Mercallite is a rare potassium hydrogen sulfate mineral that typically forms as a white efflorescent crust or coating around active volcanic fumaroles. Due to its high solubility, it is rarely found in nature unless collected immediately from dry, arid volcanic environments.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrare

Mercallite

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

Is this mercallite?

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 mercallite with a known reference. Mercallite sits at Mohs 2 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Mercallite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Mercallite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: white, colorless.
5
Look at form & habit
Crystal system: orthorhombic. Typical habit: crusts, efflorescent coatings, granular.

Often confused with

Mercallite vs. its common look-alikes — and how to tell them apart in the field.

mineral

Alunogen

Mohs 1.5-2

How to tell apart: Luster reads vitreous on Mercallite and silky on Alunogen.

Often found alongside mercallite

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

All properties

Chemical formula: KHSO₄
Mohs hardness: 2
Density: 2.26 g/cm³
Colors: White Colorless
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Orthorhombic
Crystal habit: Crusts, Efflorescent Coatings, Granular
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Volcanic Fumaroles
Typical price: $20-100 per specimen

Where rockhounds find mercallite

Classic worldwide localities

Mount Vesuvius, Italy
Kilauea, Hawaii, USA
Gromovoye fumarole, Russia

Field-hunting tip

Look in volcanic fumaroles country — that is the host setting where mercallite typically forms. If you start seeing gypsum, thenardite, alum in float, you are in the right ground. Field specimens usually show a crusts, efflorescent coatings, granular habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify mercallite?+

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

Where is mercallite found?+

Notable localities include Mount Vesuvius, Italy; Kilauea, Hawaii, USA; Gromovoye fumarole, Russia.

How much is mercallite worth?+

Typical asking prices fall in the range of $20-100 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like mercallite?+

Mercallite is most often confused with Alunogen, Misenite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with mercallite?+

Mercallite commonly co-occurs with Gypsum, Thenardite, Alum. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does mercallite form in?+

Mercallite typically forms in volcanic fumaroles. Working float back to the host body is the standard way to chase a fresh occurrence.

What is mercallite used for?+

Mercallite is used in collector.

Find mercallite on the map

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