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

1.5-3

Mohs

Luster

Vitreous

Streak

Pale blue

Transparency

Semitransparent

mineraluncommon

Posnjakite

Hardness: 1.5-3
Luster: Vitreous
Streak: Pale blue
Transparency: Semitransparent

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 1.5-3, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: pale blue.
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 blue.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: crystalline, encrustations, platy.

Often confused with

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

How to tell apart: Streak differs, so Posnjakite leaves pale blue, Langite leaves greenish blue.

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: 1.5-3
Density: 3.32 g/cm³
Colors: Blue Dark blue
Streak: Pale blue
Luster: Vitreous
Transparency: Semitransparent
Crystal system: Monoclinic
Crystal habit: Crystalline, encrustations, platy
Cleavage: Perfect
Rarity: Uncommon
Uses: Collector
Host rock: A secondary mineral formed in the oxidized zone of copper-bearing hydrothermal mineral deposits, commonly of post-mine origin; may occur in slags

Where rockhounds find posnjakite

Classic worldwide localities

Yellow Pine mine, Crisman, Boulder Co., Colorado, and at the Majuba Hill mine, Antelope district, Pershing Co., Nevada, USA
Nura-Taldy tungsten deposits, central Kazakhstan
Cornwall, as at the Drakewalls mine, Gunnislake, and the Fowey Consols mine, Tywardreath, England
Penrhiw mine, Ystumtuen, Dyfed, Wales
Muckross mine, Co. Kerry, Ireland
Salsigne mine, 15 km north of Carcassone, Aude, France

Field-hunting tip

Where it forms: A secondary mineral formed in the oxidized zone of copper-bearing hydrothermal mineral deposits, commonly of post-mine origin; may occur in slags. If you start seeing brochantite, langite, devilline in float, you are in the right ground. Typical habit: crystalline, encrustations, platy. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify posnjakite?+

Mohs hardness is 1.5-3. Luster: vitreous. The streak is pale blue. Common colors include blue, dark blue.

Where is posnjakite found?+

Notable localities include Yellow Pine mine, Crisman, Boulder Co., Colorado, and at the Majuba Hill mine, Antelope district, Pershing Co., Nevada, USA; Nura-Taldy tungsten deposits, central Kazakhstan; Cornwall, as at the Drakewalls mine, Gunnislake, and the Fowey Consols mine, Tywardreath, England; Penrhiw mine, Ystumtuen, Dyfed, Wales; Muckross mine, Co. Kerry, Ireland.

Is posnjakite safe to handle?+

Posnjakite contains copper. Contains copper; wash hands thoroughly after handling to prevent ingestion of dust or trace amounts.

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, Devilline, Serpierite, Woodwardite, Wroewolfeite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does posnjakite form in?+

Where it forms: A secondary mineral formed in the oxidized zone of copper-bearing hydrothermal mineral deposits, commonly of post-mine origin; may occur in slags. 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.