Why is my pyrite silver
Silver-coloured pyrite is usually marcasite, arsenopyrite or a surface film. A density ladder from 4.887 to 7.58 separates every candidate without ambiguity.
Pyrite is pale brass-yellow, so a silver-white specimen is usually something else. In rough order of frequency: marcasite, which is tin-white on a fresh surface; a tarnish or coating film on real pyrite; arsenopyrite; galena, which cleaves into cubes; or one of the cobalt-nickel arsenides. Galena gives itself away by perfect cubic cleavage at hardness 2.5, and density separates the rest, from marcasite at 4.887 to galena at 7.58.
In short
- Marcasite is the first thing to rule out. Same iron sulphide formula as pyrite, FeS2, different crystal structure, and the Handbook of Mineralogy records it as tin-white on a fresh surface, tarnishing to pale bronze. It is the commonest reason a “pyrite” is not yellow.
- Check whether the colour is on the surface or in the mineral. Pyrite tarnishes darker and iridescent with age, and coated “rainbow pyrite” is sold deliberately. A fresh break settles it in one move.
- Arsenopyrite and cobaltite are genuinely silver-white all through. They are denser — 6.07 and 6.33 against pyrite's 5.018 — and both are hardness 5.5 to 6, slightly softer than pyrite.
- Galena is the easy one and people still miss it. Perfect cubic cleavage in three directions, hardness 2.5 to 2.75, density 7.58. If the specimen cleaves into little steps rather than fracturing, it is galena.
- Weigh it if the eye cannot settle it. The candidates run 4.887, 5.018, 5.9, 6.07, 6.33, 6.5, 7.2, 7.58 — and the smallest gap in that ladder is 0.13, which a decent scale resolves.
| What I am seeing | Likely cause | The test that distinguishes it | Density |
|---|---|---|---|
| Pale tin-white on fresh fracture, bronze where weathered; often radiating or cockscomb | Marcasite | Habit and a fresh break; also check for crumbling, which marcasite does and pyrite rarely does | 4.887 |
| Silver or grey on the outside, brass-yellow underneath | Tarnish or an applied coating on pyrite | Break a corner, or look at a fresh chip under a lens | 5.018 |
| Silver-white throughout, sharp prisms or wedges, garlic smell when struck | Arsenopyrite | Density, and the odour of arsenic on a hammer blow | 6.07 |
| Silver-white throughout, striated pseudocubes, greyish-black streak | Cobaltite | Density; often with pink erythrite coatings nearby | 6.33 |
| Lead-grey, steps and cleavage planes rather than fracture, feels very heavy and soft | Galena | Cubic cleavage in three directions; hardness 2.5, marked by a copper coin | 7.58 |
| Tin-white with grey or iridescent tarnish, massive or as crusts | Skutterudite or safflorite | Density, and usually a Bou Azzer or Cobalt-district label | 6.5 to 7.2 |
Start with marcasite, because it is the commonest answer
Marcasite and pyrite have the same formula — FeS2 — and different structures. They occur together, and marcasite is systematically under-recognised in collections because most people expect an iron sulphide to be yellow.
The Handbook of Mineralogy gives marcasite's colour as tin-white on fresh surface, pale bronze-yellow, darkening on exposure, against pyrite's pale brass-yellow, tarnishes darker and iridescent. Fresh marcasite is genuinely silver. It is the colour of the mineral, not a film on it.
Three things point to marcasite before you weigh anything. Habit: cockscomb and spearhead groups, radiating fibrous masses, and flattened tabular crystals, where pyrite makes cubes, pyritohedra and octahedra. Setting: marcasite is the low-temperature, near-surface form and turns up in chalk, clay and coal measures. Condition: marcasite is far more prone to breaking down, and a specimen that is sweating, cracking or growing white crusts is more likely marcasite than pyrite — our note on crumbling pyrite covers that failure in full, and it is the single most destructive problem in a British collection.
Density does separate them, but the gap is the narrowest on this page: 4.887 against 5.018. That needs a careful measurement rather than a hand impression. Our note on measuring specific gravity at home shows what specimen size a 0.13 gap actually demands.
Then check whether the colour is even in the mineral
Before reaching for a scale, spend ten seconds establishing whether you are looking at a mineral or at a surface.
Pyrite tarnishes. The Handbook records it as darkening and going iridescent, and old specimens can be grey, brown, blue or a mixture. A tarnish is microns thick and a fresh chip underneath is brass-yellow.
Some of it is applied deliberately. Vapour-deposited metal coatings on pyrite, quartz and other species are a large part of the crystal-shop trade, sold under names like rainbow or flame aura. These give an even, unnaturally uniform iridescence over every surface including fractures, which natural tarnish does not. Our note on telling if a crystal is fake covers coatings in general.
The test is one fresh break, on an edge that does not show. Real brass-yellow underneath means pyrite with a film. Silver-white all the way through means you are looking at a different species, and the rest of this page applies.
A streak plate helps here too and costs almost nothing: pyrite's streak is greenish black to brownish black, and every silver-white candidate on this page streaks greyish or lead-grey. See our note on using a streak plate for doing it without marking a display face.
The density ladder, which settles everything else
Once the specimen is silver-white all through, the eye is close to useless. Colour ranges only from tin-white to steel-grey across the whole group, several of them tarnish, and pyrite, cobaltite, gersdorffite and skutterudite all form striated pseudocubes that look alike.
Density separates every one of them, and the steps are large. From the Handbook sheets: marcasite 4.887, pyrite 5.018, gersdorffite 5.9, arsenopyrite 6.07, cobaltite 6.33, skutterudite 6.5, safflorite 7.2, galena 7.58.
The smallest gap in that ladder is 0.13, between marcasite and pyrite. Every other step is 0.17 or more, and the gap between pyrite and arsenopyrite — the case that matters most often — is a full 1.05, which is enormous for this kind of test.
The practical requirement is modest. For a mineral around density 6, a scale reading to a hundredth of a gram needs roughly 7 g of clean single-species material to land within 0.05. Our specific gravity note derives that and gives the table.
The honest caveat: these species intergrow. Skutterudite, safflorite, cobaltite and arsenopyrite occur together at Bou Azzer and in the Cobalt district of Ontario, and a mixed crust returns an average that belongs to no species. A reading of 6.4 on a clean crystal means something; the same reading on a massive lump means very little.
Two shortcuts worth knowing, and one thing not to do
Arsenopyrite announces itself. Struck sharply with a steel hammer it gives off a garlic odour — arsenic, volatilised by the impact. It is a real and old diagnostic test and it is unmistakable once met. It is also the reason not to run it casually: you are volatilising arsenic. Do it outdoors, downwind, on a scrap fragment, or do not do it.
Erythrite is a free label. A pink or crimson bloom anywhere near a silver-white mineral points to cobalt, which means cobaltite, skutterudite or safflorite rather than an iron sulphide. Our erythrite note covers it; collectors have called it cobalt bloom for centuries because it works.
What not to do: do not grind, saw or ultrasonically clean a specimen you suspect is an arsenide. Arsenopyrite, cobaltite, gersdorffite, skutterudite and safflorite are all arsenic-bearing, and preparation is the step that generates the exposure rather than display. Our note on toxic minerals in a collection sets out the group rules, and we are not qualified to give health advice beyond them.
If the specimen came with a locality, use it. A silver-white mineral on a Bou Azzer label is an arsenide and almost certainly not marcasite; one from a Kent chalk pit is marcasite and almost certainly not an arsenide. Our locality notes cover the districts, and our wanted list sets out what we look for — for this group, a legible old label is worth more than the specimen's appearance, because appearance is precisely what fails here. The rest of our collecting notes cover the identification tests in the order worth running them.