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

3.5

Mohs

Luster

Metallic

Streak

Black

Transparency

Opaque

mineralrare

Modderite

Hardness: 3.5
Luster: Metallic
Streak: Black
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 3.5, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Modderite leaves a black streak.
3
Read the luster
Hold the specimen under a strong light. Modderite typically shows a metallic luster.
4
Match the color range
Compare against the expected color range: white, gray.
5
Look at form & habit
Crystal system: orthorhombic. Typical habit: massive.

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. 3.5).

Compare →

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: (Co,Fe)₂As
Mohs hardness: 3.5
Density: 6.8 g/cm³
Colors: White Gray
Streak: Black
Luster: Metallic
Transparency: Opaque
Crystal system: Orthorhombic
Crystal habit: Massive
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Hydrothermal Veins
Typical price: $50-300 per specimen

Where rockhounds find modderite

Classic worldwide localities

Witwatersrand Basin, South Africa

Field-hunting tip

Look in hydrothermal veins country, the host setting where modderite typically forms. If you start seeing cobaltite, arsenopyrite, niccolite in float, you are in the right ground. Field specimens usually show a massive habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify modderite?+

Mohs hardness is 3.5. It typically shows a metallic luster. The streak is black. Common colors include white, gray.

Where is modderite found?+

Notable localities include Witwatersrand Basin, South Africa.

How much is modderite worth?+

Typical asking prices fall in the range of $50-300 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

Is modderite safe to handle?+

It contains toxic constituents. Contains arsenic, which is toxic; wash hands thoroughly after handling and avoid dust inhalation. Handle with care, avoid grinding or breathing dust, and store separately.

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 Cobaltite, Arsenopyrite, Niccolite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does modderite form in?+

Modderite typically forms in hydrothermal veins. 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.