Alum (Potassium Alum): Physical & Optical Characteristics
The classic double sulfate in crystal form — how it grows, how it behaves in light, and how to handle it without… dissolving it 🤍🧪
📌 Overview (What “Alum” Means Here)
In mineralogy, alum usually refers to potassium alum, the hydrous double sulfate KAl(SO4)2·12H2O, the best‑known member of the alum group. It’s a cubic (isometric) mineral that commonly forms colorless to white crystals, often as octahedra grown from solution or as soft efflorescent crusts in dry environments. Its look is icy‑clean, its feel is light—and it’s decidedly water‑soluble, so handling needs a gentler touch than most display minerals.
🧾 Quick Reference (Gemological / Mineralogical)
| Property | Typical Value / Range | Notes |
|---|---|---|
| Chemistry | KAl(SO4)2·12H2O | Hydrous potassium aluminum sulfate; member of the alum group |
| Crystal system | Isometric (cubic) | Crystals often octahedral (neutral solution) or cubic (alkaline solution) |
| Hardness (Mohs) | ~2–2.5 | Fingernail can mark fresh faces |
| Specific gravity (SG) | ~1.75–1.76 | Feels unusually light in the hand |
| Refractive index (RI) | n ≈ 1.453–1.456 (single value) | Isotropic; polariscope stays dark |
| Optical character | Isotropic (cubic) | No birefringence; ADR not expected |
| Luster | Vitreous | On fresh, clean faces |
| Transparency | Transparent to translucent | Typically colorless to white |
| Cleavage / fracture | Indistinct on {111}; conchoidal to uneven fracture | Brittle; chips easily |
| UV fluorescence | Usually inert | Adhesives/dyes may fluoresce |
| Solubility | Water‑soluble; highly soluble in warm water | Avoid moisture, condensation, breath! |
| Thermal behavior | Dehydrates ~92–95 °C; decomposes on stronger heat | “Burnt alum” is the dehydrated, friable residue |
Terminology: “Alum” can name several double sulfates; here we focus on alum‑(K) (potassium alum) — the common display and lab crystal.
🧬 Crystal Chemistry & Habit (Why It Grows Like That)
Framework
Potassium and aluminum are linked by sulfate groups with abundant structural water (12 molecules per formula unit). This hydrated, open framework helps explain alum’s low density and ready solubility.
Crystal forms
Well‑formed crystals are often octahedra grown from neutral solutions; alkaline conditions can favor cubic habits. In nature, alum also occurs as drusy coatings and efflorescent crusts around volcanic fumaroles or oxidized sulfide zones.
Cleavage & toughness
Cleavage on {111} is indistinct and the material is brittle. Combine that with Mohs ~2–2.5 and you have a display mineral, not a jewelry stone.
🔎 Optical Behavior (How Alum Plays with Light)
Refractometer & Polariscope
Expect a single RI ≈ 1.45. As an isometric mineral, alum is isotropic and stays dark in the polariscope (no ADR flicker). Avoid oils and heavy contact—remember the solubility.
Luster & surface
Fresh faces show vitreous luster; humidity or incidental moisture quickly dulls surfaces as micro‑dissolution etches the polish.
Transparency
Transparent crystals can look “icy” with soft internal reflections; porous crusts read more translucent to opaque white.
🎨 Color & Appearance (Why It Looks Snowy)
- Body color: typically colorless to white.
- Cloudiness: micro‑porosity and micro‑etching scatter light → silky, “snowy” look on crusts.
- Zoning: slow solution growth can produce faint zoning; best seen with backlight.
- Surface bloom: in humid air, crystals can develop a matte bloom from incipient dissolution—store dry to preserve clarity.
🧪 Simple Bench Tests (Low‑Impact)
1) Heft & Hardness
SG ~1.75 makes alum feel light for its size. Mohs ~2–2.5 → a fingernail can mark a discreet edge. (Be gentle—this test is destructive.)
2) Solubility
A tiny water droplet will dull or pit a hidden corner within seconds—diagnostic but risky. Prefer to avoid this on specimens you care about.
3) RI / Optics
RI ≈ 1.45; isotropic. If testing, use an immersion cell with care or the immersion method visually—keep liquids away from the crystal itself.
4) Taste?
Historically described as astringent—but do not taste‑test minerals. Use instrumented or visual methods only.
🧼 Stability, Durability & Care
- Wearability: Not suitable for jewelry. Very soft, water‑soluble, and brittle.
- Storage: Airtight display box with silica gel or molecular sieve. Avoid kitchens/bathrooms or coastal humidity.
- Handling: Dry, clean hands or nitrile gloves. Minimize exhaled breath on surfaces when observing up close.
- Heat: Dehydrates and becomes friable around 92–95 °C; do not expose to hot case lights or window sun.
- Cleaning: Dust with a soft, dry brush; use a bulb blower. Never wash.
🧩 Look‑Alikes & How to Tell
| Material | How It Differs | Fast Clues |
|---|---|---|
| Halite (NaCl) | Cubic, salty, more common cleavage | Perfect cubic cleavage; SG ~2.17 (heavier than alum); avoid taste tests |
| Gypsum (CaSO4·2H2O) | Softer feel, perfect cleavage in one direction; less soluble | Hardness ~2; SG ~2.3; tabular/selenite habits, not octahedral |
| Alunite | Related sulfate but much tougher and not water‑soluble | Hardness 3.5–4; SG ~2.6–2.8; trigonal; survives water contact |
| Epsomite / Melanterite | Other soluble sulfates; different colors (green/blue for melanterite) | Characteristic hues; fibrous/needle habits common; distinct chemistry |
| Glass / Acrylic “crystals” | Man‑made display pieces | SG higher (glass ~2.5), no dissolution, mold lines/bubbles possible |
Pair habit + SG + solubility for quick IDs. Alum’s light heft and water‑sensitivity are decisive.
📸 Handling & Display Tips (Keep It Pretty)
- Light: Use soft, indirect light. LED panels on low are safer than hot halogens.
- Backgrounds: Charcoal grey or pale blue makes colorless alum read crisp; pure white can wash it out.
- Angles: Tilt octahedra to catch edge reflections; shoot quickly to avoid fogging from breath.
- Mounts: Use dry museum putty or acrylic saddles; avoid glues/solutions.
❓ FAQ
Is alum the same as “alunite”?
No. Alum here is potassium aluminum double sulfate with 12 waters; alunite is a different sulfate (harder, not water‑soluble). Easy to confuse by name—very different in hand.
Can I wear alum as jewelry?
Not recommended. Mohs ~2–2.5, brittle, and water‑soluble. Keep it as a cabinet specimen or educational crystal.
Does it fluoresce under UV?
Generally inert. If you see glow, it’s likely adhesives, impurities, or attached material—not the alum itself.
Where does natural alum occur?
As encrustations/veins around volcanic vents and in oxidized sulfide zones; also as cavern or mine efflorescences. Many clear crystals on the market are grown for teaching.
✨ The Takeaway
Alum (potassium alum) is a cubic, water‑soluble double sulfate with a single refractive index around 1.45, SG ~1.75, and Mohs ~2–2.5. It forms handsome octahedra and snowy crusts, but behaves more like a laboratory salt than a rugged rock: protect it from moisture, heat, and rough handling. Grade/display for clarity, fresh luster, and undamaged edges. Keep it dry and it will stay “ice‑clean”—no defrost cycle required.
Final wink: When photographing alum, hold your breath—literally. Your specimen will thank you for the low humidity.