Why is my pyrite crumbling?
Pyrite decay is a chemical reaction, not a disease. Above 60% relative humidity a pyrite specimen oxidises to sulphates and acid. The thresholds that stop it.
Your pyrite is not diseased, it is oxidising. Above 60% relative humidity the sulphide reacts with water and oxygen to give hydrated iron sulphates and sulphuric acid, and the new minerals take up more room than the old, so the crystal splits itself apart. Hardness 6 does not help. Below 45% RH it effectively stalls.
In short
- The single number that matters is relative humidity. The Natural Sciences Collections Association puts the reaction above 60% RH and recommends storing pyrite-bearing specimens at 45% RH or lower.
- It is not contagious in any biological sense, but the sulphuric acid and the sulphate dust it throws off will attack labels, card trays, drawer liners and any carbonate specimen sharing the box.
- It is not reversible. Nothing puts the sulphur back. Everything below is about arresting the reaction and saving what is left of the crystal.
- Marcasite and pyrrhotite behave worse than pyrite, and a pyritic matrix under a good crystal of something else is the version of this problem that catches collectors out.
- A specimen that is merely tarnished — iridescent, brassy, still solid — is not decaying. Decay produces powder, a sulphurous smell, and hairline splitting along the crystal.
| Relative humidity | What is happening | What to do |
|---|---|---|
| Below 35% | Stable for sulphides, but soluble species such as chalcanthite lose water and crumble to powder | Fine for a pyrite box; wrong for a whole mixed collection |
| 35–45% | The target band for sulphides. Oxidation is slow enough to be ignored over a collecting lifetime | Silica gel in a sealed box, recharged when the indicator turns |
| 45–60% | The general store-room band recommended for mixed geological collections | Acceptable for most material; pyrite should be boxed separately and drier |
| Above 60% | Pyrite reacts with atmospheric water to hydrated iron sulphate and sulphur trioxide, which forms a weak sulphuric acid | Move the specimens today. This is the band where damage accumulates |
| Above 65% | Mould growth in addition to sulphide decay | Unsuitable for any geological storage |
What is actually happening when pyrite crumbles
Iron disulphide is stable underground because there is no free oxygen down there. Bring it into a house and it starts to convert. Water vapour and oxygen turn the sulphide into a family of hydrated iron sulphates — melanterite, rozenite, szomolnokite, depending on how much water is available — and release sulphur oxides that dissolve in the remaining moisture as sulphuric acid.
The mechanical damage follows from the chemistry. A sulphate with eight molecules of water in it occupies considerably more space than the sulphide it replaced, and it forms inside the specimen, along grain boundaries and micro-fractures. The crystal is levered apart from within. That is why a pyrite cube can look perfect on Monday and show a split face on Friday: the reaction had been running for months before it had displaced enough volume to open a crack.
The by-products then do secondary damage. The acid eats paper, so the label goes. The efflorescent sulphates creep out and settle on whatever else is in the drawer, and any carbonate they land on — calcite, cerussite, smithsonite, malachite — will etch.
The humidity numbers, and the conflict nobody mentions
The published guidance from the Natural Sciences Collections Association is specific: above 60% RH pyrite reacts with atmospheric water, pyrite-bearing specimens should not be stored above 60% RH, and preferably should be at 45% RH or lower. A general geological store room is recommended to sit between 45% and 60% RH, at a stable 16–18°C.
Here is the part that guidance for institutions states and guidance for collectors usually skips: those two numbers are in tension with the rest of a collection. Below 35% RH, hydrated species dehydrate — the same source notes that blue copper sulphate crystals lose water and crumble to a pale blue powder below 35% RH. Sub-fossil bone should not go below 45%. So there is no single humidity that is right for everything on the shelf.
The practical resolution is not a whole-room target. It is microclimates: keep the room in the 45–60% band, then put the sulphides in individually sealed boxes with silica gel and let those run drier than the room. A sealed polystyrene box with a pierced sachet of conditioned silica gel is a better piece of conservation equipment than a dehumidifier, because it is the only approach that lets one collection hold both pyrite and chalcanthite.
How to tell decay from ordinary tarnish
Most pyrite that a collector worries about is simply tarnished. Pyrite oxidises superficially to a brassy, brown or iridescent film and then stops, and a great deal of perfectly stable Peruvian and Spanish material looks like that from the day it was mined. Tarnish is a surface colour change on a solid crystal.
Decay has four signs, and you want two of them before you act:
- Powder. A fine white, yellow, grey or green efflorescence, usually appearing first where the crystal meets the matrix. Run a fingertip along the base of the specimen in its box — grit that was not there last year is the earliest reliable sign.
- Smell. Open a long-closed box of decaying pyrite and there is a distinct sharp sulphurous note. A healthy specimen smells of nothing.
- Splitting. Hairline cracks running through the crystal rather than along an existing fracture, often with powder in them.
- Damage to the surroundings. A label going brown and brittle at the edge that touches the specimen, or a card tray softening underneath it, is the acid rather than age.
If it is only colour, leave it alone. Attempting to clean tarnish off pyrite mechanically removes the passivating film and can start the very problem you were worried about.
What to do first, and what not to do
In the first hour, and none of this costs anything: get the specimen out of the drawer it shares with anything else. Get it out of the damp room. Brush the loose powder off wet — damp-wipe the box, do not blow or dry-brush it, because iron sulphate dust is not something to breathe and the same rule covers every other reason not to make dust in a collection, which we set out on the page about toxic minerals in a collection. Then seal it dry: a lidded box, silica gel, a note of today's date on the outside.
Over the next week: buy a hygrometer before you buy anything else. It costs less than a poor specimen and it converts guesswork into a number. Check every sulphide in the collection, not just the one that failed — decay tends to arrive as a batch, because a batch of specimens shared a bad shelf.
What not to do. Do not wash it under the tap: adding liquid water to a running oxidation reaction accelerates it, and the sulphates are soluble, so you dissolve them into the fractures and they recrystallise deeper in. Do not seal a wet specimen in an airtight box. Do not varnish or lacquer it — the coating traps moisture against the crystal, obscures the very surface you need to inspect, and is regarded in the trade as an undisclosed alteration. Ammonia-based neutralising treatments and ethanolamine thioglycolate exist and are used in museum conservation, but they are reserved for material of real scientific importance and they change the surface; they are not a first response for a shelf specimen.
Which other species do the same thing
Pyrite gets the name, but it is not the worst offender.
- Marcasite — the orthorhombic dimorph of the same chemistry — decays faster and more reliably than pyrite. Much old museum marcasite has already gone. Assume any marcasite you buy is on a clock.
- Pyrrhotite, Fe1−xS, is iron-deficient by definition and correspondingly reactive; see our pyrrhotite notes for what that means when you are buying one.
- Pyritic matrix is the trap. A calcite, a fluorite or a sphalerite sitting on a shale or black-slate matrix full of framboidal pyrite will be destroyed by its own base plate while the crystal itself is untouched. Check what a specimen is standing on as carefully as what it is.
- Chalcopyrite and bornite tarnish spectacularly but are far more stable; the iridescence on a bornite is not a warning sign.
Fossil material in pyrite — ammonites from the Lower Jurassic clays especially — is the classic case, which is why the best published guidance on this problem comes out of palaeontological conservation rather than mineralogy.
When to stop conserving and start replacing
There is a point at which the honest answer is that the specimen has gone. A pyrite that has lost its faces, that leaves powder every time it is moved, and whose label is already acid-damaged is not going to come back, and keeping it in the same cabinet as sound material is an active risk to the sound material. Photograph it, keep the label separately in an inert sleeve because the label is the part with the provenance, and let the specimen go.
The economics of doing this properly are worth stating plainly rather than being talked around. A hygrometer, a box of conditioned silica gel and a set of sealed polystyrene boxes will cost you an evening and less than one decent miniature, and that is genuinely the whole of it for a normal collection — you do not need us for any of it. What that does not solve is replacement: sound, dry, well-provenanced sulphide material from the classic localities is finite, and it moves between collections rather than appearing on shelves. If a decayed piece has left a hole in a suite, that is the point at which telling us what you are looking for on the wanted list does something a dehumidifier cannot.