How Does Pyrite Form?

[Lmshoemaker]

I was talking to a fossil preparator Saturday who had an isotelus on display. He told me that the black areas on this isotelus he had were pyrite. He said that the pyrite formation had something to do with worms that clustered around the animal when it died, but I did not catch how the two correlate. Can anybody here explain this to me?

[Auspex]

I know that bacterial activity can precipitate pyrite from a low oxygen environment. I am sure there are other ways for pyrite to occur though (like pseudomorph replacement).

[painshill]

Most usually, it’s from bacterial action and it requires two different types of micro-organisms. When an organism rots after death, sulphates (in the organism, or more frequently in any surrounding water) are utilised by anaerobic bacteria as part of the oxidative process to decompose the organic matter as a nutrient source. Those kinds of processes are favoured in marine sediments. The waste products of such processes are sulphides from the reduction of the sulphates.

The classic rotten egg smell is the result of that kind of process – what you can smell is gaseous hydrogen sulphide. It’s a pretty reactive gas but normally gets dispersed into the atmosphere or is carried away in any surrounding water since it also has high solubility. In the presence of iron compounds it readily converts to iron (II) sulphide (FeS).

In the presence of aerobic bacteria, the iron (II) sulphide is oxidised to iron pyrite (FeS2) – most properly called iron persulphide, but frequently incorrectly referred to as iron bisulphide or disulphide. That may occur throughout the organism as it decomposes (depending on its size and geometry), but it happens preferentially at and near the surface.

Since the process requires both anaerobic and aerobic bacteria, pyritization by this route only occurs in sediments at interfaces between anaerobic and aerobic conditions, or where there has been a shift in conditions from anaerobic to aerobic as a result of disturbance, changes in sea level etc. It also requires specific concentrations of iron within defined minimum and maximum limits, but other metallic ions may also be bacterially mediated into pyrites (copper, for example), although much less usually so.

The extent of pyrite formation in marine sediments is therefore largely determined by the availability of dissolved sulphates and reactive iron compounds (and sometimes other metals). Deep ocean environments are therefore favourable since there can be increased concentrations of such compounds in the water column arising from things like hydrothermal vents.

All of that is a simplification and by no means the only mechanism, but it’s the most frequently encountered mode of formation in fossilization processes.

Edited March 4, 2014 by painshill

[fossilized6s]

^^^ Wow! That's quite detailed and very informative! Thank you!

[Lmshoemaker]

Most usually, it’s from bacterial action and it requires two different types of micro-organisms. When an organism rots after death, sulphates (in the organism, or more frequently in any surrounding water) are utilised by anaerobic bacteria as part of the oxidative process to decompose the organic matter as a nutrient source. Those kinds of processes are favoured in marine sediments. The waste products of such processes are sulphides from the reduction of the sulphates.

The classic rotten egg smell is the result of that kind of process – what you can smell is gaseous hydrogen sulphide. It’s a pretty reactive gas but normally gets dispersed into the atmosphere or is carried away in any surrounding water since it also has high solubility. In the presence of iron compounds it readily converts to iron (II) sulphide (FeS).

In the presence of aerobic bacteria, the iron (II) sulphide is oxidised to iron pyrite (FeS2) – most properly called iron persulphide, but frequently incorrectly referred to as iron bisulphide or disulphide. That may occur throughout the organism as it decomposes (depending on its size and geometry), but it happens preferentially at and near the surface.

Since the process requires both anaerobic and aerobic bacteria, pyritization by this route only occurs in sediments at interfaces between anaerobic and aerobic conditions, or where there has been a shift in conditions from anaerobic to aerobic as a result of disturbance, changes in sea level etc. It also requires specific concentrations of iron within defined minimum and maximum limits, but other metallic irons may also be bacterially mediated into pyrites (copper, for example), although much less usually so.

The extent of pyrite formation in marine sediments is therefore largely determined by the availability of dissolved sulphates and reactive iron compounds (and sometimes other metals). Deep ocean environments are therefore favourable since there can be increased concentrations of such compounds in the water column arising from things like hydrothermal vents.

All of that is a simplification and by no means the only mechanism, but it’s the most frequently encountered mode of formation in fossilization processes.

What a great explanation, can you go into more detail please, specifically on aerobic and anaerobic conditions, like what kind of environments these occur in? This is more interesting than I thought! Also, where does the iron come from, are there appreciable amounts in the ocean typically?

Edited March 4, 2014 by Cryptidsaurian

[painshill]

What a great explanation, can you go into more detail please, specifically on aerobic and anaerobic conditions, like what kind of environments these occur in? This is more interesting than I thought! Also, where does the iron come from, are there appreciable amounts in the ocean typically?

It's kinda complicated... but if you want the detail, there's a wealth of information in this paper:

http://eprints.whiterose.ac.uk/335/1/raiswellr2.pdf

[Lmshoemaker]

Thank you, this seems exactly what I was looking for!