Is alunite a clay mineral?
Is alunite a clay mineral? No: KAl3(SO4)2(OH)6 is a sulfate with no silicon; kaolinite is 21.76% silicon. Hardness 3.5–4, it forms with kaolinite and quartz.
No. Alunite is a potassium aluminium sulfate, KAl3(SO4)2(OH)6, with no silicon in it. The clays the British Geological Survey names, illite, kaolinite and smectite, are aluminium silicates; kaolinite is 21.76% silicon by mass. Alunite is also harder, 3.5 to 4 against kaolinite’s 2 to 2.5. It is confused with clay because it forms with kaolinite.
In short
- The verdict. Alunite is not a clay mineral. The September 2026 IMA-CNMNC Master List writes KAl3(SO4)2(OH)6: a sulfate, with no silicon. The British Geological Survey’s sedimentary rock classification uses “clay” for a grain size, “clay grade (< 4 µm)”, and for “Monomineralic aluminium silicates”, naming illite, kaolinite and smectite. Alunite is neither a silicate nor, on its sheet, a mineral defined by grain size.
- The silicon test. Worked from the IMA formulas, alunite and natroalunite are 0% silicon; kaolinite, dickite and nacrite, all Al2Si2O5(OH)4, are 21.76%; muscovite, KAl2(Si3Al)O10(OH)2, is 21.15%. The montmorillonite sheet’s Montmorillon analysis has 51.14% SiO2, 23.90% silicon. Alunite’s Marysvale analysis has 0.22% SiO2, 0.10% silicon, and 38.34% SO3.
- Why people ask. The alunite sheet says it forms by sulfate acting “on aluminous rocks, commonly accompanied by kaolinitization and silicification”, and lists kaolinite and halloysite first among its associates. The natroalunite sheet has that species forming when sulfate-bearing solutions react “with clays”. Found with clay is not the same as being clay.
- Documented failure: density. Kaolinite’s 2.61 to 2.68 lies inside alunite’s 2.6 to 2.9: a 20.00 g piece reads 12.34 to 12.54 g in water against 12.31 to 13.10 g. Montmorillonite’s 2 to 3 spans both. Hardness does better, 3.5 to 4 against 2 to 2.5 and 1 to 2, but the montmorillonite sheet warns that identifying smectites “may need data from DTA curves, dehydration curves, and X-ray powder patterns”.
- The other questions. Is alunite a sulfate? Yes, 46.38% SO4 by mass. A mineral? Yes, with its own IMA row. An ore? Its sheet gives deposits “of immense size” but no ore statement, so none is made here.
| Mineral | IMA formula | Si atoms; Si by mass | S by mass | Group (sheet) | Hardness | Density (meas.) | 20.00 g in water | “Clay” on the sheet |
|---|---|---|---|---|---|---|---|---|
| Alunite | KAl3(SO4)2(OH)6 | 0; 0% | 15.48% | Alunite group | 3.5–4 | 2.6–2.9 | 12.31–13.10 g | Not used; associates kaolinite, halloysite |
| Natroalunite | NaAl3(SO4)2(OH)6 | 0; 0% | 16.11% | Alunite group | 3.5–4 | 2.75–2.83 | 12.73–12.93 g | Forms by solutions “reacting with clays” |
| Kaolinite | Al2Si2O5(OH)4 | 2; 21.76% | 0% | Kaolinite-serpentine group | 2–2.5 | 2.61–2.68 | 12.34–12.54 g | “Clay-size fraction”; “claylike masses” |
| Dickite, nacrite | Al2Si2O5(OH)4 | 2; 21.76% | 0% | Polymorphs of kaolinite (kaolinite sheet) | — | — | — | No sheet cached |
| Halloysite | Al2Si2O5(OH)4·n H2O | 2; varies with n | 0% | Polymorph of kaolinite (kaolinite sheet) | — | — | — | No sheet cached |
| Montmorillonite | (Na,Ca)0.3(Al,Mg)2Si4O10(OH)2·nH2O | 4; 23.90% (Montmorillon analysis) | 0% | Smectite group | 1–2 | 2–3 | 10.00–13.33 g | “A common clay mineral” |
| Muscovite | KAl2(Si3Al)O10(OH)2 | 3; 21.15% | 0% | Mica group | 2.5 ∥ [001]; 4 ⊥ [001] | 2.77–2.88 | 12.78–13.06 g | — |
| Illite | No IMA row | — | — | No Handbook sheet (404) | — | — | — | Named by BGS and on the montmorillonite sheet |
Is alunite a clay? Two senses of the word
Two senses. The BGS Rock Classification Scheme, volume 3 (Research Report RR 99-03) uses clay first as a size: argillaceous sediments include “silt grade (4 to 32 µm) and clay grade (< 4 µm) particles”. It uses it second for composition: “Monomineralic aluminium silicates take the form of clays or claystones”, with three named components in its Table 19, illite, kaolinite and smectite, and with china clay as a synonym for kaolinite-claystone and fuller’s earth and bentonite for smectite-claystone.
Alunite fails the second. Its IMA formula, KAl3(SO4)2(OH)6, has aluminium but no silicon: it is an aluminium sulfate, not an aluminium silicate. The USGS Mineral Commodity Summaries 2025, Clays covers six commercial clays, ball clay, bentonite, common clay, fire clay, fuller’s earth and kaolin; alunite is not among them. Removing clay from a specimen is a separate matter, covered on how to remove clay from a mineral specimen.
And the first? Grain size describes a sediment, not a species. The alunite sheet gives crystals to 1 cm and dense, porcelaneous masses and prints no grain size, so this page makes no claim that any alunite is clay-sized. Kaolinite’s sheet, by contrast, calls it “A common constituent of the clay-size fraction of sediments”.
The silicon test: alunite against the clay minerals
Method. Silicon by mass was worked from each IMA formula on standard atomic weights, and each sheet was searched for the word clay. Alunite and natroalunite: 0% silicon, 15.48% and 16.11% sulfur. Kaolinite, dickite and nacrite, the three Al2Si2O5(OH)4 rows: 21.76% silicon and no sulfur. Muscovite, the mica group: 21.15%. Montmorillonite’s formula has variable water, so its silicon is read from the Montmorillon analysis on the sheet, 51.14% SiO2, which is 23.90%.
What the sheets say. The montmorillonite sheet puts it in the smectite group and calls it “A common clay mineral”. The kaolinite sheet gives the kaolinite-serpentine group, “claylike masses” and the clay-size fraction, and names dickite, halloysite and nacrite as polymorphs. The alunite sheet does not use the word clay at all; its group is the alunite group. Illite, the third BGS name, has no row on the IMA list and no Handbook sheet (the PDF returned 404 on 14 November 2026).
Is alunite a sulfate, then? Yes. Sulfur sits in SO4 groups, 46.38% of its mass, beside six hydroxyls. How that compares with jarosite and the other cached sulfates is on is jarosite a sulfate.
Why alunite is found with kaolinite
On the sheet. Alunite is “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. The natroalunite sheet lists alunite, kaolinite, halloysite, sillimanite and quartz, and says natroalunite forms from solutions “reacting with clays”.
Documented failure: weight. A collector holding a white, fine-grained piece may hope weight will decide. It will not: kaolinite’s 2.61 to 2.68 sits inside alunite’s 2.6 to 2.9, so 20.00 g reads 12.34 to 12.54 g in water against 12.31 to 13.10 g, on the method at how to measure specific gravity at home. Montmorillonite’s 2 to 3, 10.00 to 13.33 g, covers both.
What does separate them. Hardness, on the sheets: alunite 3.5 to 4, kaolinite 2 to 2.5, montmorillonite 1 to 2. Kaolinite is “Flexible but inelastic”; alunite is brittle. The montmorillonite sheet says positive identification of smectites “may need data from DTA curves, dehydration curves, and X-ray powder patterns”, so a mixed alteration rock is a laboratory question.
Is alunite a mineral, an ore, or toxic?
A mineral. It has its own IMA row: KAl3(SO4)2(OH)6, status Rd, 1987 s.p., country Italy / Ukraine. The test is the same as on is quartz a mineral and is gypsum a mineral: a defined composition and structure, recognised by the IMA. Its full Handbook record is on alunite.
“Alunite is an ore of”. The sheet says “Many localities, some with deposits of immense size” and gives “approximately 100 million t in the Calico Peak porphyry, Rico district” in Colorado, but makes no ore or use statement, and no source we would cite was fetched for one. Worked, alunite is 19.54% aluminium and 9.44% potassium; that is composition, not a claim about ore.
Hazard. Whether alunite is toxic is routed to toxic minerals in a collection; this is not health or safety advice. More identification 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.