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. Calumetite leaves a pale blue streak.
- 3Read the lusterHold the specimen under a strong light. Calumetite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: deep blue, sky blue.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: platy, tabular crystals or crusts.
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 — Calumetite leaves pale blue, Atacamite leaves apple green; luster reads vitreous 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 — Calumetite leaves pale blue, Paratacamite leaves apple green; luster reads vitreous on Calumetite and adamantine on Paratacamite.
How to tell apart: Connellite is the harder of the two (Mohs 3 vs. 2).
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
- Cu(OH)₃Cl
- Mohs hardness
- 2
- Density
- 3.5 g/cm³
- Streak
- Pale Blue
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Platy, Tabular Crystals or Crusts
- Cleavage
- Perfect On {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Copper-bearing Basaltic Conglomerate
- Typical price
- $50-300 per specimen depending on size and quality
Where rockhounds find calumetite
Classic worldwide localities
- Calumet and Hecla Mine, Michigan, USA
- Centennial Mine, Michigan, USA
- Kearsarge lode, Michigan, USA
Field-hunting tip
Look in copper-bearing basaltic conglomerate country — that is the host setting where calumetite typically forms. If you start seeing copper, cuprite, tenorite in float, you are in the right ground. Field specimens usually show a platy, tabular crystals or crusts habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find calumetite on the map
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