Pyrrhotite
Pyrrhotite is a non-stoichiometric iron sulphide, hardness 3.5 to 4.5, magnetic and unstable. Crystals to 40 cm, Dalnegorsk material, and why storage matters.
Pyrrhotite is iron sulphide with iron missing from the structure, Fe1-xS with x up to 0.17, hardness 3.5 to 4.5. That deficiency makes it magnetic and makes it unstable: it oxidises like pyrite but faster. Tabular crystals reach 40 cm; Dalnegorsk produces very large ones.
In short
- It is the mineral defined by what is missing. Vacancies where iron atoms should be give pyrrhotite both its magnetism and its reactivity, and the two vary together.
- Magnetism is the field test and the most useful one: intensity varies inversely with iron content, so the most iron-deficient material is the most magnetic. Troilite, the stoichiometric FeS found in meteorites, is not.
- It decays. Treat every pyrrhotite as an unstable sulphide from the day it arrives: sealed, dry, with a desiccant, checked annually.
- Bronze-yellow to pinchbeck-brown, tarnishing quickly and occasionally to iridescence, with a dark greyish-black streak and a metallic lustre.
- There are numerous polytypes — 4M, 6M, 5H, 7H and 11H are known — plus a high-temperature hexagonal polymorph. None of this is separable in the hand.
| Species | Formula | Hardness | Magnetic? | Stability in a collection |
|---|---|---|---|---|
| Pyrrhotite | Fe1-xS | 3.5 to 4.5 | Yes, variably | Poor. Oxidises readily |
| Pyrite | FeS2 | 6 to 6.5 | No | Variable. Some material stable for centuries, some not |
| Marcasite | FeS2, orthorhombic | 6 to 6.5 | No | Worst of the group. Assume it is on a clock |
| Troilite | FeS, stoichiometric | 3.5 to 4.5 | No | Meteoritic; not a normal collection species |
| Chalcopyrite | CuFeS2 | 3.5 to 4 | No | Good. Tarnish is cosmetic |
Why pyrrhotite is magnetic and pyrite is not
Most sulphides have a fixed formula. Pyrrhotite does not: it is written Fe1−xS with x running from 0 to 0.17, meaning up to about one iron site in six is empty. Those vacancies are ordered, and the ordering is what produces the magnetism — the iron atoms' magnetic moments no longer cancel out. The Handbook of Mineralogy records that the mineral is magnetic, with intensity varying inversely with iron content, which is the counterintuitive part worth remembering: the more iron is missing, the more strongly magnetic it is.
Practically, a magnet is the single best field test for pyrrhotite, and it distinguishes it instantly from pyrite and marcasite, which are not magnetic at all. Be aware that response varies from strong to barely detectable across specimens of the same species, so a weak response is not a negative result. Troilite — end-member FeS with no vacancies, essentially a meteorite mineral — is not magnetic, which is the same principle read from the other end.
Hardness reinforces the separation. Pyrrhotite is 3.5 to 4.5 and pyrite is 6 to 6.5, so a knife blade marks one and not the other. Colour helps too: pyrrhotite is bronze to pinchbeck-brown where pyrite is a paler brassy yellow, and pyrrhotite tarnishes noticeably faster.
The instability problem, stated plainly
Pyrrhotite is the most reactive of the common iron sulphides a collector will own, and it is reactive for exactly the reason it is magnetic. The vacant iron sites are chemically active sites, and the same oxidation that attacks pyrite proceeds more readily here. The products are the same — hydrated iron sulphates and sulphuric acid — and the damage pattern is the same: powder first, splitting later, acid attack on labels and neighbouring carbonates throughout.
So the storage regime is not optional, and it is the one set out in full on why is my pyrite crumbling. Sealed individual box, conditioned silica gel, below 45% relative humidity, and an annual inspection for efflorescence and for that sharp sulphurous smell. Do not wash a pyrrhotite. Do not seal it damp. Do not lacquer it.
The corollary for buying is a question worth asking directly: how has this been stored, and for how long? Fine large pyrrhotite crystals from the classic localities are old material, and old material that has spent forty years in a damp garage is a different proposition from the same specimen out of a controlled cabinet, however similar the photographs look.
Crystals, and where the large ones come from
Most pyrrhotite is massive and granular, an ore mineral rather than a specimen mineral, occurring as magmatic segregations in mafic igneous rocks and as a component of high-temperature hydrothermal and replacement veins. The collectable material is the crystallised minority: tabular or platy crystals on {0001} reaching 40 cm, steep or short pyramidal forms, and the rosettes of nearly parallel plates that are the species' most characteristic look.
The Handbook lists well-crystallised material from Herja in the Baia Mare district of Romania; from the Stari Trg mine at Trepča, Serbia; from Val Passiria and Bottino in Italy; from St Andreasberg in the Harz; from Falun in Sweden; very large crystals at Dal'negorsk in Primorskiy Kray; large crystals from the Morro Velho gold mine at Nova Lima, Minas Gerais; from the Potosí and San Antonio mines at Santa Eulalia, Chihuahua; and from the Bluebell mine at Riondel, British Columbia.
Dalnegorsk is the one to know, because it is a district we cover in its own right and because its pyrrhotite arrives with the same sulphide suite — galena, sphalerite, arsenopyrite and the rest — that makes the Dalnegorsk material recognisable. Trepča is the other name that carries weight on a label, and it is also a classic vivianite locality, so the two turn up in the same collections.
What to look for, and what to avoid
Look for: sharp plate edges and clean {0001} faces; a rosette structure with the plates genuinely intergrown rather than a single tabular crystal; an association that fixes the locality; and a base that is not itself a mass of decaying sulphide.
Be careful about: any powder in the box, any acid staining on the label, any sulphurous smell on opening, and any specimen described as having been cleaned recently — washing a pyrrhotite adds the water that the reaction needs.
Avoid entirely: a decaying pyrrhotite bought cheaply on the theory that it can be stabilised. It cannot, in any sense that restores it. The conservation treatments used in museums arrest a reaction on material of scientific importance; they do not rebuild a crystal, and they are not a reasonable project for a shelf specimen.
The one genuine bargain in this species is a well-crystallised piece that has demonstrably been stored properly — sealed, dry, documented — because most collectors do not store sulphides well and the population of sound material shrinks every year. That is a sourcing problem rather than an identification one; if it is what you are looking for, the wanted list is how to say so. We hold no stock, and on this species the useful thing is knowing what to ask about storage before anything moves.