Pyrite

Mineral Name: Pyrite (Fools Gold)

Chemical Composition: FeS2

Color: Gold, Brassy Gold, Greyish Gold.

Streak: Black with a slightly green tint.

Hardness: 6-6.5

Cleavage/Fracture: Fracture

Crystal Form: Pyrite crystals come in different forms including cubes, octahedrons, or pyritohedron (2). Cubes are six sided boxes with 90 degree angles on a side. Octahedrons are very similar to the shape of diamonds. A pyritohedron has twelve pentagonal faces, but these crystals may be shaped irregularly.

Luster: Pyrite has a Metallic luster.

Special Features: Pyrite is able to replace shell material when creating replacement fossils. It also has a different colored streak than the mineral itself. While the mineral is a brassy gold, the streak is a green tinted black. The crystal faces of many pyrite minerals have striations, meaning there are a series of ridges and linear lines (2). This is apparent in cathedral pyrite, which is a variety of pyrite.

Varieties: Varieties include rainbow pyrite, cathedral pyrite, and bravoite. Rainbow pyrite is unique in the way that it has a rainbow-like iridescence which is caused by the refraction and diffraction of light (2). This mineral has also only been found in Russia thus far. Cathedral pyrite is the mineral in larger gothic-like markings and crystals, which explains its name “cathedral” (2). Bravoite is is pyrite with a nickel-cobalt bearing, with nickel, Ni2, substituting the Fe2 in the pyrite mineral composition (2).

Mineral Group: Pyrite falls into the sulfide group, which is a group of minerals where the main anion is sulfide (S2-), hence the name iron sulfide. Pyrite is actually the most common mineral of the sulfide group.

Environment: Different rocks that contain pyrite for in different environments. In igneous rocks, pyrite crystals will be formed as the magma cools. The slower the magma cools, the bigger the crystals will be. The process of weathering cause pyrite to be found in metamorphic rocks. If found in sedimentary rocks, the mineral may have formed in a depositional environment or when the sediment was being formed into a rock. The presence of sulphate-reducing bacteria in sediments will reduce the sulphate ions to sulfides (cite). If iron is present when this occurs, then pyrite will form (10). Pyrite can also be found in the upper Midwest, near the upper northern part of the Mississippi river. Here it is very common to see pyrite occurring as vein deposits (which are sheetlike bodies of crystallized minerals that cut across other, in-situ rock). More pyrite vein deposits are also located on the border of Canada. Lastly, hydrothermal vents contain a rich source of iron, meaning particles of pyrite can also be found in these environments at the bottom of the ocean, along divergent boundaries.

Associated Rock types: Pyrite can be found in some quartz veins. It can be found in sedimentary rocks, such as shale, coal, sandstone, and limestone (2). Pyrite can also sometimes contain miniscule amounts of cobalt, nickel, silver and gold. As well it can be found in certain types of contact metamorphic rocks.

Occurrence in North America: Like it was mentioned earlier, pyrite can be found in the Upper Midwestern part of North America, in States like Wisconsin, Iowa, Pennsylvania, and Minnesota. Pyrite also can be found in states such as Illinois, Colorado, Arizona, and Utah (2). In Utah, large and well-shaped pyritohedrons and cubes of pyrite crystals can be found in the mines. In Illinois, pyrite dollars can be found, which are spherical discs of the mineral (2). Pennsylvania contains good examples of octahedral pyrite.

Economic Uses: Pyrite has been used as far back as the 16th century as an ignition source in firearms (9). Pyrite was also used by Native Americans who would polish it to make mirrors (6). Pyrite is sometimes used for jewelry as a gemstone (1). It can be polished and used in rings, necklaces and bracelets. Pyrite is usually sold being called marcasite, however marcasite itself decays too quickly under moisture to be used as jewelry (2). Pyrite is also very popular for collecting and can be used as household decor.

Industrial Uses: Pyrite was a good place to find the sulfur used to create sulfuric acid in the 1800’s through the end of World War II (6). The creation of sulfuric acid begins with the roasting of pyrite. This is done in very hot temperatures until the sulphur in the pyrite heap is gone (8). This process creates sulfur dioxide, which is then oxidized into sulfur trioxide. The sulfur trioxide’s reaction to water is what creates sulfur acid. Pyrite is sometimes mined for its association with gold, chalcopyrite, and copper. Since the sulfur content is so high it can be manufactured into copperas, or iron sulfate (8). Iron pyrite is heaped up, weathered, and the acidic runoff from the heap is boiled with iron to create the copperas. Copperas, ferrous sulfate, is used in dyeing, in the manufacturing of ink, as a wood preservative and also as a disinfectant.(8)

First Notable Identification: Though it is unknown when and who first identified pyrite, it was first used as a source for starting fire. If pyrite is struck against a hard metal is able to produce a spark and in some cases help start a fire. This fact gave the mineral its name, as “Pyr” translates into fire (2).

How We Identified It: There are four ways to identify pyrite. A streak test, the color, it’s crystal shape and the density of the mineral. Pyrite is a darkish yellow color, very similar to brass. The density of the mineral is pretty low, much lower than it’s ‘twin’ gold. For instance, if pyrite is put in water, the small flakes will wash away unlike gold (7). Another way to help identify pyrite is by using a streak plate and rubbing the mineral onto that plate which should produce a color that comes out as greenish black. Its shape is very distinct in the way the crystals form. The mineral may form in a perfect cube, for example.

Don’t Confuse It With: Gold! You can differentiate the two, pyrite and gold, by color. Gold is usually a brighter yellow-gold. Pyrite’s hardness is also much higher than gold; gold is very malleable in comparison to pyrite and can be easily scratched or bent. Gold and pyrite streaks are also different colors, gold has a yellow streak and pyrite has a black to green streak. Chalcopyrite, in its antique gold coloring, can also be confused for pyrite. This mineral however, is more yellow and has a lower hardness than pyrite.