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, so Melanothallite leaves black, Atacamite leaves apple green; luster reads resinous on Melanothallite and adamantine to vitreous on Atacamite.
How to tell apart: Streak differs, so 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, 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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