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Geology/Can you help identify this rock


I found this rock while in creek in Northern Wisconsin.  I cracked it open with a sharp tool and was amazed at the metallic look it had and in the center was a harder rock.  (kind of like a pit seed in the middle of a plum is the best way I can describe it)

I am always roaming around our woods looking for interesting rocks and have hammered others before to see the inside.  I have never seen any rock like this and certainly never saw anything that looked like the inside of this rock either.  

I tried to research the Internet for what it may be but have not found any luck figuring it out.  I included some pictures.  You can see the round looking rock which is what I found in the middle. I have pictures of the outside as well.  

If you can help me figure out what this is I would truly appreciate it :)

The mineral is marcasite.  Marcasite, sometimes called white iron pyrite, is iron sulfide (FeS2) with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite but having the same chemical formula.  Pyrite is iron sulfide with cubic crystal structure. Both structures do have in common that they contain the disulfide S22- ion having a short bonding distance between the sulfur atoms. The structures differ in how these dianions are arranged around the Fe2+ cations. Marcasite is lighter and more brittle than pyrite. Specimens of marcasite often crumble and break up due to the unstable crystal structure.

On fresh surfaces it is pale yellow to almost white and has a bright metallic luster. It tarnishes to a yellowish or brownish color and gives a black streak. It is a brittle material that cannot be scratched with a knife. The thin, flat, tabular crystals, when joined in groups, are called "cockscombs."

In marcasite jewellery, pyrite used as a gemstone is termed "marcasite". That is, marcasite jewellery is made from pyrite not from marcasite. In the late medieval and early modern eras the word "marcasite" meant both pyrite and marcasite (and iron sulfides in general).[4] The narrower, modern scientific definition for marcasite as orthorhombic iron sulfide dates from 1845.[2] The jewellery sense for the word pre-dates this 1845 scientific redefinition. Marcasite in the scientific sense is not used as a gem due to its brittleness.


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Joe Norris


I am an economic geologist. An economic geologist does mineral evaluations and appraisals of mineral or mining properties. I can tell you if your deposit has value - remember that a mineral deposit, no matter how good, only has value when mined. Any value assigned to a mineral deposit, in the ground, is only the speculative value that deposit.


I have been a economic geologist for most of my 35 year career. Although I have done work in perhaps 45 states and numerious countries much of my work has been in Appalachian coal, intermountain west gold and silver, and Arizona uranium.

Past President of the Virginia Section of the American Institute of Professional Geologists and a certified geologist in twelve states.

BS Degree from Eastern Kentucky University. Work on MS Degree @ Eastern Kentucky University, Colorad School of Mines & Marshall University Numerious short courses on the value of mineral deposits and how to value same. Also several short courses dealing with the different types of geologic processes; sedimentary, igneous, metamorphic along with the mineral associated with each.

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