Pyrite, widely known as “Fool’s Gold” due to its metallic luster and brassy-yellow color, is a common mineral. Its resemblance to precious metals often leads to questions about its physical properties, particularly its interaction with magnets, since pure iron is highly magnetic. This article explores the magnetic classification of pyrite and explains why its composition does not result in a strong magnetic attraction.
Pyrite’s Magnetic Classification
Pure pyrite (\(\text{FeS}_2\), iron disulfide) does not stick to a household magnet. The mineral is classified as either diamagnetic or very weakly paramagnetic, meaning it lacks the strong magnetic attraction characteristic of ferromagnetic materials like iron. Ferromagnetism requires the strong, cooperative alignment of electron spins within the material’s structure, which pyrite lacks. Paramagnetic materials are only very slightly drawn into a magnetic field, a pull imperceptible without specialized laboratory equipment. Diamagnetic materials, conversely, are faintly repelled. In everyday experience, pyrite behaves as if it is non-magnetic.
Why the Iron Content is Misleading
The confusion about pyrite’s magnetic properties stems from its high iron content. However, magnetism is determined not solely by the presence of an element, but by the specific arrangement and bonding of atoms within the crystal structure. In pyrite, iron atoms are tightly bonded with sulfur atoms in a cubic crystal lattice. This bonding involves a complete pairing of the iron atoms’ outer electrons, placing the iron in a low-spin divalent state. This electron configuration prevents the formation of the large, aligned magnetic domains necessary for ferromagnetism. The chemical structure of pyrite locks the iron atoms into a non-magnetic state. This contrasts with elemental iron or magnetic iron oxides, such as magnetite (\(\text{Fe}_3\text{O}_4\)), where electrons are arranged to allow for parallel alignment of their magnetic moments. The sulfur atoms in pyrite suppress the magnetic potential the iron might otherwise exhibit.
Distinguishing Pyrite from Magnetic Minerals
Since pyrite itself is not magnetic, a simple magnet test is a practical tool for distinguishing it from other iron-containing minerals that exhibit magnetic properties. The most common strongly magnetic mineral is magnetite, an iron oxide. Magnetite will readily stick to a magnet and is even found in a naturally magnetized form called lodestone.
Another iron-sulfide mineral often confused with pyrite is pyrrhotite, which has a variable formula typically written as \(\text{Fe}_{(1-\text{x})}\text{S}\). Pyrrhotite is often called “magnetic pyrite” because it is weakly to moderately magnetic, unlike true pyrite. Its magnetism is possible because its structure contains iron vacancies, or missing iron atoms, which allow for a net magnetic moment to form.
To distinguish pyrite from these magnetic cousins without a lab, one can observe physical differences. Pyrite has a Mohs hardness of 6 to 6.5 and leaves a greenish-black streak when scraped across an unglazed porcelain plate. Pyrrhotite is softer, with a hardness of 3.5 to 4.5, and typically leaves a darker, grayish-black streak. Magnetite is harder than pyrrhotite but will leave a black streak, and its magnetic attraction is immediate and strong, providing the clearest distinction.