Calumetite is a very rare copper hydroxychloride mineral known primarily from the copper mines of the Keweenaw Peninsula in Michigan. It typically appears as bright blue platy or tabular crystals forming thin crusts or linings within cavities of copper-rich host rocks.
Is this calumetite?
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 calumetite with a known reference. Calumetite sits at Mohs 2, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: blue.
- 3Read the lusterHold the specimen under a strong light. Luster: pearly on the cleavage.
- 4Match the color rangeCompare against the expected color range: azure blue, powder blue, bluish white.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: scaly, spherical, wheat sheaf.
Often confused with
Calumetite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Atacamite is the harder of the two (Mohs 3-3.5 vs. 2); streak differs, so Calumetite leaves blue, Atacamite leaves apple-green; luster reads pearly on the cleavage on Calumetite and adamantine to vitreous on Atacamite.
How to tell apart: Paratacamite is the harder of the two (Mohs 3 vs. 2); streak differs, so Calumetite leaves blue, Paratacamite leaves green; luster reads pearly on the cleavage on Calumetite and vitreous on Paratacamite.
How to tell apart: Connellite is the harder of the two (Mohs 3 vs. 2); streak differs, so Calumetite leaves blue, Connellite leaves pale greenish blue; luster reads pearly on the cleavage on Calumetite and vitreous on Connellite.
Often found alongside calumetite
Minerals reported to co-occur with calumetite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaCu₄(OH)₈Cl₂·3.5H₂O
- Mohs hardness
- 2
- Colors
- Streak
- Blue
- Luster
- Pearly on the cleavage
- Transparency
- Semitransparent
- Crystal system
- Orthorhombic
- Crystal habit
- Scaly, spherical, wheat sheaf
- Cleavage
- {001}, good
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In cavities and fractures in basalt, formed by the action of chlorine-bearing connate waters on copper, cuprite being regarded as an intermediate product in the alteration (Centennial mine, Michigan, USA)
Where rockhounds find calumetite
Classic worldwide localities
- Centennial and other nearby mines, near Calumet, Houghton Co., Michigan, USA
- Laurium, Greece, in slag
- Niederfischbach, Siegerland, and at Richelsdorf, Hesse, in slag, Germany
Field-hunting tip
Where it forms: In cavities and fractures in basalt, formed by the action of chlorine-bearing connate waters on copper, cuprite being regarded as an intermediate product in the alteration (Centennial mine, Michigan, USA). If you start seeing tremolite, quartz, epidote in float, you are in the right ground. Typical habit: scaly, spherical, wheat sheaf. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify calumetite?+
Where is calumetite found?+
Is calumetite safe to handle?+
What rocks look like calumetite?+
What minerals are found with calumetite?+
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Find calumetite on the map
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