Pyrite, or fool’s gold, has duped prospectors for millennia, and now it’s providing a tricky challenge for museums.

Pyrite is a very common mineral found worldwide and in a very large variety of rocks. The Canadian Museum of Nature collection holds about 800 pyrite specimens. Each specimen has the identical chemical formula — iron disulfide, FeS2. But slight differences in the conditions in which a pyrite specimen formed gives each a unique shape, with cubes being the most common.

A pyrite specimen in a specimen box on a cabinet shelf.
An excellent pyrite specimen with a bright gold colour and cubic shapes. Catalogue number: CMNMC 43372. Image: Christian Capehart © Canadian Museum of Nature.

With its shiny yellow colour and metallic shine, pyrite closely resembles gold. Even more confusingly, it also commonly occurs with gold deposits. Many prospectors have thought they struck it rich when they actually just found a lode of pyrite, hence its common name.

Now, museum staff are dealing with another of the mineral’s challenging characteristics: pyrite can rust.

When exposed to humid air, pyrite reacts with oxygen and water to create iron sulfide (the rust), corrosive sulfuric acid and harmful sulfur dioxide gas. This chemical reaction, called pyrite disease, causes specimens to crack and crumble. Left unchecked, pyrite disease eventually destroys a specimen.

Moreover, the corrosive acid and gas can destroy the storage containers holding the pyrite and even damage surrounding minerals. This is why it’s called pyrite disease, because it acts like a contagious infection that can spread. The acid and gas are also a health hazard for museum staff.

A pyrite specimen that is cracked and discoloured.
A specimen of pyrite damaged by pyrite disease, resulting in cracks and discolouration. Image: Christian Capehart © Canadian Museum of Nature.

Since the museum holds pyrite specimens that are of great scientific interest and stunning beauty, it is of the utmost importance that we take proactive steps to preserve them.

Pyrite oxidation is triggered by humid air, so the best ways we have found to prevent pyrite disease are to lower the humidity of the collection room and keep the specimens in dry, impermeable containers.

With these steps, the museum’s pyrite specimens have the potential to keep fooling the unwary for millennia.