The Chemistry of Fossilisation

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If you’ve ever gone combing beaches for ammonite fossils, you might have wondered about the processes which produced them. They can come in a number of shapes and forms, and their appearance can be influenced by the manner in which they were formed. This graphic takes a brief look at some of these processes!

Ammonites were around in our seas from 400 million years ago to 66 million years ago. You might think of them as quite small on the basis of the size of the fossils of them that are commonly found, but in fact, some of the largest measured 1.4 metres (4.5 feet) in diameter! Like octopus, squid, and cuttlefish, ammonites were cephalopods, though only their shells are preserved in the fossil record.

In order to be fossilised, a dead ammonite would need to settle to the seabed, where it would be buried by sediment. Various chemical processes are then involved in the formation of a fossil from the ammonite’s shell.

Over time, subtle changes occur in the ammonite’s shell. These were made from the calcium carbonate mineral aragonite, but over long time periods, this changes into the more stable mineral form of calcium carbonate, calcite. Either aragonite or calcite can be dissolved away over time, which can leave a void in the rock where the shell once was.

If the shell doesn’t dissolve, or before it does, minerals can fill the shell’s chambers and produce detailed fossils where some of the original shell can remain intact. In other cases, as the shell dissolves, minerals in the surrounding sediment can fill the cavity left behind to make a cast.

Two examples of this are silicification and pyritisation. Silicification happens when silica-rich solutions replace calcium carbonate with silica. This creates cast fossils with a grey colouration. Often these silicified fossils can be extracted using acids, as the acid will react with the surrounding rock but not the silica.

Pyritisation can happen to fossils in sediment saturated with iron sulfides. Sulfides are produced by decaying organisms and can react with iron to form pyrite (FeS2) which can take the place of the calcium carbonate. Pyrite is also known as fool’s gold and ammonite fossils that have been pyritised have a golden appearance. The downside is that these pyritised fossils can oxidise and slowly disintegrate in humid conditions, a turn of events known as ‘pyrite rot’.

This is just a short summary of some of the processes that can produce fossils; there’s a surprising amount of chemistry that’s occurred over millions of years to create the ammonite fossils you can pick up on the beach!






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References & Further Reading