Skutterudite Identification Guide

What Skutterudite Looks Like

Skutterudite is a cobalt-nickel arsenide, (Co,Ni)As3, and an important cobalt and nickel ore. It is a heavy, metallic mineral, tin-white to silver-gray, often tarnishing to gray or iridescent.

• Luster: metallic
• Color: tin-white to silver-gray, tarnishing gray, sometimes with a pinkish (cobalt bloom) or grayish coating
• Habit: cubic and octahedral crystals (isometric system), also massive and granular
• Surface clue: weathered specimens often show pink erythrite (cobalt bloom) or green annabergite (nickel bloom) coatings nearby

Step-by-Step Field-ID Checklist

1. Heft it. Skutterudite is distinctly heavy (high specific gravity) for its size.
2. Look for metallic, tin-white crystals. Cubes and octahedra with a bright silvery metallic luster are a strong clue.
3. Check the streak. It leaves a black to grayish-black streak.
4. Look for cobalt/nickel blooms. Pink (erythrite) or green (annabergite) crusts on or near the specimen strongly suggest an arsenide.
5. Arsenic smell test (careful). When struck, scratched, or heated, arsenides give off a garlicky arsenic odor — a classic confirmation; handle and wash hands afterward.

Key Diagnostic Tests

• Mohs hardness: about 5.5–6 — harder than galena, scratches with steel only with effort.
• Streak: black to grayish-black.
• Cleavage/fracture: indistinct cleavage; uneven to conchoidal fracture; brittle.
• Specific gravity: high, ~6.1–6.9 — heavier than pyrite or arsenopyrite.
• Smell: garlic (arsenic) odor when heated or struck.
• Magnetism: generally non-magnetic (distinguishes from some associated minerals).

Common Look-Alikes and How to Tell Them Apart

• Arsenopyrite: also silver-white and gives an arsenic smell, but arsenopyrite is monoclinic with wedge-shaped/prismatic crystals and is slightly harder (5.5–6) with a different habit; skutterudite is isometric (cubes/octahedra).
• Cobaltite: very similar silver-white arsenide ore; cobaltite is harder (5.5) and pseudo-cubic but is a sulfarsenide (CoAsS); precise separation often needs lab analysis, though associations and crystal form help.
• Galena: softer (2.5), with perfect cubic cleavage that breaks into cubes and a lead-gray streak; skutterudite is harder with no good cleavage and no sulfur smell.
• Pyrite/marcasite: brass-yellow versus tin-white, and pyrite gives a sulfur (not garlic) smell; skutterudite is denser.
• Native silver/platinum: these are malleable and bend, while skutterudite is brittle and shatters.

The defining combination is tin-white metallic + isometric cube/octahedron habit + very heavy + brittle + garlic arsenic odor + cobalt/nickel blooms nearby.

Where Skutterudite Is Found

Skutterudite occurs in moderate- to high-temperature hydrothermal veins with other cobalt-nickel-silver arsenides. It is named for Skutterud, Norway. Famous localities include the Cobalt district of Ontario (Canada), Bou Azzer (Morocco, a major cobalt source), the Erzgebirge of Germany and the Czech Republic, and various silver-cobalt vein districts worldwide.