Alum-(K) is a water-soluble sulfate mineral often found as crusts or powdery efflorescences in volcanic areas or near burning coal beds. It typically forms octahedral crystals that are easily recognized by their clear, vitreous luster and distinct astringent taste.
Is this alum-(k)?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch alum-(k) with a known reference. Alum-(K) sits at Mohs 2, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: colorless, white.
- 5Look at form & habitCrystal system: cubic. Typical habit: water soluble.
Often confused with
Alum-(K) vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside alum-(k)
Minerals reported to co-occur with alum-(k). Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KAl(SO₄)₂·12H₂O
- Mohs hardness
- 2
- Density
- 1.76 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Cubic
- Crystal habit
- Water soluble
- Cleavage
- Indistinct on {111}
- Rarity
- Common
- Uses
- Collector, Industrial
- Host rock
- Derived from the oxidation of sulfide minerals and potassium-bearing minerals
Where rockhounds find alum-(k)
Classic worldwide localities
- Italy
- USA
- Germany
- Czech Republic
Field-hunting tip
Where it forms: Derived from the oxidation of sulfide minerals and potassium-bearing minerals. If you start seeing gypsum, sulfur, jarosite in float, you are in the right ground. Typical habit: water soluble. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify alum-(k)?+
Where is alum-(k) found?+
What rocks look like alum-(k)?+
What minerals are found with alum-(k)?+
What kind of rock does alum-(k) form in?+
What is alum-(k) used for?+
Find alum-(k) on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
