Ferrostalderite is a rare member of the tetrahedrite group characterized by its iron-rich composition. It is typically found in rare, complex hydrothermal deposits such as those in the Binn Valley, usually appearing as small, dark tetrahedral crystals.

Hardness

3.5-4

Mohs

Luster

Metallic

Streak

Black

Transparency

Opaque

mineralrareTetrahedrite Group family

Ferrostalderite

Hardness: 3.5-4
Luster: Metallic
Streak: Black
Transparency: Opaque

No image

Is this ferrostalderite?

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 ferrostalderite with a known reference. Ferrostalderite sits at Mohs 3.5-4, 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: black.
5
Look at form & habit
Crystal system: tetragonal.

Often confused with

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

How to tell apart: Streak differs, so Ferrostalderite leaves black, Tennantite leaves reddish gray.

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Often found alongside ferrostalderite

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

All properties

Chemical formula: CuFe₂TlAs₂S₆
Mohs hardness: 3.5-4
Density: 4.528 g/cm³
Colors: Black
Streak: Black
Luster: Metallic
Transparency: Opaque
Crystal system: Tetragonal
Rarity: Rare
Uses: Collector
Host rock: Formed as massive to interstitial sulfosalt accumulations in dolostone by late stage Tl-As-Cu-Fe-rich hydrothermal fluids during upper greenschist to lower amphibolite metamorphism

Where rockhounds find ferrostalderite

Classic worldwide localities

Lengenbach quarry, Binn Valley, Wallis, Switzerland

Field-hunting tip

Where it forms: Formed as massive to interstitial sulfosalt accumulations in dolostone by late stage Tl-As-Cu-Fe-rich hydrothermal fluids during upper greenschist to lower amphibolite metamorphism. If you start seeing dolomite, realgar, baumhauerite in float, you are in the right ground.

Common questions

How do you identify ferrostalderite?+

Mohs hardness is 3.5-4. Luster: metallic. The streak is black. Common colors include black.

Where is ferrostalderite found?+

Notable localities include Lengenbach quarry, Binn Valley, Wallis, Switzerland.

Is ferrostalderite safe to handle?+

Ferrostalderite contains arsenic, copper and thallium and calls for real care. Contains copper, iron, antimony, and sulfur. Wash hands thoroughly after handling and avoid inhaling dust while trimming or polishing.

What rocks look like ferrostalderite?+

Ferrostalderite is most often confused with Tetrahedrite-(Fe), Tennantite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with ferrostalderite?+

Ferrostalderite commonly co-occurs with Dolomite, Realgar, Baumhauerite, Pyrite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does ferrostalderite form in?+

Where it forms: Formed as massive to interstitial sulfosalt accumulations in dolostone by late stage Tl-As-Cu-Fe-rich hydrothermal fluids during upper greenschist to lower amphibolite metamorphism. Working float back to the host body is the standard way to chase a fresh occurrence.

What is ferrostalderite used for?+

Ferrostalderite is used in collector.

Find ferrostalderite on the map

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