Iron sulfide is a fundamental compound found in the Earth’s crust, consisting of iron and sulfur atoms joined together. The question of whether this common mineral is magnetic does not have a simple yes or no answer. This is because “iron sulfide” refers to a group of minerals with slightly different atomic arrangements and compositions. Subtle variations in the ratio of iron to sulfur atoms profoundly change the material’s physical properties, especially its magnetic behavior.
Understanding Variable Chemical Composition
Iron sulfide exists in nature as a series of different mineral structures, each defined by a precise ratio of its constituent elements. The most common forms are represented by two distinct chemical blueprints: \(\text{FeS}_2\) and \(\text{Fe}_{1-x}\text{S}\). This difference in the number of iron atoms relative to sulfur atoms dictates whether the mineral will respond to a magnetic field.
The formula \(\text{FeS}_2\) represents a fixed, stable compound with a specific, symmetrical atomic arrangement. Conversely, \(\text{Fe}_{1-x}\text{S}\) describes a non-stoichiometric compound, meaning the ratio of iron to sulfur is variable. The ‘x’ indicates a deficit of iron atoms in the crystal lattice. These missing iron atoms create vacancies within the structure, which are the physical basis for the material’s magnetic characteristics.
Pyrite The Non-Magnetic Iron Sulfide
Pyrite, known chemically as iron disulfide (\(\text{FeS}_2\)), is the most widespread iron sulfide mineral and is often dubbed “Fool’s Gold” due to its metallic luster and pale brass-yellow color. This form of iron sulfide is fundamentally non-magnetic and will not be attracted to a common magnet. Its non-magnetic nature lies in its highly symmetrical internal arrangement, known as a cubic crystal structure.
In this structure, the iron atoms are in a low-spin state, and the sulfur atoms are paired, forming \(\text{S}_2\) units. This pairing and the overall symmetry of the lattice cause the individual magnetic moments of the iron atoms to effectively cancel each other out. Pyrite is classified as diamagnetic, though some specimens exhibit weak paramagnetism, meaning they are only slightly influenced by an external magnetic field.
Pyrrhotite The Magnetic Iron Sulfide
The forms of iron sulfide that are magnetic are collectively known as Pyrrhotite, represented by the variable formula \(\text{Fe}_{1-x}\text{S}\). This mineral is non-stoichiometric, containing a variable deficiency of iron atoms. The resulting iron vacancies in the crystal lattice are the direct cause of the mineral’s attraction to a magnet. The most strongly magnetic variety of Pyrrhotite has the specific formula \(\text{Fe}_7\text{S}_8\) and crystallizes in a monoclinic structure.
This specific arrangement of iron vacancies results in a magnetic behavior known as ferrimagnetism. Ferrimagnetism is a strong form of magnetism. The iron atoms’ magnetic moments are aligned in opposite directions, but because there are fewer iron atoms overall due to the vacancies, one direction of alignment is stronger than the other, resulting in a net magnetic attraction. Pyrrhotite’s crystal structure is typically hexagonal or monoclinic, contrasting with the cubic habit of Pyrite.
Distinguishing Between the Forms
Distinguishing between the magnetic and non-magnetic forms of iron sulfide is straightforward. The most direct test is simply to use a magnet, as Pyrrhotite will show a measurable attraction while Pyrite will not. This magnetic test is highly reliable because no other brass-colored sulfide minerals are naturally magnetic.
Beyond magnetism, the two minerals can be differentiated by several other physical characteristics:
- Pyrite is noticeably harder (6 to 6.5 on the Mohs scale), while Pyrrhotite is softer (3.5 to 4.5).
- Pyrite leaves a greenish-black to brownish-black streak when rubbed across an unglazed porcelain plate, whereas Pyrrhotite produces a darker gray to black streak.
- Pyrite typically forms distinctive cubic or twelve-sided crystals.
- Pyrrhotite often appears in massive forms or as tabular, platy crystals with a more bronze-yellow or brownish-bronze color.