What is iron sulfide?

What does iron sulfide mean, and what's its formula?

In chemistry, iron sulfide usually means iron(II) sulfide, with the formula FeS. In the simplest textbook model, iron is Fe²⁺ and sulfur is S^2-, so the compound is a 1:1 iron–sulfur solid The Chemistry of Tetragonal FeS Nano-Sized Iron Sulfide: Structure, Synthesis, Properties, and Biomedical Applications.

The names you may see for FeS

Different references use different labels for the same FeS composition. The sources describe FeS as iron(II) sulfide and also as iron monosulfide; in geological contexts, the mineral name troilite is used for one FeS form. The same review also notes tetragonal FeS_m (mackinawite), hexagonal FeS_t (troilite), and cubic sphalerite-type FeS as distinct structures within the FeS family The Chemistry of Tetragonal FeS.

Why the name can be broader than one formula

The phrase iron sulfide can also be used loosely for a wider iron–sulfur family, not only FeS. One review lists iron sulfide compounds such as FeS, Fe_1-xS, FeS₂, and Fe₃S₄, while another describes FeS as a solid phase with more than one structural form Nano-Sized Iron Sulfide: Structure, Synthesis, Properties, and Biomedical Applications The Chemistry of Tetragonal FeS.

Takeaway: if you are looking for the usual formula behind “iron sulfide,” the answer is FeS; if the context is mineral or materials science, the exact structure matters too.

Why are FeS and iron(II) sulfide the same name?

Iron(II) sulfide is the explicit chemical name for FeS: the “(II)” says iron is in the +2 oxidation state, and the formula shows a 1:1 iron-to-sulfur ratio. That is why some sources write the fuller name and others shorten it to “iron sulfide” when the context already makes the formula clear. PMC review discussing tetragonal FeS and iron(II) sulfide naming

What the formula tells you

In the sources, FeS is treated as monosulfide, while FeS₂ is iron disulfide, also called pyrite; those two names refer to different compounds with different iron-to-sulfur ratios. A separate source lists FeS₂ under ferrous disulfide, iron pyrite, or pyrite, which makes the naming split easy to see in database-style references. Iron sulfide review covering FeS and related phases FDA substance record for ferrous disulfide

Why “iron sulfide” can still be broader

Authoritative reviews use iron sulfide as an umbrella term for several iron–sulfur phases, including FeS, Fe_1−xS, FeS₂, and Fe₃S₄. They also describe FeS itself as having multiple structural forms, such as tetragonal mackinawite, hexagonal troilite, and cubic sphalerite-type FeS. PMC review of iron sulfide forms and phases PMC review of FeS polymorphs

Why real samples do not always match the textbook label

Some studies note that samples called “FeS” can be poorly defined synthetic precipitates or mixed phases, and that non-stoichiometric Fe_1−xS pyrrhotite exists with variable x. That is why mineral names, database names, and textbook formulas can overlap without being perfectly interchangeable. Non-stoichiometric and polymorphic FeS discussion

For searches such as “iron ii sulfide formula” and “iron sulfide formula,” the usual answer is FeS — but the exact formula matters whenever a source is discussing a mineral phase or a non-stoichiometric solid.

So the practical rule is simple: use iron(II) sulfide when you mean FeS, and read iron disulfide or pyrite as FeS₂. The naming difference reflects a real difference in composition, not just a style preference. FeS2 polymorph and pyrite reference

What properties and reactions identify iron sulfide?

Iron sulfide is not a single fixed material in the literature. One review describes FeS as tetragonal mackinawite, hexagonal troilite, or cubic sphalerite-type FeS, while a separate study shows that FeS can begin as soluble FeS(aq) clusters that aggregate into nanoparticles and then into solid phases (The Chemistry of Tetragonal FeS; Examining Pathways of Iron and Sulfur Acquisition, Trafficking, Deployment, and Storage in Mineral-Grown Methanogen Cells). In plain language, the sample’s appearance and behavior depend on which crystal form is present and how finely divided it is.

Phase controls the solid’s behavior

The same formula can map to different lattices. For tetragonal FeS_m, the review reports unit-cell parameters of about a ≈ 3.673 Å, c ≈ 5.033 Å, and a cell volume near 67.91 ų; for non-stoichiometric Fe_1-xS, hexagonal or monoclinic systems are reported; and FeS₂ is described as cubic pyrite or orthorhombic marcasite (The Chemistry of Tetragonal FeS; Preparation of Iron-Based Sulfides and Their Applications in Biomedical Fields; A combined experimental and theoretical study of the prototypical polymorphic transformation from marcasite to pyrite FeS2). A nano-sized iron sulfide review then quantifies how phase affects solubility, reporting pK_sp values of about 3.6 ± 0.2 for FeS, 16.4 ± 1.2 for FeS₂, 4.4 ± 0.1 for Fe₃S₄, and 5.1 ± 0.1 for Fe₇S₈ (Nano-Sized Iron Sulfide: Structure, Synthesis, Properties, and Biomedical Applications).

How air, water, and pH change the sample

Iron sulfide is also chemically changeable over time. The FeS redox study reports oxidative maturation of FeS-containing materials, with pH, redox potential, and temperature shaping transitions toward greigite or pyrite; the nano-sized iron sulfide review adds that FeS₂ surfaces react with water and oxygen to produce iron hydroxides and sulfate (Electrochemically Induced Metal- vs. Ligand-Based Redox Changes in Mackinawite: Identification of a Fe3+- and Polysulfide-Containing Intermediate; Nano-Sized Iron Sulfide: Structure, Synthesis, Properties, and Biomedical Applications). That means a sample can shift in composition and reactivity as conditions change, especially when oxygenated water is involved.

Takeaway: iron sulfide is identified more safely by its phase, solubility, and redox behavior than by formula alone.

For readers comparing samples, the key point is that iron sulfide behaves as a family of solids with different structures and stability, not as one uniform substance (The Chemistry of Tetragonal FeS; Nano-Sized Iron Sulfide: Structure, Synthesis, Properties, and Biomedical Applications).

Where does iron sulfide occur in nature and industry?

Named mineral phases are not one single material

The evidence separates “iron sulfide” into several named phases rather than one universal mineral. FeS can appear as mackinawite, troilite, or sphalerite-type material; pyrrhotite is written as Fe_1-xS because its composition varies; and FeS₂ is a different sulfide altogether, occurring as pyrite or marcasite. Other listed iron–sulfur phases include greigite and smythite. The Chemistry of Tetragonal FeS Nano-Sized Iron Sulfide: Structure, Synthesis, Properties, and Biomedical Applications A combined experimental and theoretical study of the prototypical polymorphic transformation from marcasite to pyrite FeS2

That distinction matters because a sample labeled “iron sulfide” may be a specific mineral phase, a variable-composition family, or a broader set of related sulfides rather than one fixed formula. Nano-Sized Iron Sulfide: Structure, Synthesis, Properties, and Biomedical Applications

How iron sulfide forms in oxygen-poor waters

One pathway is aqueous precipitation under anoxic, sulfide-rich conditions: Fe(II) reacts with HS^- to form soluble FeS(aq) clusters, which then aggregate into nanoparticles and solid FeS phases such as mackinawite. Examining Pathways of Iron and Sulfur Acquisition, Trafficking, Deployment, and Storage in Mineral-Grown Methanogen Cells

The reviewed sources describe the chemistry in terms of precipitation, clustering, and phase formation, not as a single immutable mineral product. Examining Pathways of Iron and Sulfur Acquisition, Trafficking, Deployment, and Storage in Mineral-Grown Methanogen Cells The Chemistry of Tetragonal FeS

How the material can change after it forms

FeS-containing materials can undergo oxidative maturation, and the cited review says that pH, redox potential, and temperature help determine whether the material shifts toward greigite or pyrite. The same source notes that the FeS-to-pyrite reaction is thermodynamically favorable, with a reported ΔG° of about −62.8 kJ/mol for pyrite and −60.9 kJ/mol for marcasite. Electrochemically Induced Metal- vs. Ligand-Based Redox Changes in Mackinawite: Identification of a Fe3+- and Polysulfide-Containing Intermediate The Chemistry of Tetragonal FeS

A bare formula does not always tell you whether the sample is a named mineral phase, a variable-composition family, or a fresh precipitate.

The literature in this section therefore points to a simple rule: when iron and sulfide meet in oxygen-poor water, the first product can be a precipitate, but the final solid may later mature into a different iron sulfide phase. Examining Pathways of Iron and Sulfur Acquisition, Trafficking, Deployment, and Storage in Mineral-Grown Methanogen Cells Electrochemically Induced Metal- vs. Ligand-Based Redox Changes in Mackinawite: Identification of a Fe3+- and Polysulfide-Containing Intermediate

What is iron sulfide used for?

The retrieved literature places iron sulfide mainly in scientific and process chemistry: studies of how it forms, how it changes phase, how it reacts at surfaces, and how different iron–sulfur solids compare with one another. One review describes FeS forming under anoxic, sulfide-rich conditions from Fe(II) and HS^-, with soluble FeS(aq) clusters aggregating into nanoparticles and then solid phases such as mackinawite Examining pathways of iron and sulfur acquisition, trafficking, deployment, and storage in mineral-grown methanogen cells. Another review on tetragonal FeS emphasizes that iron sulfide is not one single solid, but a family of phases that include mackinawite, troilite, pyrrhotite, pyrite, greigite, and smythite The Chemistry of Tetragonal FeS.

Mineralogy and geochemistry

In geology and environmental chemistry, iron sulfide is studied because it appears in low-oxygen, sulfur-rich settings and can move between solid forms as conditions change. The FeS review notes that oxidative maturation depends on pH, redox potential, and temperature, with FeS-containing materials able to transition toward greigite or pyrite Electrochemically induced metal- vs. ligand-based redox changes in mackinawite The Chemistry of Tetragonal FeS.

Materials and catalytic research

The same sources show why iron sulfide matters in surface chemistry. A review reports that FeS phases can catalyze the reduction of H⁺, CO₂, and NO₃^-, while FeS₂ surfaces react with water and oxygen to form iron hydroxides and sulfate Electrochemically induced metal- vs. ligand-based redox changes in mackinawite Nano-Sized Iron Sulfide: Structure, Synthesis, Properties, and Biomedical Applications. In practice, that means iron sulfide is discussed as a reactive surface and process material in catalysis and materials science, with behavior that depends strongly on the phase.

Why phase identity matters

Different iron sulfides do not behave the same way, so uses are phase-specific rather than generic. The comparative literature reports approximate room-temperature pK_sp values of 3.6 ± 0.2 for FeS, 16.4 ± 1.2 for FeS₂, 4.4 ± 0.1 for Fe₃S₄, and 5.1 ± 0.1 for Fe₇S₈ Nano-Sized Iron Sulfide: Structure, Synthesis, Properties, and Biomedical Applications. That spread is one reason the literature keeps distinguishing mackinawite, troilite, pyrite, greigite, and non-stoichiometric pyrrhotite instead of treating all iron sulfide as interchangeable.

Iron sulfide is best understood as a family of phases with different formation conditions, reactions, and applications, not as one uniform material.

Is iron sulfide safe to handle, and who is at risk?

The chemistry papers retrieved for this article do not read like a safety data sheet, but they do show why iron sulfide should be treated as a condition-sensitive material rather than a fixed, inert solid.

What changes the material

The main practical point is that iron sulfide can change phase when the surroundings change. The mackinawite study describes transitions toward oxidized phases such as greigite or pyrite, while the nano-sized iron sulfide review notes that FeS2 surfaces react with water and oxygen and produce iron hydroxides and sulfate. That means wet, oxygen-bearing, or otherwise changing conditions are chemically relevant, not incidental. FeS maturation under changing conditions and FeS2 surface reactivity

Who is most likely to encounter these risks

When the phase can change with pH, redox potential, temperature, water, or oxygen, the label on the container should be treated as a working hypothesis, not a guarantee.

If you meant dietary iron, what does the range show?

This is a catalog buying guide for nutritional iron, not iron sulfide chemistry. The figures below come from Tigoo’s own range snapshot, and the product amounts are catalog declarations, not recommended doses and not evidence of superiority, efficacy, or safety.

What the snapshot shows

The median declared product amount is the middle listed amount across the category, which gives a quick read on how the range is positioned without turning any one label into a recommendation.

How to read a supplement label

The snapshot does not include a product-level breakdown of forms, so no form comparison should be invented from the data given. If you are comparing labels, stay with the information actually printed on pack: ingredient form, declared amount, serving size, and tolerability.

• Ingredient form: check the exact iron form named on the label.
• Declared amount: compare the amount listed per serving, not a guessed target.
• Serving size: confirm how many capsules, tablets, or scoops make one serving.
• Tolerability: read the stated directions and constraints as written on the pack.

What this range can and cannot tell you

One snapshot can show how broad the assortment is, but it cannot tell you which product is best for a particular shopper. It also cannot tell you anything about effectiveness or safety from the milligram figure alone, because the listed amount is a catalog declaration, not a verified outcome.

Use the catalog to compare labels, not to turn a listed milligram amount into a promise.

If your goal is simply to navigate the range, start with the declared amount and the ingredient form, then check the serving size before comparing products side by side. That is the clearest way to read the category without inventing a form ranking that the snapshot does not provide.

For readers who were searching for iron sulfide by mistake, this is the point at which the paths diverge: the iron category here is a supplement assortment, while iron sulfide is a chemical topic. The catalog can help you shop; it cannot answer chemistry questions.

What are the common naming mistakes around iron sulfides?

“Iron sulfide” does not always point to one single substance. So the same words can mean a specific compound, a mineral name, or a whole group of iron-sulfur solids, depending on the source.

FeS, FeS2, and Fe1-xS are the names that get crossed

The simplest answer to “iron ii sulfide formula” is FeS. That is the formula used for iron(II) sulfide, and it is the label that appears when sources mean the 1:1 iron-to-sulfur compound FeS review. The confusion begins when readers move to iron disulfide, which is FeS₂, or to pyrrhotite, written as Fe_1-xS, where x varies and the composition is not fixed at 1:1 FeS2 polymorph study iron-based sulfides review.

If the question is “iron ii sulfide formula,” the usual answer is FeS.

Mineral names add another layer

FeS itself can appear under several mineral or structural names: tetragonal mackinawite, hexagonal troilite, and cubic sphalerite-type FeS are all listed as FeS forms in the literature FeS review. FeS₂ is different again, with cubic pyrite and orthorhombic marcasite as its two polymorphs FeS2 polymorph study. That is why “troilite” belongs with FeS, while “pyrite” belongs with FeS₂, and “pyrrhotite” belongs with Fe_1-xS.

Why formulas alone can be misleading

Some sources use “FeS” without specifying the crystal form or stoichiometry, and the literature also notes that real samples can be mixtures of phases rather than a single clean compound FeS review iron-based sulfides review. In other words, a short formula is a useful label, but it may not tell you which solid is actually being discussed.

Read the context first: chemistry class, geology paper, safety data sheet, or supplement page. Then match the name to the formula and the phase name before deciding what “iron sulfide” means in that source.