Pyrite Deteriorates Through a Process Call Pyrite Oxidation, Known Informally As Pyrite

Pyrite decay

Pyrite deteriorates through a process call pyrite oxidation, known informally as “pyrite decay”. This is caused when pyrite is exposed to high relative humidity (RH). At around 60% RH the pyrite will react with oxygen in the air to form iron sulphate and sulphuric acid. Where the sulphate forms it cracks the specimen open.

To minimise pyrite decay, minimise expose of pyrite specimens to water and oxygen. Dry wet specimens slowly and thoroughly and avoid washing specimens. If you need to remove the mud from a specimen then picking it away dry is best if possible. If you have an electric engraver then you could use this to remove the mud. Its best to avoid extremes of temperature and humidity. In a museumk, these can often be controlled but in a home, this might mean storing specimens as far away as possible from the kitchen, bathroom, laundry area, radiators or direct sunlight.

Storing specimens in a tupperware box with a good seal with dry silica gel helps to prevent or slow pyrite decay by removing water from the air surrounding the specimens. Keeping the specimen dry should also reduce any salt growth on the surface as the salt cannot move through the specimen. Salts move through objects in solution and form when the solution dries out leaving the salt behind. The salts lever and crack the specimen open when they form inside the specimen or on its surface.

In museums pyrite specimens may be stored in an oxygen free atmosphere, but this is outside the resources of most collectors! Lacquering the specimen with a clear varnish has been a treatment used by museums in the past and is recommended by some people. Recent research shows though that in the medium to long-term this usually speeds up pyrite decay as the lacquer doesn’t form a complete oxygen or moisture barrier and can cause moisture to build up on the surface of the specimen causing oxidation.

Pyrite specimens that have oxidised give off a sulfurous smell, and become fragile and eventually crumble. The pyrite first loses its gold shine and then discolours to white or yellow. Pyrite decay can never be completely stopped. Some specimens are stable over long periods of time, whereas others can decay very quickly. It is advisable to keep specimens that are decaying physically separate from other specimens and their labels, as the sulphur gas that is released can damage other specimens and paper. Photography at least is a means of preserving the appearance of your pyrite specimens forever!