Stillwaterite is a very rare palladium arsenide typically found as microscopic grains within platinum-group element deposits. Collectors prize it primarily as a rare ore mineral associated with ultramafic layered intrusions like the Stillwater Complex.

Hardness

4.5

Mohs

Luster

Metallic

Streak

Black

Transparency

Opaque

mineralrare

Stillwaterite

Hardness: 4.5
Luster: Metallic
Streak: Black
Transparency: Opaque

No image

Is this stillwaterite?

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 stillwaterite with a known reference. Stillwaterite sits at Mohs 4.5, 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: gray.
5
Look at form & habit
Crystal system: trigonal. Typical habit: microscopic crystals.

Often confused with

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

mineral

Vysotskite

Mohs 1.5

How to tell apart: Stillwaterite is noticeably harder (Mohs 4.5 vs. 1.5).

Often found alongside stillwaterite

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

All properties

Chemical formula: Pd₈As₃
Mohs hardness: 4.5
Density: 10.4 g/cm³
Colors: Gray
Streak: Black
Luster: Metallic
Transparency: Opaque
Crystal system: Trigonal
Crystal habit: Microscopic crystals
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: In layered ultramafic igneous bodies

Where rockhounds find stillwaterite

Classic worldwide localities

Banded and Upper zones of the Stillwater complex, Montana, USA (type locality)
Roby zone, Lac des Iles complex, Ontario, Canada
Siikakämä intrusion, about 50 km southeast of Rovaniemi, northern Finland
Lukkulaisvaara layered intrusion, Karelia, Russia

Field-hunting tip

Where it forms: In layered ultramafic igneous bodies. If you start seeing gold, palladoarsenide, sperrylite 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 stillwaterite?+

Mohs hardness is 4.5. Luster: metallic. The streak is black. Common colors include gray.

Where is stillwaterite found?+

Notable localities include Banded and Upper zones of the Stillwater complex, Montana, USA (type locality); Roby zone, Lac des Iles complex, Ontario, Canada; Siikakämä intrusion, about 50 km southeast of Rovaniemi, northern Finland; Lukkulaisvaara layered intrusion, Karelia, Russia.

Is stillwaterite safe to handle?+

Stillwaterite contains arsenic and calls for real care. Contains arsenic, which is toxic if inhaled or ingested. Wash hands thoroughly after handling and avoid creating dust or powder.

What rocks look like stillwaterite?+

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

What minerals are found with stillwaterite?+

Stillwaterite commonly co-occurs with Gold, Palladoarsenide, Sperrylite, Braggite, Hollingworthite, Chalcopyrite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does stillwaterite form in?+

Where it forms: In layered ultramafic igneous bodies. Working float back to the host body is the standard way to chase a fresh occurrence.

What is stillwaterite used for?+

Stillwaterite is used in collector.

Find stillwaterite on the map

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