Alum-(Na) is a rare hydrated sodium aluminum sulfate member of the alum group, typically forming as white powdery efflorescences or delicate octahedral crystals. It is highly water-soluble and primarily found in arid environments or volcanic fumaroles, making it a challenging mineral to maintain in private collections.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrareAlum Group family

Alum-(Na)

Hardness: 3
Luster: Vitreous
Streak: White
Transparency: Transparent

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Is this alum-(na)?

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-(na) with a known reference. Alum-(Na) sits at Mohs 3, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: white.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous.
4
Match the color range
Compare against the expected color range: colorless.
5
Look at form & habit
Crystal system: cubic. Typical habit: fibrous.

Often confused with

Alum-(Na) vs. its common look-alikes, and how to tell them apart in the field.

mineral

Alum-(K)

Mohs 2

How to tell apart: Alum-(Na) is noticeably harder (Mohs 3 vs. 2).

How to tell apart: Alum-(Na) is noticeably harder (Mohs 3 vs. 1.5).

How to tell apart: Alum-(Na) is noticeably harder (Mohs 3 vs. 1.5).

Compare →

Often found alongside alum-(na)

Minerals reported to co-occur with alum-(na). Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: NaAl(SO₄)₂·12H₂O
Mohs hardness: 3
Density: 1.67 g/cm³
Colors: Colorless
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Cubic
Crystal habit: Fibrous
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: fumaroles and oxidized sulfide deposits

Where rockhounds find alum-(na)

Classic worldwide localities

Italy
USA
Russia
Chile

Field-hunting tip

Where it forms: fumaroles and oxidized sulfide deposits. If you start seeing gypsum, jarosite, copiapite in float, you are in the right ground. Typical habit: fibrous. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify alum-(na)?+

Mohs hardness is 3. Luster: vitreous. The streak is white. Common colors include colorless.

Where is alum-(na) found?+

Notable localities include Italy; USA; Russia; Chile.

What rocks look like alum-(na)?+

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

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

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

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

Where it forms: fumaroles and oxidized sulfide deposits. Working float back to the host body is the standard way to chase a fresh occurrence.

What is alum-(na) used for?+

Alum-(Na) is used in collector.

Find alum-(na) on the map

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