Modderite is a rare cobalt arsenide mineral primarily identified from South African cobalt-nickel-arsenic deposits. It typically presents as metallic, massive grains and is highly sought after by systematic mineral collectors due to its scarcity.

Hardness

Mohs

Luster

Metallic

Streak

Gray

Transparency

Opaque

mineralrare

Modderite

Hardness: ~4
Luster: Metallic
Streak: Gray
Transparency: Opaque

Is this modderite?

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 modderite with a known reference. Modderite sits at Mohs ~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: gray.
3
Read the luster
Hold the specimen under a strong light. Luster: metallic.
4
Match the color range
Compare against the expected color range: bluish white, steel gray.
5
Look at form & habit
Crystal system: orthorhombic. Typical habit: granular, microscopic crystals.

Often confused with

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

mineral

Skutterudite

Mohs 5.5-6

How to tell apart: Skutterudite is the harder of the two (Mohs 5.5-6 vs. ~4); streak differs, so Modderite leaves gray, Skutterudite leaves black.

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Often found alongside modderite

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

All properties

Chemical formula: CoAs
Mohs hardness: ~4
Density: 8.28 g/cm³
Colors: Bluish white Steel gray
Streak: Gray
Luster: Metallic
Transparency: Opaque
Crystal system: Orthorhombic
Crystal habit: Granular, microscopic crystals
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: In heavy mineral concentrates (Witwatersrand, South Africa)

Where rockhounds find modderite

Classic worldwide localities

Dashkesan deposit, Middle Caucasus Mountains, Azerbaijan

Field-hunting tip

Where it forms: In heavy mineral concentrates (Witwatersrand, South Africa). If you start seeing nickeline, alloclasite, safflorite in float, you are in the right ground. Typical habit: granular, microscopic crystals. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify modderite?+

Mohs hardness is ~4. Luster: metallic. The streak is gray. Common colors include bluish white, steel gray.

Where is modderite found?+

Notable localities include Dashkesan deposit, Middle Caucasus Mountains, Azerbaijan.

Is modderite safe to handle?+

Modderite contains arsenic and cobalt and calls for real care. Contains arsenic, which is toxic; wash hands thoroughly after handling and avoid dust inhalation.

What rocks look like modderite?+

Modderite is most often confused with Skutterudite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with modderite?+

Modderite commonly co-occurs with Nickeline, Alloclasite, Safflorite, Glaucodot, Cobaltite, Pentlandite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does modderite form in?+

Where it forms: In heavy mineral concentrates (Witwatersrand, South Africa). Working float back to the host body is the standard way to chase a fresh occurrence.

What is modderite used for?+

Modderite is used in collector.

Find modderite on the map

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