Melanothallite is a rare copper chloride mineral found primarily in volcanic fumaroles as a product of volcanic gases. It typically occurs as small, dark, resinous crystals or crusts and is highly susceptible to atmospheric moisture, requiring careful preservation by collectors.
Is this melanothallite?
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 melanothallite with a known reference. Melanothallite 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. Melanothallite leaves a black streak.
- 3Read the lusterHold the specimen under a strong light. Melanothallite typically shows a resinous luster.
- 4Match the color rangeCompare against the expected color range: black, dark blue.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: tabular crystals, crusts, lamellar.
Often confused with
Melanothallite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Melanothallite leaves black, Atacamite leaves apple green; luster reads resinous on Melanothallite and adamantine to vitreous on Atacamite.
How to tell apart: Streak differs — Melanothallite leaves black, Paratacamite leaves apple green; luster reads resinous on Melanothallite and adamantine on Paratacamite.
Often found alongside melanothallite
Minerals reported to co-occur with melanothallite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Cu₂OCl₂
- Mohs hardness
- 3
- Density
- 4.2 g/cm³
- Streak
- Black
- Luster
- Resinous
- Transparency
- Opaque
- Crystal system
- Orthorhombic
- Crystal habit
- Tabular Crystals, Crusts, Lamellar
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Volcanic Fumaroles
- Typical price
- $50-300 per specimen
Where rockhounds find melanothallite
Classic worldwide localities
- Vesuvius, Italy
Field-hunting tip
Look in volcanic fumaroles country — that is the host setting where melanothallite typically forms. If you start seeing tenorite, eriocalcite, cotunnite in float, you are in the right ground. Field specimens usually show a tabular crystals, crusts, lamellar habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find melanothallite on the map
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