Pyrite is one of the most common sulfide minerals found across the globe, recognized for its striking metallic luster and brassy yellow hue. This mineral is a compound of iron and sulfur, giving it the chemical classification of iron sulfide (FeS2).
Defining Pyrite: Chemistry and Crystal Structure
Pyrite is chemically known as iron disulfide, represented by the formula FeS2. This composition means that each iron atom within the mineral is bonded to two sulfur atoms. The sulfur atoms in pyrite exist as diatomic pairs (S2 2-), a distinct characteristic separating it from other iron-sulfur compounds.
The internal arrangement of atoms in pyrite follows the isometric crystal system, which often results in the formation of near-perfect cubes. These cubic crystals sometimes exhibit fine parallel lines, known as striations, on their faces. Another common crystal habit for pyrite is the pyritohedron, a twelve-faced shape named specifically after the mineral.
Pyrite possesses a relatively high density, with a specific gravity typically ranging between 4.95 and 5.10. On the Mohs scale of mineral hardness, pyrite scores between 6 and 6.5, meaning it is hard enough to scratch glass.
The Identity Crisis: Pyrite Versus Gold
Pyrite is most famously known by the misleading moniker “Fool’s Gold,” a name earned during historical gold rushes. This nickname arose because inexperienced prospectors often mistook the mineral’s bright, metallic, brassy-yellow color for genuine gold. The visual similarity is most striking when the minerals are observed in low light conditions.
One reliable way to differentiate pyrite from gold is by performing a streak test, which involves rubbing the mineral across an unglazed porcelain plate. Pyrite leaves a distinct greenish-black or brownish-black powder streak. In sharp contrast, genuine gold always leaves a true yellow or golden streak, regardless of the size or shape of the sample.
The physical response to force is another clear differentiator between the two minerals. Gold is highly malleable and ductile, meaning it can be hammered into thin sheets, bent, or dented without breaking. Pyrite, however, is brittle and will fracture, shatter, or crumble when struck with a hammer or a steel tool.
Gold is also significantly heavier than pyrite, possessing a specific gravity of approximately 19.3. This difference means that a piece of gold will feel much heavier for its size than a comparable piece of pyrite. Furthermore, gold rarely forms distinct cubes; instead, it typically occurs as irregular flakes, nuggets, or wires.
The tendency of pyrite to form distinct, sharp geometric crystals is a significant giveaway. While gold maintains a rich, deep yellow color that does not tarnish, pyrite often has a paler, brassy-yellow tint and can develop a dull surface tarnish or iridescent coloring upon exposure to air.
Formation Environments and Industrial Utility
Pyrite forms across a wide variety of geological settings, often where oxygen levels are low, creating a chemically reducing environment. It is commonly found in sedimentary rocks, such as shales and coal seams, where organic matter consumes oxygen and promotes the reaction between iron and sulfur. The mineral also precipitates from hot, mineral-rich fluids in hydrothermal veins, frequently alongside other sulfide minerals like galena and sphalerite.
Today, the primary commercial purpose of pyrite capitalizes on its high sulfur content. When heated in the absence of oxygen, pyrite decomposes to release sulfur dioxide gas (SO2). This gas is then collected and used as a precursor in the industrial contact process to manufacture sulfuric acid (H2SO4). The resulting acid is an important source of industrial sulfur used in fertilizer production and chemical manufacturing.
Historically, the mineral was valued as a source of ignition due to its ability to generate sparks when struck against steel. This property made it a component in early firearms, specifically in the wheel-lock mechanisms.
In the early 20th century, pyrite also played a role in the development of radio technology. Small crystals of the mineral were used in “cat’s whisker” detectors in crystal radio sets. The semiconducting properties of pyrite allowed it to rectify radio waves and convert the signal into audible sound.
The stability of pyrite changes when it is exposed to the atmosphere and water during mining operations. When iron disulfide reacts with oxygen and water, oxidation occurs. This reaction generates sulfuric acid and releases dissolved iron, leading to the environmental issue known as acid mine drainage. This drainage is characterized by its low pH, which can dissolve heavy metals from the surrounding rock, contaminating local water sources.