Pyrite is a naturally occurring mineral celebrated for its striking metallic luster and distinct brass-yellow color. This appearance earned it the nickname, “Fool’s Gold,” due to frequent misidentification by inexperienced prospectors. The mineral is highly abundant, found in igneous, metamorphic, and sedimentary rocks worldwide. Pyrite is important in mining history as a source of sulfur and sometimes hosts trace amounts of other valuable metals.
The Core Elements
Pyrite is chemically classified as an iron sulfide, a compound formed exclusively from iron (\(\text{Fe}\)) and sulfur (\(\text{S}\)). The precise chemical formula is \(\text{FeS}_2\), confirming that for every iron atom, there are two sulfur atoms. Pyrite is an iron disulfide, representing the most common sulfide mineral found on Earth.
By weight, the pure mineral is approximately 46.5% iron and 53.5% sulfur. This fixed 1:2 atomic ratio defines it as a distinct mineral species. The combination of these two elements gives pyrite its characteristic properties, including its metallic sheen and relatively high density. Trace amounts of other elements, such as nickel, cobalt, arsenic, or gold, can sometimes substitute for iron atoms within the crystalline structure.
Internal Arrangement
The chemical formula \(\text{FeS}_2\) represents a precise internal organization of atoms that determines the mineral’s physical properties. Unlike simpler sulfides, the sulfur atoms in pyrite are bonded together in pairs, creating a distinct \(\text{S}_2^{2-}\) unit. These diatomic sulfur units are a defining characteristic of the pyrite structure.
Each iron atom bonds to six different sulfur atoms, forming a distorted octahedron. The arrangement of these units creates a highly symmetrical, repeating crystal lattice. This internal organization causes pyrite to crystallize in the isometric system, typically forming cubes, octahedrons, or pyritohedrons. The strong, three-dimensional bonding network is responsible for the mineral’s relative hardness and brittle nature.
Pyrite vs Gold
The confusion between pyrite and gold is rooted in their visual similarities, as both exhibit a metallic luster and a yellow to brassy hue. The fundamental difference lies in their chemical composition: Pyrite is a compound (\(\text{FeS}_2\)), while gold (\(\text{Au}\)) is a pure elemental metal.
This compositional disparity results in several distinguishing physical characteristics. Pyrite is significantly harder (6 to 6.5 on the Mohs scale) and brittle, meaning it shatters when struck. Gold, by contrast, is very soft and malleable (2.5 to 3 on the Mohs scale), allowing it to be easily scratched. A definitive test is the streak color: pyrite leaves a greenish-black streak when rubbed across an unglazed porcelain plate, while gold leaves a bright, golden-yellow streak.
Gold is also dramatically denser than pyrite, with a specific gravity of 19.3 compared to pyrite’s 4.9 to 5.1. This means gold feels noticeably heavier than a piece of pyrite of the same size.