Kröhnkite is a rare hydrated sodium copper sulfate characterized by its distinct bright azure-blue color and vitreous luster. It forms primarily as a secondary mineral in the arid oxidation zones of copper deposits, often found as tabular crystals or fibrous crusts associated with other copper sulfates.

Hardness

2.5-3

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineraluncommon

Kröhnkite

Hardness: 2.5-3
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this kröhnkite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch kröhnkite with a known reference. Kröhnkite sits at Mohs 2.5-3, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: white.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous.
4
Match the color range
Compare against the expected color range: blue, dark sky blue, greenish blue, yellowish green.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: encrustations, fibrous, massive.

Often confused with

Kröhnkite vs. its common look-alikes, and how to tell them apart in the field.

How to tell apart: Streak differs, so Kröhnkite leaves white, Brochantite leaves pale green.

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Often found alongside kröhnkite

Minerals reported to co-occur with kröhnkite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: Na₂Cu(SO₄)₂·2H₂O
Mohs hardness: 2.5-3
Density: 2.90 g/cm³
Colors: BlueDark sky blueGreenish blue Yellowish green
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Monoclinic
Crystal habit: Encrustations, fibrous, massive
Cleavage: Perfect on {010}; poor on {101}
Rarity: Uncommon
Uses: Collector
Host rock: An uncommon secondary mineral formed in the oxidized zone of copper deposits, typically in very arid climates

Where rockhounds find kröhnkite

Classic worldwide localities

Chuquicamata, at Quetena, west of Calama, and at Collahuasi, Antofagasta, Chile
El Cobre de Mejillones and in the Incahuasi district, Atacama, Chile
Wheal Hazard, St. Just, Cornwall, England
Recsk copper deposit, Mátra Mountains, Hungary
Capo Calamita, Elba, Italy
Broken Hill, New South Wales, Australia

Field-hunting tip

Where it forms: An uncommon secondary mineral formed in the oxidized zone of copper deposits, typically in very arid climates. If you start seeing atacamite, blödite, chalcanthite in float, you are in the right ground. Typical habit: encrustations, fibrous, massive. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify kröhnkite?+

Mohs hardness is 2.5-3. Luster: vitreous. The streak is white. Common colors include blue, dark sky blue, greenish blue, yellowish green.

Where is kröhnkite found?+

Notable localities include Chuquicamata, at Quetena, west of Calama, and at Collahuasi, Antofagasta, Chile; El Cobre de Mejillones and in the Incahuasi district, Atacama, Chile; Wheal Hazard, St. Just, Cornwall, England; Recsk copper deposit, Mátra Mountains, Hungary; Capo Calamita, Elba, Italy.

Is kröhnkite safe to handle?+

Kröhnkite contains copper. Wash your hands afterwards, never grind or breathe the dust, and store it apart from specimens you handle often.

What rocks look like kröhnkite?+

Kröhnkite is most often confused with Chalcanthite, Brochantite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with kröhnkite?+

Kröhnkite commonly co-occurs with Atacamite, Blödite, Chalcanthite, Antlerite, Natrochalcite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does kröhnkite form in?+

Where it forms: An uncommon secondary mineral formed in the oxidized zone of copper deposits, typically in very arid climates. Working float back to the host body is the standard way to chase a fresh occurrence.

What is kröhnkite used for?+

Kröhnkite is used in collector.

Find kröhnkite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.