Johillerite is an exceptionally rare arsenate mineral known primarily from the fumaroles of the Tolbachik volcano in Russia. It typically appears as small, tabular, blue-violet crystals formed during high-temperature volcanic activity.
Is this johillerite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch johillerite with a known reference. Johillerite sits at Mohs ~3, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white to very pale violet.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: violet.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: aggregates, radial.
Often confused with
Johillerite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Tilasite is the harder of the two (Mohs 5 vs. ~3); luster reads vitreous on Johillerite and resinous, vitreous on cleavages on Tilasite.
How to tell apart: Berzeliite is the harder of the two (Mohs 4.5-5 vs. ~3); streak differs, so Johillerite leaves white to very pale violet, Berzeliite leaves nearly white to yellow-orange; luster reads vitreous on Johillerite and resinous on Berzeliite.
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
- Density
- 4.15 g/cm³
- Colors
- Streak
- White to very pale violet
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Aggregates, radial
- Cleavage
- Perfect on {010}; good on {100} and {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A very rare secondary mineral in oxidized copper ore
Where rockhounds find johillerite
Classic worldwide localities
- Found at Tsumeb, Namibia
Field-hunting tip
Where it forms: A very rare secondary mineral in oxidized copper ore. If you start seeing cuprian adamite, conichalcite, chalcocite in float, you are in the right ground. Typical habit: aggregates, radial. Train your eye for that shape before scanning the outcrop.
Common questions
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Find johillerite on the map
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