Is jarosite a sulfate?
Is jarosite a sulfate? Yes: KFe3+3(SO4)2(OH)6, a secondary mineral oxidized from pyrite, a sulfide. Yet SO4 is only 38.36% of its mass, and iron 33.45%.
Yes. Jarosite is KFe3+3(SO4)2(OH)6 on the IMA list: potassium and ferric iron with two sulfate groups and six hydroxyls, and no other anion group, so it is a sulfate. Worked by mass, though, SO4 is only 38.36% of it and iron 33.45%. It forms from pyrite, a sulfide, in oxidized rock. Hardness 2.5 to 3.5, density 2.91 to 3.26.
In short
- The verdict. Jarosite is a sulfate. The September 2026 IMA-CNMNC Master List writes KFe3+3(SO4)2(OH)6, status Rd, 1987 s.p.; the Handbook of Mineralogy sheet writes the same and prints “Mineral Group: Alunite group”. The anion groups are sulfate and hydroxyl only, with no silicate, phosphate, carbonate or sulfide beside them. One of the sheet’s own references is a paper titled “Sulfate studies IV”.
- The SO4 share ledger. A script took every Handbook sheet cached for this site on 14 November 2026, 628, and kept the 41 whose IMA formula contains SO4. 29 have SO4 as their only oxyanion. Worked on standard atomic weights, their SO4 share runs from 70.56% (anhydrite) down to 2.51% (connellite). Jarosite, at 38.36%, ranks 14th of 29.
- Documented failure: “a sulfate is mostly sulfate”. 23 of the 29 are under half SO4 by mass; only anhydrite, glauberite, bassanite, gypsum, coquimbite and celestine are over. Every one of the 19 with hydroxyl in the formula, jarosite included, is under half. Jarosite carries more iron (33.45%) than sulfur (12.80%). Class follows the anion group, not the largest share by mass.
- Second failure: SO4 in the formula is not the whole test. Of the 41, 12 also carry a silicate, phosphate, arsenate or carbonate group, or sulfide: lazurite, nosean, leadhillite and tsumebite among them. Jarosite is not one of the 12. How sulfate differs from sulfide by charge is worked for lead on is anglesite a sulfate and not repeated here.
- How and where it forms. The sheet: “typically altering from pyrite” in oxidized sulfide-bearing rocks, and around hot springs. The Queensland Government’s acid sulfate soils guidance names jarosite as the yellow mottle in oxidised acid sulfate soils.
| Rank of 29 | Mineral | IMA formula | Formula mass | SO4 by mass | S by mass | Largest metal by mass | OH in formula? |
|---|---|---|---|---|---|---|---|
| 1 | Anhydrite | Ca(SO4) | 136.13 | 70.56% | 23.55% | Ca 29.44% | No |
| 4 | Gypsum | Ca(SO4)(H2O)2 | 172.16 | 55.79% | 18.62% | Ca 23.28% | No |
| 6 | Celestine | Sr(SO4) | 183.68 | 52.30% | 17.45% | Sr 47.70% | No |
| 9 | Alunite | KAl3(SO4)2(OH)6 | 414.20 | 46.38% | 15.48% | Al 19.54% | Yes |
| 10 | Baryte | Ba(SO4) | 233.39 | 41.16% | 13.74% | Ba 58.84% | No |
| 13 | Natrojarosite | NaFe3+3(SO4)2(OH)6 | 484.68 | 39.64% | 13.23% | Fe 34.57% | Yes |
| 14 | Jarosite | KFe3+3(SO4)2(OH)6 | 500.79 | 38.36% | 12.80% | Fe 33.45% | Yes |
| 15 | Melanterite | Fe(SO4)(H2O)6·H2O | 278.01 | 34.55% | 11.53% | Fe 20.09% | No |
| 16 | Plumbojarosite | Pb0.5Fe3+3(SO4)2(OH)6 | 565.29 | 33.98% | 11.34% | Fe 29.64% | Yes |
| 18 | Anglesite | Pb(SO4) | 303.26 | 31.67% | 10.57% | Pb 68.33% | No |
| 21 | Brochantite | Cu4(SO4)(OH)6 | 452.28 | 21.24% | 7.09% | Cu 56.20% | Yes |
| 29 | Connellite | Cu36(SO4)(OH)62Cl8·6H2O | 3829.84 | 2.51% | 0.84% | Cu 59.73% | Yes |
| All 29 | SO4 the only oxyanion | — | — | 6 over 50%; 23 under | — | — | 19 with OH, all under 50% |
| Excluded 12 | SO4 plus Si, P, As, CO3 or S3 | e.g. Lazurite, Nosean, Leadhillite, Tsumebite | — | — | — | — | — |
Is jarosite a sulfate or a sulfide?
A sulfate. The IMA formula, KFe3+3(SO4)2(OH)6, puts every sulfur atom in an SO4 group, beside six hydroxyls; there is no sulfide sulfur and no other anion group. The Handbook of Mineralogy jarosite sheet prints the same formula, places the species in the alunite group, and cites Brophy and Sheridan (1965), “Sulfate studies IV: The jarosite-natrojarosite-hydronium jarosite solid solution series”. The charge-balance test that separates sulfate from sulfide is worked on is anglesite a sulfate; this page does not repeat it.
Why people ask. Jarosite grows out of a sulfide. Its sheet calls it “A secondary mineral in oxidized portions of sulfide-bearing rocks, typically altering from pyrite”, and lists alunite and pyrite as its associates. Pyrite, FeS2, is iron sulfide; jarosite is what some of that iron and sulfur become once oxygen gets in. The yellow crust and the brassy cube are two different classes of mineral on the same specimen.
A word on “hydrated”. The NASA JPL release of 2 March 2004 calls jarosite “a hydrated iron sulfate mineral”. The IMA and sheet formulas carry hydroxyl, (OH)6, and no H2O molecules; counted as water, the hydroxyl is 10.79% of the mass, which is the sheet’s ideal H2O figure. Both are describing the same formula in different words.
How much of jarosite is sulfate? The SO4 share ledger
Method. Every Handbook sheet cached for this site on 14 November 2026, 628 of them, was matched by name to the September 2026 IMA list. 41 have SO4 in the IMA formula. 12 of those also carry silicon, phosphorus, arsenic, a carbonate group or sulfide (S3), and were set aside; 29 have SO4 as their only oxyanion. For each of the 29 a script parsed the formula and computed SO4 as a share of the formula mass on standard atomic weights. The set is a working cache, not every sulfate that exists.
Result. The share runs from 70.56% for anhydrite to 2.51% for connellite. Six are over half: anhydrite, glauberite, bassanite, gypsum (55.79%), coquimbite and celestine (52.30%). Alunite is 46.38%, baryte 41.16%. Jarosite is 38.36%, 14th of 29, between natrojarosite (39.64%) and melanterite (34.55%). Anglesite is 31.67%, brochantite 21.24%, linarite 23.97%.
What that means for jarosite. It is 33.45% iron, 12.80% sulfur, 7.81% potassium and 38.36% SO4: more iron than sulfur by mass. All 19 sheets with hydroxyl in the formula are under half SO4, the highest being ammonioalunite at 48.87%. A sulfate, in other words, is named for its anion group, not for being mostly sulfate.
Where the shortcuts fail
Documented failure: “mostly sulfate”. 23 of the 29 sheets are under 50% SO4, jarosite among them. A reader who expects a sulfate to be largely sulfate by weight will be wrong about most of this set, and badly wrong about connellite at 2.51%.
Documented failure: “SO4 in the formula”. Twelve of the 41 sheets with SO4 also carry another anion group. Lazurite and nosean carry Al6Si6O24 silicate units, leadhillite and caledonite carbonate, hinsdalite and tsumebite phosphate, chalcophyllite and parnauite arsenate. Spotting SO4 is the start of the question, not the end; jarosite passes because SO4 and OH are all it has.
The relatives. Natrojarosite (Na), ammoniojarosite (NH4), hydroniumjarosite (H3O), argentojarosite (Ag) and plumbojarosite (Pb0.5) sit at 40.05% to 33.73% SO4, close to jarosite, because they share its Fe3+3(SO4)2(OH)6 framework. The aluminium version, alunite, is 46.38%; the clay question about it is on is alunite a clay mineral.
How is jarosite formed, and in which soil?
On the sheet. “A secondary mineral in oxidized portions of sulfide-bearing rocks, typically altering from pyrite; less common as a low-temperature, primary hydrothermal mineral, including as deposits around hot springs.” Oxidized sulfide outcrops are the subject of what is a gossan.
In soil. The Queensland Government’s acid sulfate soils guidance says oxidised acid sulfate soils “will often contain yellow and orange mottling. The yellow mottle is the mineral jarosite and the orange colours are other iron oxide minerals.” Its table gives a pH F reading below 4 as “Oxidised acid sulfate soil (extremely acidic)—jarosite may be present”. That is the answer to “which type of soil”, quoted; this page adds nothing to it.
Hazard, and the rest. Whether jarosite is toxic, dangerous or radioactive is routed to toxic minerals in a collection and is my mineral radioactive; this is not health or safety advice. Its Handbook numbers and localities are on jarosite, and more guides are in the collecting guides. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.