Sperrylite Identification Guide
Identify sperrylite, the rare platinum arsenide, by its tin-white metallic cubes, extreme density, high hardness, and sulfide host ores.
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What Sperrylite Looks Like
Sperrylite (PtAs₂) is the most important platinum mineral and one of the few platinum-group minerals that forms visible crystals. It is tin-white to silvery, with a bright metallic luster, and typically forms small but sharp cubic and cubo-octahedral crystals. Crystals are usually only millimeters across, set in or on dark sulfide ore.
- Color: tin-white, sometimes with a slight steel tint or dark tarnish film
- Luster: metallic and bright
- Transparency: opaque
- Habit: isometric — cubes, octahedra, pyritohedra and combinations; also grains
Step-by-Step Field-ID Checklist
- Look for tiny bright cubes with sharp faces on a dark sulfide matrix.
- Judge the heft. Even a small specimen feels surprisingly heavy because of platinum.
- Test the streak on unglazed porcelain — it should be black to brownish-black, not metallic-silver smear.
- Check hardness against pyrite and quartz; sperrylite is harder than both.
- Confirm the geological setting — it is found in platinum-bearing nickel-copper sulfide deposits.
Key Diagnostic Tests
- Mohs hardness: 6–7 — distinctly hard for a metallic mineral; it scratches glass easily and resists a steel knife.
- Streak: black to brownish-black.
- Density: very high, ~10.6 g/cm³ — the single most diagnostic property; it feels far heavier than pyrite (~5) or galena (~7.5).
- Cleavage: indistinct; fracture conchoidal to uneven and brittle.
- Not magnetic itself (though associated pyrrhotite/magnetite may be).
- No acid reaction.
Common Look-Alikes and How to Tell Them Apart
- Pyrite ("fool's gold"): pyrite is brass-yellow, not tin-white, and is much lighter (density ~5). Sperrylite's silver color and extreme weight separate them.
- Arsenopyrite: silvery-white like sperrylite but lower density (~6) and gives off a garlic (arsenic) odor when struck or heated; sperrylite does not produce the strong garlic smell from a light scratch.
- Cobaltite / skutterudite: silvery metallic arsenides but much lighter than sperrylite; chemistry and density distinguish them.
- Platinum nuggets / native platinum: malleable and hackly, not brittle cubic crystals; native platinum has no crystal faces and is even denser but dents rather than chips.
- Galena: lead-gray with perfect cubic cleavage (breaks into cubes) and lower hardness (2.5); sperrylite is much harder and lacks cleavage.
Where It Is Found
Sperrylite occurs in magmatic nickel-copper-platinum sulfide deposits. Classic localities include the Sudbury Basin, Ontario (its type-related occurrence), the Bushveld Complex, South Africa, and the Norilsk deposits, Russia. Hunt for it in dark pyrrhotite-chalcopyrite-pentlandite ore.
Frequently asked questions
How can you tell if it's real sperrylite?
Real sperrylite is tin-white with a metallic luster, forms small sharp cubes, is hard (6–7, scratches glass), gives a black streak, and is extremely dense (~10.6) — it feels far heavier than pyrite of the same size. Confirm with its platinum-sulfide host ore.
What is sperrylite worth?
As the chief ore of platinum, sperrylite is economically valuable, and crystallized specimens are prized by collectors because well-formed platinum-mineral crystals are rare. Value depends on crystal size, sharpness, and matrix.
Sperrylite vs pyrite — how do I tell them apart?
Pyrite is brass-yellow and light (density ~5); sperrylite is silvery tin-white and about twice as dense (~10.6). The heft and silver color are the giveaways.
Is sperrylite magnetic?
No, sperrylite is not magnetic, though it commonly occurs alongside magnetic minerals like pyrrhotite and magnetite, so test the actual silvery crystals.
What does sperrylite look like?
Tiny bright tin-white metallic cubes or octahedra perched on or in dark nickel-copper sulfide ore.