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Curious Little Rock...Help with ID?
  
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adam Smith




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PostPosted: May 06, 2019 11:47    Post subject: Curious Little Rock...Help with ID?  

I live in central Illinois and found this in my back yard. I'm curious what it could be, and would appreciate any help in identifying it.


20190506_094459.jpg
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 Viewed:  1692 Time(s)

20190506_094459.jpg



20190506_094420.jpg
 Mineral: Unknown
 Description:
Illinois River Valley
4x2x2 inches
 Viewed:  1683 Time(s)

20190506_094420.jpg


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Bob Harman




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PostPosted: May 06, 2019 19:16    Post subject: Re: Curious Little Rock...Help with ID?  

Some of the most interesting and varied mineral combinations are found as part of the southern Illinois fluorite district, but your example is not like anything that I ever saw. And, unless transported northward by someone, being from central Illinois is some distance from that area. Central Illinois, itself, is just not blessed with that many really interesting examples.

The empty spaces and nearby partially occupied spaces show what appears to be etched out calcite, separated by more resistant material.

I wonder if the example is natural or a piece of old construction debris. Just not sure. BOB
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alfredo
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PostPosted: May 06, 2019 20:23    Post subject: Re: Curious Little Rock...Help with ID?  

Barite?
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Don Lum




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PostPosted: May 06, 2019 20:36    Post subject: Re: Curious Little Rock...Help with ID?  

The shape of the cavities remind me of some of the polyhedral agates I have from Brazil. One idea on the formation of these agates is that quartz fills the cavities in calcite and then the calcite dissolves away. I know Illinois is a long way from Minas Gerais, but just a thought. You could test the rock with 10% HCl or vinegar and see if it fizzes.
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Bergur_E_Sigurdarson




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PostPosted: May 06, 2019 20:43    Post subject: Re: Curious Little Rock...Help with ID?  

What I find curious is the dark, glassy looking material between the blades... has bubbles in it (slag or tar?)
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SteveB




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PostPosted: May 07, 2019 00:10    Post subject: Re: Curious Little Rock...Help with ID?  

Strange indeed. First thought was gypsum, second thought involved plywood and beavers. Its hard to tell from a photo really either way. Intinct tells me this is not the natural form, its too mixed up with not structure really. It would be plywood thats been through a chipper maybe and solidifeid with muds? Likewise gypsum or similar formation that was broken through a natural destructive force like earthquake or landslide so the parts were broken in place and formation continued and leakage of other mineral salts conglomerated it all together? Certainly looks like like it was originally in one form then broken and mixed and re-set with new cementing medium. The structural forms I see are common to a laminated formation which gets broken. Can you try some acid and scratch tests on the sheet portions? Any indication the inner portions are softer than the sheet parts?
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Kevin Schofield




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PostPosted: May 07, 2019 08:05    Post subject: Re: Curious Little Rock...Help with ID?  

adam Smith wrote:
I live in central Illinois and found this in my back yard. I'm curious what it could be, and would appreciate any help in identifying it.


It is possible that this is a chunk of material from a "fault gouge". The flat faces in the second picture have a slightly "polished" look of material that I have seen from similar settings. For those non-geologists among us (and I assume that you are one Adam), the gouge is the material that gets ground up along a fault plane when it moves deep underground. It is often reduced to a rock flour, which cements together with whatever minerals are in the waters circulating up and down the fault (calcite, quartz and barite are common). When the fault moves again, the cemented gouge gets torn up and eventually may be reduced again to rock flour, or if it comes to rest in a "kink" in the fault plane may form a breccia of pieces. This rock may possibly be something of that sort.

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Pete Richards
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PostPosted: May 07, 2019 08:11    Post subject: Re: Curious Little Rock...Help with ID?  

I believe this is a natural object.

I think Don Lum is on the right track. The so-called polyhedral geodes I have seen (from New York State and South Africa) are typical (non-agate) geodes - deposits of quartz around the boundary of a void, with crystal becoming coarser inward. The mystery is what caused the void to have a polyhedral shape. In the case of New York - and I think probably elsewhere - these geodes occur because an open vein was criss-crossed by nearly paper-thin blades of calcite (papierspat) which intersected to produce polygonal voids, within which the quartz was deposited. Later, the calcite dissolved away, and weathering allowed the polyhedral geodes to be released as separate objects. In some examples, remains of the calcite are still preserved. The vein is now be composed of a network of these "polyhedral geodes" fitting together like a jig-saw puzzle.

This could also happen in voids of other shapes and origins. The feature that is needed is the intersecting network of planar crystals that form the borders of the geodes.

This object appears to be equivalent to the first step in the process described above - crystals of some sort have grown into an open or perhaps mud-filled vein, mostly perpendicular to the vein walls (of which only one is present on the specimen). Polyhedral geodes have not formed here, only the crystals which would be the bounding partitions.

The crystals could be calcite, barite, or any of several other minerals that can form platy crystals, and as Bergur observed, the center of each blade might be one mineral and the outer parts might be a different mineral.

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