Johillerite is an exceptionally rare arsenate mineral known primarily from the fumaroles of the Tolbachik volcano in Russia. It typically appears as small, tabular, yellow to yellow-green crystals formed during high-temperature volcanic activity.

Hardness

3.5

Mohs

Luster

Vitreous

Streak

Yellow

Transparency

Translucent

mineralrare

Johillerite

Hardness: 3.5
Luster: Vitreous
Streak: Yellow
Transparency: Translucent

Is this johillerite?

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 johillerite with a known reference. Johillerite sits at Mohs 3.5 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Johillerite leaves a yellow streak.
3
Read the luster
Hold the specimen under a strong light. Johillerite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: yellow, yellow-green.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: tabular crystals.

Often confused with

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

mineral

Tilasite

Mohs 5

How to tell apart: Tilasite is the harder of the two (Mohs 5 vs. 3.5); streak differs — Johillerite leaves yellow, Tilasite leaves white.

How to tell apart: Berzeliite is the harder of the two (Mohs 5 vs. 3.5); streak differs — Johillerite leaves yellow, Berzeliite leaves white.

Compare →

Often found alongside johillerite

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

All properties

Chemical formula: NaCuMg₃(AsO₄)₃
Mohs hardness: 3.5
Density: 3.44 g/cm³
Colors: Yellow Yellow-green
Streak: Yellow
Luster: Vitreous
Transparency: Translucent
Crystal system: Monoclinic
Crystal habit: Tabular Crystals
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Fumarolic Deposits
Typical price: $100-500 thumbnail

Where rockhounds find johillerite

Classic worldwide localities

Tolbachik volcano, Russia

Field-hunting tip

Look in fumarolic deposits country — that is the host setting where johillerite typically forms. If you start seeing tenorite, hematite, paratacamite in float, you are in the right ground. Field specimens usually show a tabular crystals habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify johillerite?+

Mohs hardness is 3.5. It typically shows a vitreous luster. The streak is yellow. Common colors include yellow, yellow-green.

Where is johillerite found?+

Notable localities include Tolbachik volcano, Russia.

How much is johillerite worth?+

Typical asking prices fall in the range of $100-500 thumbnail. Quality, size, and provenance can move individual specimens well outside that range.

Is johillerite safe to handle?+

It contains toxic constituents. Contains arsenic, which is toxic. Handle with care, wash hands after touching, and do not ingest or inhale dust. Handle with care, avoid grinding or breathing dust, and store separately.

What rocks look like johillerite?+

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

What minerals are found with johillerite?+

Johillerite commonly co-occurs with tenorite, hematite, paratacamite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does johillerite form in?+

Johillerite typically forms in fumarolic deposits. Working float back to the host body is the standard way to chase a fresh occurrence.

What is johillerite used for?+

Johillerite is used in collector.

Find johillerite on the map

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