collector. It is sometimes used as gemstone by being faceted and polished for use as a side jewel in a ring, necklace, or bracelet. Pyrite is many times wrongly called "Marcasite" in the gem trade. Although the mineral Marcasite has the same composition as Pyrite, it is a different mineral. Marcasite is not suitable for gem use, because it powders and may disintegrate into a powder.

Also see the gemstone section on Pyrite

Striking Features

Hardness, color, well shaped crystals, heaviness, and streak

Popularity (1-4)

Prevalence (1-3)

Demand (1-3)

Distinguishing
Similar Minerals

Chalcopyrite - softer (3½ - 4), more intense yellow
Cobaltite - lacks the yellow color of Pyrite, softer (3½)
Marcasite - crystallizes in different crystal system (orthorhombic)
Pyrrhotite - softer (3½ - 4½), darker color
Small, gold colored, massive or flaky Pyrite may be confused with Gold, but is easily distinguished by Pyrite's black streak and Gold's yellow streak (and by Gold's low hardness of 2½ - 3)

Commonly
Occurs With

Quartz, Azurite, Microcline, Biotite, Albite, Hornblende, Barite, Chalcopyrite, Silver, Sphalerite

Noteworthy
Localities

Pyrite is a very common mineral, and occurs in numerous localities. Only the best are mentioned here. Enourmous amounts of small Pyrite crystal clusters come from Oruro and Colavi, Bolivia. In Navajun (Logroño), Spain, large cubic crystals, are abundant. They are frequently embedded in a light brown matrix, and are occasionally twinned. Perfect crystals occur in Rio Marina on the island of Elba, Italy.
In the U.S., there are also many fine localities. In Park City, Bingham Co., Utah, very large, well shaped Pyritohedrons and Pyrite Cubes have been found, as well as in the American Mine in the Bingham Canyon, Salt Lake Co., Utah. Large, intergrown cubes, many times partially octahedral, occur in abundance at Leadville, Lake Co., Colorado. Pyrite "Dollars" are mostly found in Sparta, Randolph Co., Illinois. The French Creek Mine in Chester Co., Pennsylvania is famous for the octahedral crystals that occur there, although most are distorted. Many interesting nodules were recently discovered in Alden, Monroe Co., New York.

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Links

Picture Links

1. Single Pyrite cube
2. Pyrite "Pyritohedron"
3. Mass of Pyrite crystals
4. Pyrite "Dollars"
5. Intergrown cubic Pyrite twins