Cumengeite is a highly prized secondary mineral known for its distinctive deep indigo-blue, pseudo-cubic or pyramidal crystals. It is typically found in oxidized lead-copper deposits and is often associated with the mineral boleite, from which it is sometimes distinguished by its more elongated, star-like crystal formations.
Is this cumengeite?
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 cumengeite with a known reference. Cumengeite sits at Mohs 2.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: sky-blue.
- 3Read the lusterHold the specimen under a strong light. Luster: weakly vitreous to adamantine.
- 4Match the color rangeCompare against the expected color range: indigo blue.
- 5Look at form & habitCrystal system: tetragonal. Typical habit: pyramidal crystals, pseudo-cubic, stellate groups.
Often confused with
Cumengeite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Cumengeite leaves sky-blue, Boleite leaves blue, with greenish tint.
How to tell apart: Streak differs, so Cumengeite leaves sky-blue, Pseudoboleite leaves light green; luster reads weakly vitreous to adamantine on Cumengeite and pearly on cleavages on Pseudoboleite.
Often found alongside cumengeite
Minerals reported to co-occur with cumengeite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Pb₂₁Cu₂₀Cl₄₂(OH)₄₀·6H₂O
- Mohs hardness
- 2.5
- Density
- 4.656 g/cm³
- Colors
- Indigo blue
- Streak
- Sky-blue
- Luster
- Weakly vitreous to adamantine
- Transparency
- Translucent
- Crystal system
- Tetragonal
- Crystal habit
- Pyramidal crystals, pseudo-cubic, stellate groups
- Cleavage
- {101}, good; {110}, distinct; {001}, poor
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A secondary mineral formed through reaction of chloride ions with primary sulfides in the oxidized zone of some Pb-Cu deposits; in smelter slag immersed in and leached by sea water
Where rockhounds find cumengeite
Classic worldwide localities
- Amelia mine, Boleo, near Santa Rosalía, Baja California, Mexico
- Laurium, Greece, in slag
- Baratti Beach, Tuscany (in slag) and at Vesuvius (near a fumarole), Italy
- Cornwall, from Tolcarne Beach, Great Western Beach, and Watergate Bay, Newquay, England
- Newporth Beach, Falmouth, England
- Porthilly mine, St. Minver, England
Field-hunting tip
Where it forms: A secondary mineral formed through reaction of chloride ions with primary sulfides in the oxidized zone of some Pb-Cu deposits; in smelter slag immersed in and leached by sea water. If you start seeing boleite, pseudoboleite, atacamite in float, you are in the right ground. Typical habit: pyramidal crystals, pseudo-cubic, stellate groups. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify cumengeite?+
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Find cumengeite on the map
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