Tuesday, 22 December 2020


We sometimes find fossils preserved by pyrite. They are prized as much for their pleasing gold colouring as they are for their scientific value as windows into the past. 

Sometimes folk add a coating of brass to increase the aesthetic appeal — a practice is frowned upon in paleontological communities.

Pyrite, sometimes called Fool's Gold, is a brass-yellow mineral with a bright metallic lustre. I popped a photo of some pyrite below so you can see the characteristic shape of its cubic crystal system.

Fool's Gold has a chemical composition of iron sulfide (FeS2) and is the most common sulfide mineral. It forms at high and low temperatures usually in small quantities, in igneous, metamorphic, and sedimentary rocks. If these sulfide minerals are close at hand when a fossil is forming, they can infuse specimens, replacing their mineral content to beautiful effect.

When we find a fossil preserved with pyrite, it tells us a lot about the conditions on the seabed where the organism died. Pyrite forms when there is a lot of organic carbon and not much oxygen in the vicinity. 

The reason for this is that bacteria in sediment usually respire aerobically (using oxygen), however, when there is no oxygen, they respire without oxygen (anaerobic) typically using sulphate. 

Sulphate is a polyatomic anion with the empirical formula SO2−4. It is generally highly soluble in water. Sulfate-reducing bacteria, some anaerobic microorganisms, such as those living in sediment or near deep-sea thermal vents, use the reduction of sulfates coupled with the oxidation of organic compounds or hydrogen as an energy source for chemosynthesis.

The sulfide mineral Pyrite, FeS2
High quantities of organic carbon in the sediment form a barrier to oxygen in the water. This also works to encourage anaerobic respiration. Anaerobic respiration using sulphate releases hydrogen sulphide, which is one of the major components in pyrite. 

So, when we find a fossil preserved in pyrite, we know that it died and was buried in sediment with low quantities of oxygen and high quantities of organic carbon. 

If you have pyrite specimens and want to stop them from decaying, you can give them a 'quick' soak in water (hour max) then wash them off, dry thoroughly in a warm oven. 

Cool, then soak in pure acetone for a couple of days. Then soak in paraloid, a thermoplastic resin surface coating or acetone for a couple of days. Keep in a sealed container with a desiccant pack afterwards to keep them dry — or leave them out on display to enjoy knowing that the decay will come in time. We do this with cut flowers so why not fossils sometimes.

I have a friend who gives her pyrite fossils on display a quick thumb wipe with vasoline or petroleum jelly. I'm not sure if the hydrocarbons there will play nice over time but it will act as a protective barrier.