Pyrite is absolutely a crystal, a classification confirmed by its highly ordered atomic arrangement. This common mineral, an iron sulfide with the chemical formula FeS2, possesses the precise internal structure required for this designation. To understand why pyrite is a true crystal, one must examine the fundamental organization of its constituent atoms. The geometry displayed on the mineral’s surface is a large-scale reflection of this precise, microscopic order.
What Defines a Crystal?
A crystal is defined by the arrangement of its internal components, not just its outward shape. A solid material is considered crystalline if its atoms, ions, or molecules are organized in a highly ordered, repeating pattern. This regular arrangement forms a crystal lattice, which extends in all directions. This long-range order is the fundamental requirement for any substance to be classified as a crystal.
This internal structure differentiates true crystals from amorphous solids, such as glass. Amorphous solids lack this repeating, three-dimensional blueprint, meaning their atoms are arranged more randomly, similar to a frozen liquid. The unit cell is the smallest repeating block of the lattice, which, when stacked, builds the entire crystal structure. The defining characteristic is the invisible, periodic arrangement of the components, which dictates all of the material’s physical properties.
Pyrite’s Internal Atomic Structure
Pyrite, chemically known as iron disulfide (FeS2), perfectly satisfies the requirement for a highly ordered internal structure. The iron (Fe) and sulfur (S) atoms are arranged in a precise, repeating pattern that solidifies its status as a true crystal. Its internal lattice belongs to the isometric, or cubic, crystal system, which is the most symmetrical of the seven crystal systems.
Within this structure, each iron atom is bonded to six sulfur atoms in a specific geometric configuration. The iron atoms occupy octahedral sites, while the sulfur atoms, which exist as pairs (S2 2-), occupy tetrahedral sites in the lattice. This consistent, regular packing confirms its crystalline nature. The regularity of this arrangement is so specific that marcasite, which shares the exact same chemical formula (FeS2), is classified as a different mineral because its atoms are arranged in a less symmetrical system.
Pyrite’s External Crystal Habit
The internal atomic structure of pyrite directly controls its external shape, known as its crystal habit. Pyrite is renowned for forming exceptionally well-defined, geometric crystals, which are a visible manifestation of its cubic internal lattice. The most recognized habit is the perfect cube, where the faces meet at precise 90-degree angles.
Pyrite’s external form is not limited to simple cubes; it can also grow into 12-sided shapes called pyritohedrons, octahedrons (eight-sided), or combinations of these forms. The specific external shape a pyrite crystal develops depends on the conditions—such as temperature and pressure—under which it grew. Many pyrite crystals also feature fine parallel grooves, called striations, on their faces, which are a visible clue to the mineral’s underlying atomic symmetry.
Pyrite: Fool’s Gold and Other Confusion
The common nickname “Fool’s Gold” is the primary source of confusion about pyrite, often leading people to question its classification. Pyrite’s metallic luster and pale brass-yellow color cause it to be mistaken for real gold, especially by inexperienced prospectors. Several distinct physical differences easily separate the two minerals.
Pyrite is much harder than gold, registering 6 to 6.5 on the Mohs hardness scale, meaning a knife cannot scratch it. Gold, conversely, is soft, with a hardness of 2.5 to 3, and is easily scratched. The streak test is another key difference: rubbing pyrite against an unglazed porcelain plate leaves a greenish-black or brownish-black streak, while real gold leaves a metallic yellow one. Finally, pyrite is brittle and shatters when struck, whereas gold is malleable and simply flattens.