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Why are some fossils made of iron pyrite?


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Just recently I have been spending alot of my time collecting fossils and I've noticed that where fossil beds made up of organic material there is a lot of iron pyrite. Some of the fossils from these beds are pyrite as well but not all, why is this and why is there large deposits of iron in these beds?

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All fossils are actually made of rock or mineral which seeps in a replaces the actual animal. In pyrite fossils, it's the same process, just with iron pyrite. I even recall seeing an ammonite made of opal.

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  • 2 weeks later...

Maybe it is just the area that you are looking in.

 

Also iron pyrite does not erode that easily (i believe) so what you are looking at is harder minerals standing the test of time where softer ones erode away revealing the fossils you are finding.

 

Cay

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  • 4 months later...

Some of the best depositional environments - especially where shales are concerned - occur deep underwater where there can be increased concentrations of metals in the water column. Pyrite forms frequently in shales. Not sure what type of rock your formation is made up of, but that'd be my guess. Part of the reason behind the high concentration of metals is due to the presence of deep-sea vents - the metals seep into the water from vertical hydrothermal fluid movements.

 

On a related note, there are some fossils made out of pyrite that did not form from the process of fossilization. Some shelled creatures are chemotrophs that feed off of the same deep-sea vents, and in the process, they absorb a lot of the metals present in their food supply (on an unrelated note, as a palaeontologist I have at times used the metallic isotope 'fingerprint', if you will, to identify where an organism spent its life). In any case, certain bivalves, gastropods, and molluscs secrete their shells directly out of their skin using whatever extra components they pull out of the seawater, and this is usually calcium, but you can get some weird forms like pyritized shells too. They look really cool =)

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In any case, certain bivalves, gastropods, and molluscs secrete their shells directly out of their skin using whatever extra components they pull out of the seawater, and this is usually calcium, but you can get some weird forms like pyritized shells too. They look really cool =)

 

Sweet I want one! I'm guessing they don't tend to show up on the seashore though...

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Sweet I want one! I'm guessing they don't tend to show up on the seashore though...

 

Not that I've seen - like I said, it's usually only in shale deposits, and those can be found all over the place... I have several in my possession I collected from a shale deposit on the side of a mountain at 10,000 feet. A lot of the time the fossils in shale ended up there after dying and settling all the way down to the ocean floor, but the ones that seem to secrete the pyrite actually lived on the ocean floor itself. Unfortunately I have them all in storage right now... 1,800 miles away... But when I have them back in my possession I'll take pictures and post them to this thread.

 

Have a good weekend!

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