Alum: Formation & Geology Varieties

Alum: Formation & Geology Varieties

Alum (Potassium Alum): Formation, Geology & Varieties

Where alum grows in nature, how those snowy crusts and crisp octahedra form, and how to tell the family members apart 🤍🧪

📌 Overview (What Geologists Mean by “Alum”)

In mineralogy, “alum” typically means the mineral alum‑(K), the natural form of potassium alum with formula KAl(SO4)2·12H2O. It’s a hydrous double sulfate of potassium and aluminium that crystallizes in the isometric system and often appears as soft, snowy efflorescences or (more rarely) crisp octahedra. Alum belongs to a broader alum group defined by the general formula XAl(SO4)2·12H2O where X is a monovalent cation (K, Na, NH4, etc.). 1

Plain‑talk tip: Alum grows where acidic, sulfate‑rich waters meet aluminium + potassium sources, then dry out. Think fumaroles, acid mine walls, and dry caves. 2

🌋 Geologic Settings (Where Alum Occurs)

  • Volcanic fumaroles & solfataras: Acidic vapors/condensates precipitate alum on crater walls and scoria around active vents (e.g., Vesuvius and Solfatara, Campania, Italy). 3
  • Supergene zones in argillaceous sediments and coal beds: Oxidation of pyrite/marcasite generates sulfuric acid; where K and Al are available from clays or feldspars, evaporating porewaters crystallize alum as efflorescent crusts. 4
  • Cave and sheltered microclimates: Sulfuric acid from sulfide oxidation (or H2S degassing) reacts with Al‑bearing rocks; ammonia from guano can produce the ammonium analogue (tschermigite). 5

Textbook octahedra are rare in nature; most field finds are drusy/porous coatings and stalactitic masses formed by repeated wetting & drying. 6

🧪 Formation Pathways (From Acid to Alum)

1) Fumarolic Precipitation

Acidic, sulfate‑rich vapors condense on cool surfaces; where K and Al ions are present, alum‑(K) crystallizes as coatings or tiny octahedra. RRUFF‑catalogued specimens from Bolivian fumaroles show alum with native sulfur and sodium alum hydrates — a classic vent assemblage. 7

2) Supergene “Acid Mine” Route

Oxidizing pyrite produces sulfuric acid and Fe‑sulfates; acid percolates through K‑rich clays/feldspars and mobilizes Al. During dry spells, alum crystallizes as efflorescence alongside alunogen, pickeringite, epsomite, melanterite, and gypsum. 8

3) Cave/Guano Chemistry

In sulfuric‑acid caves, ammonia from guano can shift the cation budget to NH4+, favoring tschermigite (ammonium alum) as transparent crusts. Reports from Serpents Cave document tschermigite with alunogen and jurbanite on acid‑etched walls. 9

Geochem nutshell: Acid + Al + SO42− + K/NH4/Na → alum‑type double sulfates on evaporation. Keep it dry, and you’ll keep it pretty. 10

🧬 Paragenesis & Textures (How It Grows)

  • Sequence: Acid formation → metal/alkali uptake from host → evaporation → early fibrous/porous sulfates → alum overgrowths in drier phases. In mine walls and dumps, suites evolve seasonally as humidity swings. 11
  • Habit controls: Neutral solutions favor octahedra; alkaline solutions can produce cubic habits — but both are delicate and short‑lived outdoors. 12
  • Textures: Commonly drusy coatings, stalactitic “icicles,” and powdery blooms (micro‑dissolution/re‑precipitation). Vent‑proximal crusts can show zoning from temperature/chemistry gradients. 13

Translation: alum is a fair‑weather friend — gorgeous when dry, fussy when damp. (Same, honestly.)

🧼 Stability & Alteration (Hydration States Matter)

  • Water‑soluble & humidity‑sensitive: Even breath can fog/etch fresh faces; repeated wet/dry cycles dull luster. 14
  • Thermal behavior: Heating drives off structural water; controlled studies show melting/dehydration begins well below 100 °C on lab timescales. 15
  • Hydrate shifts: K‑alum can dehydrate/transform; related Na and K phases exist at lower hydrations (e.g., mendozite, kalinite), which form or overprint in very arid niches. 16

🧩 Associated Minerals (The Company Alum Keeps)

In fumarolic and supergene settings, alum is often found with alunogen (Al2(SO4)3·17H2O), pickeringite, epsomite, melanterite, gypsum, and native sulfur. This sulfate‑rich cast is a reliable fingerprint of acid sulfate environments. 17

Collector clue: If you spot alunogen’s silky masses plus greenish melanterite in a dry adit or dump, look for delicate alum blooms nearby — then photograph fast and store dry. 18

🔬 Varieties & Close Relatives (Alum Group Snapshot)

Species Formula Environment / Notes Fast ID Clues
Alum‑(K) (potassium alum) KAl(SO4)2·12H2O Fumaroles, supergene efflorescences, cave walls; rare octahedra in neutral solutions. Type area: Campania, Italy. 19 Very light SG; water‑soluble; isotropic; drusy crusts common. 20
Alum‑(Na) (sodium alum) NaAl(SO4)2·12H2O Similar settings; part of the alum series. More soluble; occurs with Na‑rich suites. 21 Cubic dodecahydrate; fragile efflorescences; low SG. 22
Tschermigite (alum‑(NH4)) (NH4)Al(SO4)2·12H2O Caves & mines where ammonia (guano) is present; rare but diagnostic. 23 Transparent crusts; forms with alunogen/jurbanite in acid caves. 24
Kalinite (K‑alum undecahydrate) KAl(SO4)2·11H2O Arid efflorescences; fibrous, monoclinic; historically debated but approved species. 25 Fibrous habits; lower hydrate than alum‑(K). 26
Mendozite (Na‑alum undecahydrate) NaAl(SO4)2·11H2O Evaporites in very dry regions; can effloresce to tamarugite (hexahydrate). 27 Prismatic/pseudo‑rhombohedral; very soluble. 28
Tamarugite (Na‑alum hexahydrate) NaAl(SO4)2·6H2O Widespread but sparse in arid/saline sites; often alteration of Na‑alum hydrates. 29 Biaxial; tabular/prismatic crystals; still water‑soluble. 30
“Chrome alum” (KCr(SO4)2·12H2O) Cr3+ analogue Industrial/synthetic double sulfate; natural mineral occurrences are not established in the IMA‑approved alum group list. 31 Dark violet lab crystals; educational demos. 32

Alums can adopt different α/β/γ structure types; natural dodecahydrates are the commonest in the field. 33

🗺️ Notable Localities (Snapshot)

Campania, Italy — Vesuvius & Solfatara

Classic fumarolic assemblages with alum‑(K) on scoria and crater walls; type area for the species. 34

Alum Cave Bluff, Tennessee, USA

Sheltered cliff/cave environment producing sulfate efflorescences; alum‑(K) recorded among the suite. 35

El Desierto Fumaroles, Potosí, Bolivia

Alum‑(K) with sulfur and tamarugite documented by RRUFF (confirmed by single‑crystal XRD). 36

Monte Arsiccio Mine, Tuscany, Italy

Acid‑sulfate secondary suite; alum‑(K) in granoblastic aggregates with other sulfates. 37

These sites highlight the two big themes: volcanic acid condensates and supergene acid drainage. 38

🧭 Field & Display Tips (For a Mineral That Melts in a Stare)

  • Document fast: Photograph in situ; humidity alters surfaces quickly. Bag with desiccant if you collect. 39
  • Don’t wash: Use a bulb blower/soft dry brush; water will pit or erase delicate crusts. 40
  • Storage: Airtight micro‑cases with silica gel; avoid kitchens/bathrooms and coastal humidity. (Yes, alum hates beach weather.) 41

❓ FAQ

Is alum always natural?

No. Many clear octahedra sold for teaching are grown from solution. Natural alum‑(K) does occur but more often as crusts/efflorescences than as perfect single crystals. Always label natural vs. lab‑grown for clarity. 42

What’s the difference between alum and alunite?

Alum here = a hydrous double sulfate (e.g., alum‑(K)); alunite is a much harder potassium aluminum sulfate hydroxide that often acts as a K/Al source for alum solutions in nature or industry. 43

Does “chrome alum” occur as a mineral?

Chromium potassium alum is a well‑known synthetic double sulfate used industrially; it’s not listed as a natural IMA‑approved alum‑group species. Treat purple crystals as chemical curiosities, not field minerals. 44

✨ The Takeaway

Alum‑(K) thrives where acid sulfate waters encounter K + Al sources and the air does the finishing work by evaporation. Expect it at fumaroles, acid‑mine walls, and dry caves, often with alunogen, epsomite, and melanterite. Within the alum family, K/Na/NH4 members and lower‑hydrate cousins (kalinite, mendozite, tamarugite) reflect local chemistry and humidity. Keep it dry, label it clearly, and enjoy the paradox: a “mineral” that’s basically a polite, crystalline laboratory salt — gorgeous, but please don’t add water. 45

Final wink: Alum dissolves faster than gossip in a small town — display under a cover and everyone’s happier. 😄

📚 Sources & Notes

  1. Alum‑(K) definition & alum group formula. Add a mineralogy reference (e.g., RRUFF/Mindat, textbook). ↩︎
  2. Occurrence summary. Fumaroles, acid mine walls, caves/microclimates. ↩︎
  3. Campania fumaroles. Vesuvius/Solfatara alum notes. ↩︎
  4. Supergene efflorescences. Pyrite oxidation → sulfuric acid → alum with K/Al sources. ↩︎
  5. Caves & guano to tschermigite. Ammonium alum contexts. ↩︎
  6. Habit frequency. Octahedra rare; drusy/porous coatings common. ↩︎
  7. Bolivian fumaroles (RRUFF). Alum with sulfur & Na‑alum hydrates. ↩︎
  8. Acid mine route associations. Alunogen, pickeringite, epsomite, melanterite, gypsum. ↩︎
  9. Serpents Cave reports. Tschermigite with alunogen/jurbanite. ↩︎
  10. Geochemical summary. Reaction pathway to alum‑type double sulfates. ↩︎
  11. Paragenetic sequence. Seasonal evolution of sulfate suites. ↩︎
  12. Habit controls. Neutral → octahedra; alkaline → cubes. ↩︎
  13. Textures & zoning. Druse/stalactites; vent‑proximal zoning. ↩︎
  14. Humidity sensitivity. Water solubility; luster loss. ↩︎
  15. Thermal dehydration. Onset below ~100 °C (lab timescales). ↩︎
  16. Hydrate relatives. Kalinite, mendozite, etc. ↩︎
  17. Associated minerals list. Alunogen, pickeringite, epsomite, melanterite, gypsum, sulfur. ↩︎
  18. Collector clue. Use of alunogen/melanterite as indicators. ↩︎
  19. Alum‑(K) environments & type area. Campania references. ↩︎
  20. Alum‑(K) fast ID. Isotropic; drusy; very light SG. ↩︎
  21. Alum‑(Na) environments. Na‑rich suites; higher solubility. ↩︎
  22. Alum‑(Na) ID. Dodecahydrate; fragile efflorescences. ↩︎
  23. Tschermigite environments. Caves/mines with ammonia. ↩︎
  24. Tschermigite ID. Transparent crusts; cave associations. ↩︎
  25. Kalinite notes. Monoclinic undecahydrate; arid settings. ↩︎
  26. Kalinite ID. Fibrous; lower hydrate. ↩︎
  27. Mendozite notes. Na‑alum undecahydrate; alteration to tamarugite. ↩︎
  28. Mendozite ID. Prismatic; very soluble. ↩︎
  29. Tamarugite notes. Hexahydrate occurrences/alteration. ↩︎
  30. Tamarugite ID. Biaxial; tabular/prismatic. ↩︎
  31. Chrome alum status. Synthetic; not an IMA‑approved natural alum‑group species. ↩︎
  32. Chrome alum crystals. Dark violet lab crystals for demos. ↩︎
  33. Structure types. α/β/γ notes; natural dodecahydrates most common. ↩︎
  34. Campania localities. Vesuvius/Solfatara. ↩︎
  35. Alum Cave Bluff. Suite of sulfates including alum‑(K). ↩︎
  36. El Desierto (RRUFF). XRD‑confirmed alum‑(K) with sulfur/tamarugite. ↩︎
  37. Monte Arsiccio. Acid‑sulfate secondary suite. ↩︎
  38. Locality theme summary. Fumarolic vs. supergene acid drainage. ↩︎
  39. Field: document fast. Photograph + desiccant. ↩︎
  40. Field: don’t wash. Dry tools only. ↩︎
  41. Storage. Airtight micro‑cases; silica gel; avoid humidity. ↩︎
  42. FAQ: natural vs. lab‑grown. Disclosure guidance. ↩︎
  43. FAQ: alum vs. alunite. Chemistry & hardness difference. ↩︎
  44. FAQ: chrome alum as mineral. Synthetic status. ↩︎
  45. Summary note. Where/why alum forms; family variants. ↩︎

Tip: Fill these with solid sources (RRUFF, Mindat with literature citations, museum/conservation notes, peer‑reviewed geochem papers, USGS/GSJ reports). Avoid unsourced blogs.

Back to blog