Ondrušite is a rare hydrated calcium copper arsenate mineral found in the hydrothermal veins of the Jáchymov district. It typically occurs as small tabular crystals or crusts that form in association with other arsenic-bearing secondary minerals.

Hardness

~2-3

Mohs

Luster

Vitreous

Streak

White to grayish white with a greenish tint

Transparency

Translucent

mineralrareArmenite Group family

Ondrušite

Hardness: ~2-3
Luster: Vitreous
Streak: White to grayish white with a greenish tint
Transparency: Translucent

Is this ondrušite?

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 ondrušite with a known reference. Ondrušite 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. Streak: white to grayish white with a greenish tint.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous.
4
Match the color range
Compare against the expected color range: white, whitish green, gray-green, apple-green.
5
Look at form & habit
Crystal system: triclinic. Typical habit: tabular crystals, aggregates.

Often confused with

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

Often found alongside ondrušite

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

All properties

Chemical formula: CaCu₄(AsO₄)₂(AsO₃OH)₂·10H₂O
Mohs hardness: ~2-3
Density: 3.26 g/cm³
Colors: WhiteWhitish greenGray-green Apple-green
Streak: White to grayish white with a greenish tint
Luster: Vitreous
Transparency: Translucent
Crystal system: Triclinic
Crystal habit: Tabular crystals, aggregates
Cleavage: Perfect on {001}
Rarity: Rare
Uses: Collector
Host rock: Secondary in the weathered zone of a vein-type Ag-As-Co-Ni-Bi-U hydrothermal deposit

Where rockhounds find ondrušite

Classic worldwide localities

Geister vein, Rovnost mine and the Geschieber vein, Svornost mine, Jáchymov (St. Joachimsthal) ore district, Czech Republic

Field-hunting tip

Where it forms: Secondary in the weathered zone of a vein-type Ag-As-Co-Ni-Bi-U hydrothermal deposit. If you start seeing lindackerite, geminite, lavendulan in float, you are in the right ground. Typical habit: tabular crystals, aggregates. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify ondrušite?+

Mohs hardness is ~2-3. Luster: vitreous. The streak is white to grayish white with a greenish tint. Common colors include white, whitish green, gray-green, apple-green.

Where is ondrušite found?+

Notable localities include Geister vein, Rovnost mine and the Geschieber vein, Svornost mine, Jáchymov (St. Joachimsthal) ore district, Czech Republic.

Is ondrušite safe to handle?+

Ondrušite contains arsenic and copper and calls for real care. Contains arsenic, which is toxic if ingested or inhaled as dust. Always wash hands after handling and keep away from children or pets.

What rocks look like ondrušite?+

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

What minerals are found with ondrušite?+

Ondrušite commonly co-occurs with Lindackerite, Geminite, Lavendulan, Slavkovite, Strashimirite, Olivenite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does ondrušite form in?+

Where it forms: Secondary in the weathered zone of a vein-type Ag-As-Co-Ni-Bi-U hydrothermal deposit. Working float back to the host body is the standard way to chase a fresh occurrence.

What is ondrušite used for?+

Ondrušite is used in collector.

Find ondrušite on the map

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