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322 Views 1 Reply Last post: Aug 29, 2014 10:55 PM by MikeHardman RSS
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Aug 29, 2014 6:42 PM

What kind of rock is this?

Hey I live in Colorado.

Someone sent me these photos and this is on the eastern plains. I asked if I can see if in person to figure out other things about it. But maybe it is a dead give away on what it is based on what it looks like.

 

I haven't seen a small nubbin of rock with this kind of pattern.

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    Aug 29, 2014 10:55 PM (in response to Cassomophone)
    Re: What kind of rock is this?

    Welcome to NaturePlus, Colorado.

     

    Well, it is almost certainly banded ironstone, in a general descriptive sense.

     

    But it is curious in that it shows a mix of features of two main types:

     

    - Sedimentary banded ironstone - flattish bands, perhaps with folding.

    See http://en.wikipedia.org/wiki/Banded_iron_formation

    and do  a Google image search for 'banded ironstone'.

     

    - Concretionary banded ironstone - bands forming closed surfaces, often concentric.

    Sometimes the shapes of the surfaces can be seen to parallel the sides of blocks of rock defined by fractures, eg.http://blogs.agu.org/georneys/2011/01/22/what-type-of-weathering/.

    Such banding, representing variations in mineralogy and hence potentially hardness, may come to stand out as a result of weathering. ...Or not, eg. http://paleo.cc/paluxy/nevada.htm

     

    Because the surfaces in yours are at least partly closed, I can rule out the former.

     

    A third possibility is that the iron-rich layers represent a fossil, the original mineral having been replaced with an iron-rich one. But what sort of fossil? None comes to mind that is a good fit. And I can't see any organic details.

     

    I don't get much insight from the location, but thanks anyway.

     

    So I think it is probably a concretionary banded ironstone, but there are some aspects that leave me unconvinced. That's the best I can do from the photos.


    Somebody else here might have an idea. So stay tuned a while.

     

    Mike

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