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.
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.
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.
Canadian Museum of Nature Blog 
Unfortunately, in many pyrite specimens weathering already started – which might not be obvious at the first sight. The oxidation starts from tiny cracks inside, where moisture makes oxygen transfer much faster and first sulfates appear. Sulfates are brutal oxidizing agent and speed up the decomposition by several orders. This runs inside the specimen and when you start seeing “rotten” surface, it might be already too late as the decomposition is already running at full speed and internal structure of specimen might be severely damaged.
Keeping specimens dry at constant temperature helps. But it can not completely stop the decomposition if sulfates are already formed inside the specimens – which is very likely for most specimens. You need to use chelates and special procedures to properly conserve pyrites (and marcasites) but this is quite time consuming and expensive. You need to completely remove sulfates from specimen, reduce Fe3+ and then properly conserve the surface to prevent re-formation of sulfates and Fe3+ oxides.
Pyrite seems to present a convergence of collections conservation, mineralogy, and palaeontology. See this article from The Economist https://www.economist.com/science-and-technology/2019/01/26/shifts-in-earths-orbit-increase-the-chances-of-spectacular-fossils
Can you put like a w-D type on the areas that are infected? Or an anti crossion oil ?
Hi Michael, thanks for your question! Our mineralogists say that there are different treatments that can be applied, however they would not recommend WD or anti-corrosion oil.
Hi – I know this is a year later but I’m looking for answers! Would it be safe to store pyrite in a clear oil? I’d prefer not to keep replacing dessicants.
Hello Nate
Here’s what our conservators had to say:
“We do not recommend storing pyrite in oil. I do not believe it will damage the specimen, but it is not a method we use since it makes the specimen less accessible and not suitable for display.
We do recommend storing pyrite specimens in dry conditions. This is especially the case for specimens which have active Pyrite degradation. Humidity accelerates the chemical reaction which damages the pyrite. Dry conditions will help to slow down the process. Specimens can be stored in a well-sealed container, such as food storage container, with a desiccant to create a dry microenvironment. Many desiccants can be reactivated in an oven at low temperature. ”
All the best
R&C Blog Coordinator
Hey, I own a jaw dropping 4 inch pyrite sphere as well as a smaller egg and sphere and I really love the pieces. I can see white chalky residue on all 3 pieces in the internal crevice’s as well as some tarnished appearance. Not sure how to proceed, do you have any advice?
My plan was to put them in a plexiglass box or something with some packets that absorb moisture and oxygen. Is there a way to purge most of the white residue safely is hopes of preserving them longer? I also read bacteria can accelerate the deterioration can you sterilize it with UV light or something?
Thanks any help appreciated.