Maybe fossil

[Mayorman]

 I recently dug this rock out of a pile I have next to my house near Grand Rapids MI. I know I put it there many years ago but I can't be sure it originated from this area. There are a lot of things I don't know about this thing. I posted it on a rock forum and showed it to some rock hound friends and they all seem mystified. Some of them suggested that it could be a fossil. Maybe a type of sponge? That could have been a wild guess but I'm hoping someone here can confirm or deny that for me.

It seems to be all quartz of one sort or another but the interesting thing is that it seems to be made up of pieces like a conglomerate but with almost no material between them. It reminds me of monkey bread, if you know what that is. Also, a lot of the pieces are solid on the outside but porous and sort of grainy on the inside.

Enough rambling! Please let me know what you think. And let me know if you need any more photos or other information. Thanks!

   

[fossilus]

Weathered quartzite that has been cemented? Doesn't look fossil to me especially if it is truly quartz.

[Herb]

does not look like a fossil to me either

[Ludwigia]

No fossils there. Metamorphized conglomerate?

[FranzBernhard]

Its not a fossil, indeed.

 

It would be important to known if it is really mainly quartz or mainly some kind or carbonate.

 

- Are you able to scratch a freshly broken pebble with a steel nail? What happens?

(I know, I know hardness tests on rocks are, well, equivocal. But you can get at least an indication if the pebbles are a carbonate-rich rock or a quartz-rich rock).

 

- Further, put a drop of vinegar or (better) muriatic/hydrochloric acid on a freshly brocken surface of a pebble. What happens? If nothing, try to make some powder from a pebble and test it again with acid.

 

Looking forward to your results, @Mayorman!

Franz Bernhard

[FossilNerd]

I agree that it is not a fossil. I’m not sure what it is, but it’s very neat. I think @FranzBernhard has some good suggestions. 
 

@ynot may also have an idea the next time he pops in. 

[ynot]

Looks like opalite (common opa;).

Is it light for it's size?

[supertramp]

it could be a brecciated marble

ciao

https://www.google.com/search?q=brecciated+marble&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjZ1tnOx9boAhUt2aYKHftaB1AQ_AUoAXoECAwQAw&biw=1366&bih=576

[Mayorman]

Thanks for all the input!

It is pretty clearly quartz. No reaction to acid and it won't scratch with a knife or leave a streak on porcelain. Could it be metamorphic something? It seems too finely grained to be quartzite and a conglomerate would have more than one kind of stone in it. What could have morphed into this strange configuration of pieces/cells/globs, some with porous centers, all seemingly fitted or pressed together like this?

I've been puzzling over this rock, and others like it, for a long time now and I would do a happy dance if someone identified it for me.  But alas! My search continues. I guess it wouldn't be fun if it was easy.

 

[Mayorman]

I just saw the posts from ynot and supertramp. I'd say it's actually slightly heavy for it's size and it's too hard to be marble. Marble would react to acid too. Wouldn't it? Thanks for the suggestions.

[FranzBernhard]

Thanks for the additional infos, @Mayorman!

Ok, its essentially something SiO₂-based.

- Monomict (composed of one kind of rock) breccias do occur, especially if they are created by tectonic forces (fault breccia).

- The porous centers were obviously created after the breccia/conglomerate has formed. It may point to some dissolution or replacement, eg. replacement of calcite by dolomite. Or maybe also replacement of a carbonate by something SiO₂-based?

- The individual clasts in your specimen touch each other often in a concave-convex pattern; in some places also in a somewhat zig-zag-fashion? Especially the later would point to a strong diagenetic, possible low-grade metamorphic, overprint, causing some pressure solution.

- But it could also be that there is not much pressure solution involved. Instead, an existing quartz vein or very pure quartzite has been broken up by tectonic forces without much displacement of the resulting clasts and with the narrow gaps between them filled with something.

@ynot, @FossilNerd. @Ludwigia, @Herb, @fossilus

So, still no conclusive answer from me! Especially the porous centers are puzzling me!
Franz Bernhard

[fossilus]

One other possibility that crossed my mind was that it could maybe be a sample from a quartz filled fault/fracture zone. I remember seeing those in college but that was over 40 years ago.

[Mayorman]

You've certainly given me enough to think about @FranzBernhard  Do you think it's possible that the clasts, if that's the right term, could have formed individually around some impurity or maybe like minerals deposited in a vug in another rock? Just guessing on my part but maybe that will to some other thought on your part.

I also attached some other examples of these porous centers hoping they might lead to something. BTW a US penny measures 19mm across.

[FranzBernhard]

4 hours ago, Mayorman said:

could have formed individually around some impurity or maybe like minerals deposited in a vug in another rock

That´s not impossible, indeed! But not very probable (that´s just a gut feeling!)

 

Nice to have some more specimens of this kind, thanks! They give at least some context. There is quite a variability in it, and I am getting the feeling, that it is something in that line:

17 hours ago, fossilus said:

a sample from a quartz filled fault/fracture zone.

There can be various structures in those settings within one fracture zone / hydrothermal vein. 

But I am still not certain, what it really is!

@ynot, @Ludwigia, @FossilNerd

Franz Bernhard

[UndercoverN]

Almost looks like gold veins?

[Mayorman]

Thank you all for indulging me. You have helped me eliminate the fossil possibility and that's really all I can ask from a fossil forum. All the info I got beyond that was just a nice bonus. My quest continues!

@UndercoverN Don't I wish!

[Herb]

Have I missed the test results? Does it fizz ? How hard is it?

[Pippa]

I suggest you post your rock ID question on Mindat.org 

Plenty of geo / mineral experts there to help you.

(Make sure you read their ID thread requirements before posting)

 

Good luck!

[supertramp]

On 7/4/2020 at 6:16 PM, Mayorman said:

No reaction to acid and it won't scratch with a knife or leave a streak on porcelain.

That certainly rules out marble (if limestone, my second guess would have been nodular limestone)

 

An observation that could help clarify the reason for the porous centers.

As Mayrman said, the “nodular” structure that we can see on a centimeter scale, is replicated within the individual “clasts", which seem to be made up of even smaller (a few millimeters or less) interlocked grains.

But, at the center of the larger fragment, on fresh cut surfaces, you can see that these smaller grains are not interlocked, but rather “floating” in a matrix (perhaps more erodible), whose removal has probably produced the porous  and concave appearance on weathering surfaces.

Regards