Strange disc like metal object found in creek

[Stasdab]

Hello , found this in a creek in will county Illinois. Its metalic with some kind of layers on top that looks more rock like. It has a reddish hue to it but when scraped of its silvery . Its very dense and fairly hard but not magnetic, I smacked it a couple times with a hammer and no dent in the metal looking area but the top area broke showing a grey grainy under layer.its roughly 3x3x1 with a 9 inch circumference and weighs close to 2 pounds. I'm clueless thanks if you can help

[CMD]

I believe it's a concretion:

http://crystal.isgs.uiuc.edu/maps-data-pub/publications/rocks-min/sub-rocks.shtml

http://en.m.wikipedia.org/wiki/Concretion

[Stasdab]

Hello , well with further research on concretions that cmd gave me a tip on I think its a pyrite concretion. Fellow member painshill had a post about Kansas pop rocks that looks almost identical to my find. I did smack it with a hammer and a small chip came off as you can see with new photo. I also chipped away the oxidized area to show more of the shiny pyrite surface, while doing this it did produce a few sparks. My only ? Is what's this doing in a Illinois creek, maybe there's another source somewhere up north that the glaciers pushed down. Thanks for looking and any of input.

[painshill]

It is for sure an iron-rich concretion and likely a mix of oxides, although there could well be some pyritisation (sulphides). Although these things have various names (like Kansas Pop-Rocks for example) which indicate a particular locality, they can form anywhere there is iron rich groundwater flowing through sediments and silts that are destined to become rocks like sandstones, mudstones and shales. The nucleus for the concretion is frequently a fragment of organic material or a marine shell such as a brachiopod.
They only acquire names like "Kansas Pop-Rocks" because they are commonly found in those localities, but what you found could well be local to your area rather than deposited from elsewhere... but just less commonly found and so not has earned its own nick-name. The Kansas examples are generally of a particular composition which includes the oxide magnetite, present as tiny interior crystals.

[Stasdab]

Thanks Painshill, would it be worth trying to bust it in half to try to find the fossilized nucleus?

[painshill]

Stasdab wrote:

Thanks Painshill, would it be worth trying to bust it in half to try to find the fossilized nucleus?

  No... whether it was fragmentary or even a whole brachiopod to begin with, there'll likely be little of it remaining as recognisable material now. If it were a nodule rather than a concretion then yes, those are often splittable along a sedimentary plane to reveal a compression fossil. Generally, the concretionary process is too slow to preserve/replace organic material before it decays unless there are exceptional circumstances.