The role of pyrite in fossilisation and its potential instability

Pyrite often forms in organic-rich marine sediments through bacterial action. Bacteria reduce organics to form bisulphide, which reacts in turn with dissolved iron in seawater to form pyrite. Consequently, it is frequently found in stratigraphic formations that consist of marine clays. Pyrite can be involved in fossilisation through a number of ways, including complete replacement, as infill in permineralised bone, or simply as microcrystals finely disseminated through the fossil and matrix. Replacement by pyrite can often be easily recognised: the fossil may have a gold or brassy metallic lustre. Infill is harder to recognise, and disseminated pyrite even more so. Pyrite in fossils may be stable or unstable- in its stable form, pyritised fossils will generally retain their shiny, metallic appearance. Pyrite preservation and stability can vary even within the same specimen. Fossils preserved in pyrite can be prone to oxidation, particularly at high relative humidities. There are a number of signs that indicate oxidation is occurring, depending on the severity of the condition. One or more of the following may be present: a sulphurous smell, white or yellow powdery crystals on the surface of the specimen, expansion cracks, as well as acid burns on associated labels, boxes and drawers. Such burns often have a characteristic ovoid appearance. If left unchecked, pyrite oxidation can completely destroy a specimen and its labels.