Jarosite
Jarosite, KFe3+3(SO4)2(OH)6: amber-yellow to brown, streak pale yellow, hardness 2.5–3.5, 33.45% iron, altered from pyrite; named for the Barranco del Jaroso.
Jarosite is a potassium iron sulfate hydroxide, KFe3+3(SO4)2(OH)6 on the September 2026 IMA list: hexagonal, amber-yellow to dark brown, streak pale yellow, hardness 2.5 to 3.5, density 2.91 to 3.26. Worked from the formula it is 33.45% iron. Its Handbook sheet says it typically forms by alteration of pyrite in oxidized sulfide-bearing rocks, and names it after the Barranco del Jaroso, Spain.
In short
- The species. The September 2026 IMA-CNMNC Master List lists jarosite as KFe3+3(SO4)2(OH)6, status Rd, 1987 s.p., Spain. The Handbook of Mineralogy sheet gives the same formula: hexagonal, R3̄m, crusts of minute crystals or pseudocubic rhombohedra to 25 cm, cleavage {0001} distinct, hardness 2.5 to 3.5, density 2.91 to 3.26, amber-yellow to dark brown with a pale yellow streak. A 20.00 g piece reads 13.13 to 13.87 g in water.
- How it forms. “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.” Its associates on the sheet are alunite and pyrite. Pyrite decay in a collection, whose listed signs include a yellow efflorescence, is on why did my pyrite break; that page does not name the products as jarosite, and this page does not either.
- The Saint Félix recast. The sheet’s analysis from Saint Félix de Pallières, France, recast on 2 sulfur, gives K 0.823, Fe 3.046 and 5.89 OH, matching the printed K0.82 and Fe3.05. Sodium comes out at 0.089, twice the printed Na0.04, which equals the Na2O count (0.044). So the alkali site holds 0.91 of its 1, not 0.86, and K/(K+Na) is 0.90: well on the jarosite side of the natrojarosite series.
- Documented failure: the cell. The printed cell, a 7.29 and c 17.16 Å with Z = 3, gives a density of 3.159 against the printed calculated 3.127, about 1% high. Two natrojarosite analyses, recast the same way, give Na/(Na+K) of 0.93 (Kundip) and 1.00 (Tintic Standard). Weight in water cannot sort the series: natrojarosite’s 13.57 to 13.71 g sits inside jarosite’s 13.13 to 13.87 g.
- Soil and Mars. The Queensland Government’s acid sulfate soils guidance says the yellow mottle in oxidised acid sulfate soils “is the mineral jarosite”. The NASA JPL release of 2 March 2004 reports that the Opportunity rover’s Mössbauer spectrometer “detected a hydrated iron sulfate mineral called jarosite” at Meridiani Planum.
| Row | K | Na | Fe | OH (from H2O) | K + Na | K/(K+Na) | Printed on the sheet | Density printed; from cell | 20.00 g in water |
|---|---|---|---|---|---|---|---|---|---|
| Ideal KFe3+3(SO4)2(OH)6 | 1 | 0 | 3 | 6 | 1 | 1 | Ideal column (2): SO3 31.97, Fe2O3 47.83, K2O 9.41, H2O 10.79 | — | — |
| Jarosite (1), Saint Félix de Pallières, France; total 99.85 with 0.29 insol. | 0.823 | 0.089 | 3.046 | 5.89 | 0.912 | 0.903 | (K0.82Na0.04)Σ=0.86Fe3.05(SO4)2(OH)6: K and Fe reproduce; Na does not | 2.91–3.26 meas., 3.127 calc.; 3.159 | 13.13–13.87 g |
| Natrojarosite (1), Kundip, Western Australia | 0.072 | 1.011 | 3.107 | 6.02 | 1.083 | 0.066 | (Na0.94K0.06)Σ=1.00Fe3(SO4)2(OH)6: printed with the alkali site set to 1 | 3.11–3.18 meas., 3.29 calc.; 3.095–3.122 | 13.57–13.71 g |
| Natrojarosite (2), Tintic Standard mine, Utah; microprobe, H2O by difference | 0 | 1.089 | 2.954 | 5.84 | 1.089 | 0 | Na1.09Fe2.95(SO4)2.00(OH)5.88: reproduces | — | — |
| Alunite (1), Marysvale, Utah (aluminium in place of iron) | 0.927 | 0.044 | Al 3.046 | 6.02 | 0.972 | 0.954 | No formula printed for the analysis | 2.6–2.9 meas., 2.82 calc.; 2.821 | 12.31–13.10 g |
What is jarosite? Its formula and IMA status
Jarosite is KFe3+3(SO4)2(OH)6 on the September 2026 IMA list, status Rd (redefined), 1987 s.p., with Spain as the country of first description and American Mineralogist 95 (2010), 1590 as its reference. The Handbook of Mineralogy jarosite sheet prints the same formula and places it in the alunite group. Worked on standard atomic weights, the formula mass is 500.79: iron 33.45%, potassium 7.81%, sulfur 12.80%. As oxides that is SO3 31.97, Fe2O3 47.83, K2O 9.40 and H2O 10.79, the sheet’s ideal column apart from K2O, printed 9.41. The iron figure is the one on is goethite magnetic.
What the sheet records. Hexagonal, point group 3̄ 2/m, space group R3̄m, a = 7.29 and c = 17.16 Å, Z = 3. Well-formed crystals are pseudocubic rhombohedra {01-12} or tabular on {0001}, to 25 cm, but it typically occurs as minutely crystalline crusts, and also fibrous, nodular, granular, pulverulent and earthy. Cleavage on {0001}, distinct; fracture uneven to conchoidal; brittle. Hardness 2.5 to 3.5; density 2.91 to 3.26 measured, 3.127 calculated; strongly pyroelectric. Amber-yellow to dark brown, streak pale yellow, lustre subadamantine to vitreous, resinous on fractures. Uniaxial (–), typically anomalously biaxial, ω 1.791 to 1.820, ε 1.705 to 1.715.
Its family on the IMA list. A scan of the formula column finds seven rows ending Fe3+3(SO4)2(OH)6: jarosite (K), natrojarosite (Na), ammoniojarosite (NH4), hydroniumjarosite (H3O), argentojarosite (Ag), plumbojarosite (Pb0.5) and dorallcharite (Tl). The sheet says jarosite “Forms a series with natrojarosite”. Its aluminium counterpart is alunite, KAl3(SO4)2(OH)6. The phosphate and arsenate relatives are on crandallite and beudantite.
How is jarosite formed? From pyrite, and around hot springs
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.” The association line is short: alunite and pyrite. Oxidized iron-sulfide outcrops of this kind are the subject of what is a gossan, and the iron oxides that keep company with it are on goethite and hematite.
In soil. The Queensland Government describes oxidised acid sulfate soils that “will often contain yellow and orange mottling. The yellow mottle is the mineral jarosite and the orange colours are other iron oxide minerals.” Its field-test table reads, for a pH F reading below 4, “Oxidised acid sulfate soil (extremely acidic)—jarosite may be present”. That is the source for the soil question; this page adds nothing to it.
On Mars. The NASA JPL release of 2 March 2004 (release 2004-74) says that at Meridiani Planum the Opportunity rover’s Moessbauer spectrometer “detected a hydrated iron sulfate mineral called jarosite”, and that jarosite “may point to the rock’s wet history having been in an acidic lake or an acidic hot springs environment.” That is NASA’s reading, quoted; the Handbook sheet itself does not mention Mars.
The Saint Félix recast: what the analysis says
Method. Each oxide on the sheet was divided by its formula weight, and the atoms set to 2 sulfur. The Saint Félix de Pallières analysis (SO3 32.02, Fe2O3 48.63, Na2O 0.55, K2O 7.75, H2O 10.61, insol. 0.29) gives K 0.823, Na 0.089, Fe 3.046 and H for 5.89 OH. The sheet prints (K0.82Na0.04)Σ=0.86Fe3.05(SO4)2(OH)6.
Documented failure: the sodium. Potassium and iron reproduce. Sodium does not: 0.55% Na2O is 0.089 sodium atoms per 2 sulfur, and the printed 0.04 matches the count of Na2O units, 0.044. With 0.089, the alkali site holds 0.912 of 1, not 0.86. The sheet’s second reference is titled “The jarosite-natrojarosite-hydronium jarosite solid solution series”, but the sheet does not say what fills the remaining site, and this page does not guess. The arithmetic is reported; the sheet is not corrected.
Second documented failure: the cell. The printed cell holds 789.8 cubic ångströms and, with Z = 3, gives 3.159 against the printed 3.127. The same method reproduces alunite’s printed 2.82 (2.821). Either way the measured range, 2.91 to 3.26, is wide: 13.13 to 13.87 g in water for a 20.00 g piece, on the method at how to measure specific gravity at home. Natrojarosite’s 3.11 to 3.18 (13.57 to 13.71 g) sits inside it, so weight cannot separate the two; the K/(K+Na) column in the table can, and it needs an analysis.
Where is jarosite found? The sheet’s localities
Distribution, as printed. “Widespread. A few localities for studied material include”: the Barranco del Jaroso, Sierra Almagrera, Almería Province, Spain; Horní Slavkov (Schlaggenwald), Czech Republic; the Frisches Glück mine, Fürstenberg, near Schwarzenberg, Saxony, Germany; the Kamariza mine, Laurium, Greece, for which see Laurium (Lavrion). In the USA, Marysvale and the Mammoth mine, Tintic, Utah; the Vulture mine near Wickenburg, Tombstone “and many other places in Arizona”; the Meadow Valley mines, Pioche district, Nevada. “Immense crystals from the La Domitila mine, Sierra Peña Blanca uranium district, Sierra del Cuervo, Chihuahua, Mexico.” In Chile, Chuquicamata, Quetena and Alcaparrosa, Antofagasta. Volcanoes on the Kamchatka Peninsula, Russia.
Name and type material. The name is for the Barranco del Jaroso, Spain, “from where the first scientifically studied specimens were obtained.” Type material is at the Mining Academy, Freiberg, Germany, 17.966. The sheet marks no type locality and the IMA list gives only the country, Spain; the term is explained at what is a type locality.
Uses and hazard. The sheet names no use, and no source we would cite was fetched for one, so this page states none. 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. Iron sulfates and their relatives are in the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.