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. Streak: black.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: black, bluish black, greenish black.
- 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 vitreous on Melanothallite and adamantine to vitreous on Atacamite.
How to tell apart: Streak differs, so Melanothallite leaves black, Paratacamite leaves green.
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
- 4.08 g/cm³
- Streak
- Black
- Luster
- Vitreous
- Transparency
- Opaque, transparent in thin fragments
- Crystal system
- Orthorhombic
- Crystal habit
- Tabular crystals, crusts, lamellar
- Cleavage
- One direction, perfect, probably prismatic
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Sublimed on crater walls formed in 1868 and 1906 (Vesuvius, Italy); in volcanic fissures (Tolbachik volcano, Russia)
Where rockhounds find melanothallite
Classic worldwide localities
- Vesuvius, Campania, Italy
- Tolbachik fissure volcano, Kamchatka Peninsula, Russia
Field-hunting tip
Where it forms: Sublimed on crater walls formed in 1868 and 1906 (Vesuvius, Italy); in volcanic fissures (Tolbachik volcano, Russia). If you start seeing eriochalcite, chalcocyanite, euchlorine in float, you are in the right ground. Typical habit: tabular crystals, crusts, lamellar. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify melanothallite?+
Where is melanothallite found?+
Is melanothallite safe to handle?+
What rocks look like melanothallite?+
What minerals are found with melanothallite?+
What kind of rock does melanothallite form in?+
What is melanothallite used for?+
Find melanothallite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
