Alunite
Alunite, KAl3(SO4)2(OH)6: white streak, hardness 3.5–4, density 2.6–2.9, 19.54% aluminium, formed by sulfate acting on aluminous rocks; Tolfa, Marysvale.
Alunite is a potassium aluminium sulfate hydroxide, KAl3(SO4)2(OH)6 on the September 2026 IMA list: hexagonal, colourless to white, grey, yellow or reddish brown, streak white, hardness 3.5 to 4, density 2.6 to 2.9. Worked, it is 19.54% aluminium and 9.44% potassium. Its Handbook sheet lists Tolfa, Italy, first and says it forms a series with natroalunite, its sodium analogue.
In short
- The species. The September 2026 IMA-CNMNC Master List lists alunite as KAl3(SO4)2(OH)6, status Rd, 1987 s.p., Italy / Ukraine. The Handbook of Mineralogy sheet gives the same formula: hexagonal, R3̄m, pseudocubic or tabular crystals to 1 cm, but commonly granular to dense massive or porcelaneous; cleavage {0001} perfect; hardness 3.5 to 4; density 2.6 to 2.9; streak white. A 20.00 g piece reads 12.31 to 13.10 g in water.
- How it forms. “Formed between 15 °C and 400 °C by the action of sulfate, which may be generated from pyrite or solfataric action, on aluminous rocks, commonly accompanied by kaolinitization and silicification.” Its associates are kaolinite, halloysite, diaspore, pyrite, gypsum and quartz. Why that does not make it a clay is on is alunite a clay mineral.
- The aluminium swap. Jarosite has the same R3̄m structure and Z = 3 with ferric iron where alunite has aluminium. Worked from the two sheets, the swap makes the formula 20.91% heavier (414.20 to 500.79) and the cell 7.97% larger (731.5 to 789.8 ų: a +4.43%, c −0.99%). Cell density rises from 2.821 to 3.159, and a 20.00 g piece goes from 12.91 g in water at the printed 2.82 to 13.60 g at 3.127.
- Documented failure: alunite or natroalunite? The Marysvale analysis on the alunite sheet recasts to K/(K+Na) 0.954; the National Belle analysis on the natroalunite sheet to 0.389, so it is sodium-led. But natroalunite’s 2.75 to 2.83 (12.73 to 12.93 g in water) sits inside alunite’s 2.6 to 2.9, hardness is 3.5 to 4 on both, and both streak white: the series is placed by analysis, not in the hand.
- Ore and use. The sheet records “Many localities, some with deposits of immense size”, including “approximately 100 million t in the Calico Peak porphyry” in Colorado, but it does not call alunite an ore of anything, and no source we would cite was fetched for a use, so this page states none.
| Quantity | Alunite | Jarosite | Natroalunite |
|---|---|---|---|
| IMA formula; status | KAl3(SO4)2(OH)6; Rd, 1987 s.p., Italy / Ukraine | KFe3+3(SO4)2(OH)6; Rd, 1987 s.p., Spain | NaAl3(SO4)2(OH)6; Rd, 1987 s.p., USA |
| Formula mass | 414.20 | 500.79 (+20.91%) | 398.09 |
| Cell a, c (Å) | 6.981, 17.331 | 7.29, 17.16 | 6.982–6.992, 16.725–16.905 |
| Cell volume (ų) | 731.5 | 789.8 (+7.97%) | 706.1–715.7 |
| Density from cell, worked | 2.821 | 3.159 | 2.771–2.809 |
| Density printed | 2.6–2.9 meas., 2.82 calc. | 2.91–3.26 meas., 3.127 calc. | 2.75–2.83 meas., 2.819 calc. |
| 20.00 g piece in water | 12.31–13.10 g | 13.13–13.87 g | 12.73–12.93 g |
| Metal by mass, worked | Al 19.54%, K 9.44% | Fe 33.45%, K 7.81% | Al 20.33%, Na 5.78% |
| Analysis recast, K/(K+Na) | Marysvale: 0.954 (Al 3.046 on S = 2) | Saint Félix: 0.903 | National Belle: 0.389; Matanumad: 0.040 |
| Hardness | 3.5–4 | 2.5–3.5 | 3.5–4 |
| Colour; streak | Colourless if pure, white, grey, yellow, red to reddish brown; white | Amber-yellow to dark brown; pale yellow | White, less commonly grey, yellow, green, brown; white |
| Optics | Uniaxial (+); ω 1.572, ε 1.592 | Uniaxial (–); ω 1.791–1.820, ε 1.705–1.715 | Uniaxial (+); ω 1.574, ε 1.590 |
| USGS Table 1, best range (samples) | >1.70 amp (1) | 0.40–0.50 amp (3) | Not listed |
What is alunite? Its formula and IMA status
Alunite is KAl3(SO4)2(OH)6 on the September 2026 IMA list, status Rd (redefined), 1987 s.p., with “Italy / Ukraine” as the country of first description and Mineralogical Magazine 76 (2012), 313 as its reference. The Handbook of Mineralogy alunite sheet prints the same formula and places it in the alunite group. Worked on standard atomic weights, the formula mass is 414.20: aluminium 19.54%, potassium 9.44%, sulfur 15.48%. As oxides, SO3 38.66, Al2O3 36.92, K2O 11.37 and H2O 13.05, the sheet’s ideal column to the hundredth.
What the sheet records. Hexagonal, point group 3̄ 2/m or 3m, space group R3̄m or R3m (synthetic), a = 6.981 and c = 17.331 Å, Z = 3. Crystals are typically pseudocubic {01-12} or tabular {0001} with flat vicinal rhombohedra, to 1 cm; it is also fibrous to columnar and porcelaneous, and “commonly granular to dense massive”. Cleavage on {0001}, perfect; fracture conchoidal; brittle. Hardness 3.5 to 4; density 2.6 to 2.9 measured, 2.82 calculated; strongly pyroelectric. “Colorless if pure; may be white, pale shades of gray, yellow, red, to reddish brown.” Streak white; lustre vitreous, somewhat pearly on {0001}. Uniaxial (+), ω 1.572, ε 1.592.
Its family on the IMA list. A scan of the formula column finds six rows ending Al3(SO4)2(OH)6: alunite (K), natroalunite (Na), ammonioalunite (NH4), schlossmacherite (H3O), huangite (Ca0.5) and walthierite (Ba0.5). The iron side is on jarosite. The beudantite page already gives alunite’s hardness, density and ω as above, and crandallite covers the phosphate relatives; the gorceixite sheet’s “Alunite supergroup” wording is quoted there, while the alunite sheet prints “Alunite group”.
How does alunite form? Sulfate acting on aluminous rock
On the sheet. “Formed between 15 °C and 400 °C by the action of sulfate, which may be generated from pyrite or solfataric action, on aluminous rocks, commonly accompanied by kaolinitization and silicification.” The association line reads kaolinite, halloysite, diaspore, pyrite, gypsum, quartz. That is a reading of the sheet, not a model; how associations are read is on what is mineral paragenesis.
Why it is met with clay. Kaolinite and halloysite are its first two associates, and the natroalunite sheet says that species forms when sulfate-bearing solutions react “with clays”. Alunite itself has no silicon in its formula; the full case is on is alunite a clay mineral.
What alunite crystals look like. Pseudocubic rhombohedra or tabular plates to 1 cm, colourless if pure, but more often the mineral is granular, dense or porcelaneous, white to grey, yellow, red or reddish brown. The white streak separates it from jarosite’s pale yellow; see how to use a streak plate.
The aluminium swap: alunite against jarosite
Same structure, different metal. Both sheets give R3̄m, Z = 3 and the alunite group, and the IMA formulas differ only in Al3 against Fe3+3. Worked, jarosite’s formula is 20.91% heavier, 500.79 against 414.20. Its cell is 7.97% larger, 789.8 against 731.5 cubic ångströms: a grows 4.43% while c shrinks 0.99%. The cell densities follow, 2.821 against 3.159, a ratio of 1.120; the printed calculated densities, 2.82 and 3.127, give 1.109.
In the hand. A 20.00 g piece reads 12.91 g in water at alunite’s printed 2.82 and 13.60 g at jarosite’s 3.127, on the method at how to measure specific gravity at home. The measured ranges barely touch, 12.31 to 13.10 g against 13.13 to 13.87 g. Streak and colour do more: white against pale yellow, white-to-reddish against amber-yellow to dark brown. USGS Open-File Report 99-529, Table 1 lists alunite at >1.70 amp (one sample) and jarosite at a best range of 0.40 to 0.50 amp (three samples); how to read those currents is on is goethite magnetic.
Documented failure: the sodium twin. The Marysvale analysis on the alunite sheet recasts on 2 sulfur to K 0.927, Na 0.044 and Al 3.046 (2.995 if its 0.58% P2O5 is counted with sulfur), K/(K+Na) 0.954. The natroalunite sheet’s National Belle analysis gives 0.389 and its Matanumad analysis 0.040; Matanumad is “stated to correspond to” Na0.96, but recasts to Na 0.838. Natroalunite’s density, 2.75 to 2.83, lies inside alunite’s, and hardness and streak match. Only the analysis places a piece in the series.
Where is alunite found? Tolfa, Marysvale and the rest
Distribution, as printed. “Many localities, some with deposits of immense size.” Tolfa, 14 km east-northeast of Civitavecchia, Lazio, Italy. Mukacheve (Muzijeva) and Beregavo, near Mukachevo, Ukraine. Rodalquilar, Almería Province, Spain. Décazeville, Aveyron, France. In the USA, “a very large deposit near Marysvale, Ohio district, Piute Co., Utah”; in Colorado, South River, Red Mountain, the Rosita Hills and “approximately 100 million t in the Calico Peak porphyry, Rico district”; Meiklejohn Mountain near Beatty, Nevada; Lassen Volcanic National Park, California. Hickory’s Pond, Newfoundland, Canada. Hilton, Bernborough and the Mount Isa district, Queensland, and Bullah Delah, New South Wales, Australia.
Name. “A contraction of the earlier name aluminilite.” The sheet marks no type locality and names no type material; the IMA list gives Italy / Ukraine, matching Tolfa and Mukacheve at the head of the Distribution list, but this page does not call either the type locality. See what is a type locality.
In English, and as an ore. Alunite is the English and IMA name. The sheet gives tonnages but no ore or use statement, so none is made here. Hazard questions go to toxic minerals in a collection; this is not health or safety advice. More sulfates are in the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.