Chalcothallite is an extremely rare copper-thallium sulfide/selenide mineral that is primarily known from the unique alkaline rocks of the Ilímaussaq complex. Collectors typically find it in small, opaque, dark metallic grains or thin lamellar masses associated with other rare thallium species.

Hardness

2-2.5

Mohs

Luster

Metallic

Streak

Black

Transparency

Opaque

mineralrare

Chalcothallite

Hardness: 2-2.5
Luster: Metallic
Streak: Black
Transparency: Opaque

Is this chalcothallite?

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 chalcothallite with a known reference. Chalcothallite sits at Mohs 2-2.5, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: black.
3
Read the luster
Hold the specimen under a strong light. Luster: metallic.
4
Match the color range
Compare against the expected color range: lead gray, black.
5
Look at form & habit
Crystal system: tetragonal. Typical habit: aggregates, lamellar, tarnishes.

Often confused with

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

mineral

Galena

Mohs 2.5-2.75

How to tell apart: Streak differs, so Chalcothallite leaves black, Galena leaves lead-gray.

How to tell apart: Streak differs, so Chalcothallite leaves black, Chalcocite leaves blackish lead-gray.

Compare →

Often found alongside chalcothallite

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

All properties

Chemical formula: (Tl,K)₂(Cu,Cu,Fe,Ag)₆.₃SbS₄
Mohs hardness: 2-2.5
Density: 6.6 g/cm³
Colors: Lead gray Black
Streak: Black
Luster: Metallic
Transparency: Opaque
Crystal system: Tetragonal
Crystal habit: Aggregates, lamellar, tarnishes
Cleavage: Perfect on {001}, indistinct on {100} and {010}
Rarity: Rare
Uses: Collector
Host rock: In ussingite veins cutting poikilitic sodalite syenite

Where rockhounds find chalcothallite

Classic worldwide localities

Mt. Nákâlâq, northwest of Taseq Lake, in the Ilímaussaq intrusion, southern Greenland

Field-hunting tip

Where it forms: In ussingite veins cutting poikilitic sodalite syenite. If you start seeing galena, vrbaite, cuprostibite in float, you are in the right ground. Typical habit: aggregates, lamellar, tarnishes. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify chalcothallite?+

Mohs hardness is 2-2.5. Luster: metallic. The streak is black. Common colors include lead gray, black.

Where is chalcothallite found?+

Notable localities include Mt. Nákâlâq, northwest of Taseq Lake, in the Ilímaussaq intrusion, southern Greenland.

Is chalcothallite safe to handle?+

Chalcothallite contains antimony, copper and thallium and calls for real care. Contains thallium and antimony; handle with extreme caution and wash hands thoroughly after contact. Do not inhale dust or ingest.

What rocks look like chalcothallite?+

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

What minerals are found with chalcothallite?+

Chalcothallite commonly co-occurs with Galena, Vrbaite, Cuprostibite, Thalcusite, Silver, Gudmundite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does chalcothallite form in?+

Where it forms: In ussingite veins cutting poikilitic sodalite syenite. Working float back to the host body is the standard way to chase a fresh occurrence.

What is chalcothallite used for?+

Chalcothallite is used in collector.

Find chalcothallite on the map

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