Gudmundite is a relatively uncommon iron antimony sulfide that typically appears as metallic, steel-gray masses. It is most easily identified in laboratory settings due to its striking resemblance to arsenopyrite, though it is often found in antimony-rich ore deposits rather than simple arsenic veins.

Hardness

~5-6

Mohs

Luster

Metallic

Streak

Grayish black

Transparency

Opaque

mineraluncommon

Gudmundite

Hardness: ~5-6
Luster: Metallic
Streak: Grayish black
Transparency: Opaque

Is this gudmundite?

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 gudmundite with a known reference. Gudmundite sits at Mohs ~5-6, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: grayish 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: silvery white, steel gray.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: crystalline, massive.

Often confused with

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

mineral

Arsenopyrite

Mohs 5.5-6

How to tell apart: Streak differs, so Gudmundite leaves grayish black, Arsenopyrite leaves black.

Often found alongside gudmundite

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

All properties

Chemical formula: FeSbS
Mohs hardness: ~5-6
Density: 6.72 g/cm³
Colors: Silvery white Steel gray
Streak: Grayish black
Luster: Metallic
Transparency: Opaque
Crystal system: Monoclinic
Crystal habit: Crystalline, massive
Cleavage: None
Rarity: Uncommon
Uses: Collector, Scientific research
Host rock: A late-stage hydrothermal mineral formed in sulfide deposits

Where rockhounds find gudmundite

Classic worldwide localities

Gudmundstorp, 3 km north of Sala, Västmanland (type locality), Sweden
Boliden, Holmtjärn, Malånäs, Tunaberg, and in the Vena mines, near Askersund, Örebro, Sweden
Kalliolampi, near Nurma, Finland
Sulitjelma, Norway
Waldsassen, Bavaria, Germany
Salsigne mine, 15 km north of Carcassone, Aude, France

Field-hunting tip

Where it forms: A late-stage hydrothermal mineral formed in sulfide deposits. If you start seeing pyrite, pyrrhotite, chalcopyrite in float, you are in the right ground. Typical habit: crystalline, massive. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify gudmundite?+

Mohs hardness is ~5-6. Luster: metallic. The streak is grayish black. Common colors include silvery white, steel gray.

Where is gudmundite found?+

Notable localities include Gudmundstorp, 3 km north of Sala, Västmanland (type locality), Sweden; Boliden, Holmtjärn, Malånäs, Tunaberg, and in the Vena mines, near Askersund, Örebro, Sweden; Kalliolampi, near Nurma, Finland; Sulitjelma, Norway; Waldsassen, Bavaria, Germany.

Is gudmundite safe to handle?+

Gudmundite contains antimony. Contains antimony and sulfur; handle with care and avoid inhalation of dust when polishing or breaking.

What rocks look like gudmundite?+

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

What minerals are found with gudmundite?+

Gudmundite commonly co-occurs with Pyrite, Pyrrhotite, Chalcopyrite, Lead sulfantimonides, Antimony, Bismuth. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does gudmundite form in?+

Where it forms: A late-stage hydrothermal mineral formed in sulfide deposits. Working float back to the host body is the standard way to chase a fresh occurrence.

What is gudmundite used for?+

Gudmundite is used in collector, scientific research.

Find gudmundite on the map

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