Chlorothionite is a very rare copper-potassium sulfate chloride found almost exclusively in volcanic fumarole environments. It is characterized by its distinct bright blue to blue-green color, typically appearing as delicate crusts or efflorescences around volcanic vents.

Hardness

2.5

Mohs

Luster

Vitreous

Streak

White

Transparency

Translucent

mineralrare

Chlorothionite

Hardness: 2.5
Luster: Vitreous
Streak: White
Transparency: Translucent

Is this chlorothionite?

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 chlorothionite with a known reference. Chlorothionite sits at Mohs 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. Chlorothionite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Chlorothionite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: blue, blue-green.
5
Look at form & habit
Crystal system: orthorhombic. Typical habit: crusts, granular.

Often confused with

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

Often found alongside chlorothionite

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

All properties

Chemical formula: K₃Cu(SO₄)Cl
Mohs hardness: 2.5
Density: 2.71 g/cm³
Colors: Blue Blue-green
Streak: White
Luster: Vitreous
Transparency: Translucent
Crystal system: Orthorhombic
Crystal habit: Crusts, Granular
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Fumaroles
Typical price: $50-300 per specimen

Where rockhounds find chlorothionite

Classic worldwide localities

Vesuvius, Italy

Field-hunting tip

Look in fumaroles country, the host setting where chlorothionite typically forms. If you start seeing tenorite, sylvite, halite in float, you are in the right ground. Field specimens usually show a crusts, granular habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify chlorothionite?+

Mohs hardness is 2.5. It typically shows a vitreous luster. The streak is white. Common colors include blue, blue-green.

Where is chlorothionite found?+

Notable localities include Vesuvius, Italy.

How much is chlorothionite worth?+

Typical asking prices fall in the range of $50-300 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

Is chlorothionite safe to handle?+

It contains toxic constituents. Contains copper which is toxic if ingested or inhaled as dust. Wash hands thoroughly after handling specimens. Handle with care, avoid grinding or breathing dust, and store separately.

What rocks look like chlorothionite?+

Chlorothionite is most often confused with Chalcanthite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with chlorothionite?+

Chlorothionite commonly co-occurs with Tenorite, Sylvite, Halite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does chlorothionite form in?+

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

What is chlorothionite used for?+

Chlorothionite is used in collector.

Find chlorothionite on the map

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