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.

Hardness

2-2.5

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralcommonAlum Group family

Alum-(K)

Also: Potash Alum, Tschermigite-like

Hardness: 2-2.5
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this alum-(k)?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch alum-(k) with a known reference. Alum-(K) sits at Mohs 2-2.5 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Alum-(K) leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Alum-(K) typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: colorless, white, grayish.
5
Look at form & habit
Crystal system: cubic. Typical habit: octahedral crystals, crusts, efflorescences.

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: 2-2.5
Density: 1.75 g/cm³
Colors: Colorless White Grayish
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Cubic
Crystal habit: Octahedral Crystals, Crusts, Efflorescences
Cleavage: None
Rarity: Common
Uses: Collector, Industrial
Host rock: Volcanic Fumaroles, Burning Coal Seams
Typical price: $10-30 for small specimens

Where rockhounds find alum-(k)

Classic worldwide localities

Italy
USA
Germany
Czech Republic

Field-hunting tip

Look in volcanic fumaroles, burning coal seams country — that is the host setting where alum-(k) typically forms. If you start seeing gypsum, sulfur, jarosite in float, you are in the right ground. Field specimens usually show a octahedral crystals, crusts, efflorescences habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify alum-(k)?+

Mohs hardness is 2-2.5. It typically shows a vitreous luster. The streak is white. Common colors include colorless, white, grayish.

Where is alum-(k) found?+

Notable localities include Italy; USA; Germany; Czech Republic.

How much is alum-(k) worth?+

Typical asking prices fall in the range of $10-30 for small specimens. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like alum-(k)?+

Alum-(K) is most often confused with Tschermigite, Halite, Mendozite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with alum-(k)?+

Alum-(K) commonly co-occurs with Gypsum, Sulfur, Jarosite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does alum-(k) form in?+

Alum-(K) typically forms in volcanic fumaroles, burning coal seams. Working float back to the host body is the standard way to chase a fresh occurrence.

What is alum-(k) used for?+

Alum-(K) is used in collector, industrial.

Find alum-(k) on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.