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Black Rock in Wisconsin


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My first guess would be "Taconite" the famous Precambrian, iron ore (Biwabik Iron Formation) found in upper Minnesota and parts of Wisconsin, Michigan & Canada.  Defiantly not basalt.

Edited by 4meter
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I'm in Madison which is pretty far south and a long way from that range. It's confusing because I don't see any rust. I chipped a piece off, ran it under water and left it sit for a few days and it doesn't show any signs of rust. I'm just curious, why do you say that it's definitely not basalt?

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boblien,  basalt is not magnetic, there is no magnetite in basalt.  From the photos, the rock sample does not display any features that are characteristic of a basalt.  A quick check of the surface geologic map around Madison, Wis does not show any basalt deposits, mostly limestone, dolomite, sandstone and some glacial deposits.  The closest volcanic rocks are to the north, north-east and these are rhyolites and associated volcanic-clastic rocks.  The iron found in Taconite is mixed with quartz and other things so wetting it or leaving it outside in the rain will not cause it rust.  I live in southern Arizona and there are lots of basalt of various ages found in AZ & NM, plus I have spent time in Hawaii which is mostly made of basalt, even did some work on the precambrian meta-basalts of Shenandoah National Park in N Virginia;  your rock is not a basalt.  Maybe you can take the pics and sample to a local Community College with a Geology program and have the Geologist ID the sample.  What is the history of the are where you found the rock?  Any mining, refining or industrial use?

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IMG_0854.thumb.JPG.af882ba0e71ced681451656f588a335e51_IMG_08551.thumb.JPG.7753fb

The photos are of a few pieces that we were able to chip off.

The second photo is of the bigger rock above but turned over to show the side that was exposed to the elements. The top picture really doesn't do a good job of showing how dark these rocks are. The shiny flakes in the rock look like metal to me.

The land has been owned by my family for over a hundred years and this rock is in a heavily wooded area next to a marsh. There hasn't been any mining or industrial processes nearby. The rock is unlike any others I've seen in the area.

I wasn't questioning what you had said but wanted to be able to explain why it wasn't basalt to a friend of mine. I appreciate any ideas as to what this could be. 

Edited by boblien
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boblien, I know you were not questioning me.  I was trying to type out my thought process.  Sorry if it came out sounding that I was offended, I am not. 

Here is a wild thought, maybe the rock is a piece of jacupirangite.  These are unusual igneous rocks, very uncommon, tend to be dark colored and magnetic due to a high percentage of magnetite found in them.  Does your sample look like the rocks photos in the top of the linked website?  The only place I know where this type of rock is found in the USA is at Magnet Cove, Arkansas.  I have a sample somewhere in my collection or at least I use to.  :idunno:

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