Posnjakite is a secondary copper sulfate mineral that forms beautiful, vibrant blue, platy crystal clusters. It is often found as a crust or coating in the oxidized zones of copper mines and is frequently associated with other copper minerals like brochantite and langite.

Hardness

2-3

Mohs

Luster

Vitreous

Streak

Pale Blue

Transparency

Translucent

mineraluncommon

Posnjakite

Hardness: 2-3
Luster: Vitreous
Streak: Pale Blue
Transparency: Translucent

Is this posnjakite?

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 posnjakite with a known reference. Posnjakite sits at Mohs 2-3 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Posnjakite leaves a pale blue streak.
3
Read the luster
Hold the specimen under a strong light. Posnjakite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: blue, greenish-blue.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: platy crystals, crusts, rosettes, radiating aggregates.

Often confused with

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

mineral

Brochantite

Mohs 3.5-4

How to tell apart: Streak differs — Posnjakite leaves pale blue, Brochantite leaves pale-green.

Often found alongside posnjakite

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

All properties

Chemical formula: Cu₄(SO₄)(OH)₆·H₂O
Mohs hardness: 2-3
Density: 3.84 g/cm³
Colors: Blue Greenish-blue
Streak: Pale Blue
Luster: Vitreous
Transparency: Translucent
Crystal system: Monoclinic
Crystal habit: Platy Crystals, Crusts, Rosettes, Radiating Aggregates
Cleavage: Perfect in One Direction
Rarity: Uncommon
Uses: Collector
Host rock: Oxidized Zones of Copper-rich Sulfide Deposits
Typical price: $10-60 for small specimens

Where rockhounds find posnjakite

Classic worldwide localities

Kastelorizo, Greece
Bisbee, Arizona, USA
Cornwall, England
Schneeberg, Germany
Mina Profunda, Chile

Field-hunting tip

Look in oxidized zones of copper-rich sulfide deposits country — that is the host setting where posnjakite typically forms. If you start seeing brochantite, langite, chalcopyrite in float, you are in the right ground. Field specimens usually show a platy crystals, crusts, rosettes, radiating aggregates habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify posnjakite?+

Mohs hardness is 2-3. It typically shows a vitreous luster. The streak is pale blue. Common colors include blue, greenish-blue.

Where is posnjakite found?+

Notable localities include Kastelorizo, Greece; Bisbee, Arizona, USA; Cornwall, England; Schneeberg, Germany; Mina Profunda, Chile.

How much is posnjakite worth?+

Typical asking prices fall in the range of $10-60 for small specimens. Quality, size, and provenance can move individual specimens well outside that range.

Is posnjakite safe to handle?+

It contains toxic constituents. Contains copper; wash hands thoroughly after handling to prevent ingestion of dust or trace amounts. Handle with care, avoid grinding or breathing dust, and store separately.

What rocks look like posnjakite?+

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

What minerals are found with posnjakite?+

Posnjakite commonly co-occurs with Brochantite, Langite, Chalcopyrite, Malachite, Gypsum. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does posnjakite form in?+

Posnjakite typically forms in oxidized zones of copper-rich sulfide deposits. Working float back to the host body is the standard way to chase a fresh occurrence.

What is posnjakite used for?+

Posnjakite is used in collector.

Find posnjakite on the map

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