Thalfenisite is a rare thallium-bearing sulfide mineral typically found in complex nickel-copper ore deposits. It usually occurs as microscopic grains embedded within other sulfide minerals, making it a challenging find for even advanced collectors.

Hardness

1-1.5

Mohs

Luster

Metallic

Streak

Black

Transparency

Opaque

mineralrare

Thalfenisite

Hardness: 1-1.5
Luster: Metallic
Streak: Black
Transparency: Opaque

Is this thalfenisite?

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 thalfenisite with a known reference. Thalfenisite sits at Mohs 1-1.5, 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: bronze brown, greenish brown.
5
Look at form & habit
Crystal system: cubic. Typical habit: microscopic crystals.

Often confused with

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

mineral

Djerfisherite

Mohs <3.5

How to tell apart: Djerfisherite is the harder of the two (Mohs <3.5 vs. 1-1.5); luster reads metallic on Thalfenisite and submetallic on Djerfisherite.

How to tell apart: Thalcusite is the harder of the two (Mohs 2.5-3 vs. 1-1.5).

Compare →

Often found alongside thalfenisite

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

All properties

Chemical formula: Tl₆(Fe,Ni)₂₅S₂₆Cl
Mohs hardness: 1-1.5
Density: 5.26 g/cm³
Colors: Bronze brown Greenish brown
Streak: Black
Luster: Metallic
Transparency: Opaque
Crystal system: Cubic
Crystal habit: Microscopic crystals
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: In pentlandite-galena-chalcopyrite ores, localized at the contact of chalcopyrite and galena and included in pentlandite

Where rockhounds find thalfenisite

Classic worldwide localities

Oktyabr mine, Talnakh area, Noril’sk region, western Siberia, Russia (type locality)

Field-hunting tip

Where it forms: In pentlandite-galena-chalcopyrite ores, localized at the contact of chalcopyrite and galena and included in pentlandite. If you start seeing pentlandite, galena, chalcopyrite in float, you are in the right ground. Typical habit: microscopic crystals. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify thalfenisite?+

Mohs hardness is 1-1.5. Luster: metallic. The streak is black. Common colors include bronze brown, greenish brown.

Where is thalfenisite found?+

Notable localities include Oktyabr mine, Talnakh area, Noril’sk region, western Siberia, Russia (type locality).

Is thalfenisite safe to handle?+

Thalfenisite contains nickel and thallium and calls for real care. Contains thallium, a highly toxic heavy metal. Handle with gloves, avoid inhalation of dust, and wash hands thoroughly after handling.

What rocks look like thalfenisite?+

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

What minerals are found with thalfenisite?+

Thalfenisite commonly co-occurs with Pentlandite, Galena, Chalcopyrite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does thalfenisite form in?+

Where it forms: In pentlandite-galena-chalcopyrite ores, localized at the contact of chalcopyrite and galena and included in pentlandite. Working float back to the host body is the standard way to chase a fresh occurrence.

What is thalfenisite used for?+

Thalfenisite is used in collector.

Find thalfenisite on the map

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