I have this unknown fossil or ?

[Bobstreasures]

I found this fossil on the Elk River in Tennessee. Any help identifying would be appreciated . Thanks

[doushantuo]

i don't think this is evidence of past life("fossil")

[Innocentx]

As far as I can tell you have an exceptional septarian nodule.  See here:  https://www.google.com/search?hl=en&tbm=isch&source=hp&biw=1366&bih=664&ei=aZctXPrmBemGggfY-ZbABw&q=septarian+nodule&oq=septarian+nodule&gs_l=img.12..0l8.3094.3094..4453...0.0..0.87.87.1......0....1j2..gws-wiz-img.....0.qpPGdYTnAnE

[Bobstreasures]

6 hours ago, Bobstreasures said:

I found this fossil on the Elk River in Tennessee. Any help identifying would be appreciated . Thanks

 

[ynot]

Welcome to TFF!

I agree with it being a septarian nodule.

[DeepTimeIsotopes]

29 minutes ago, Innocentx said:

As far as I can tell you have an exceptional septarian nodule.  See here:  https://www.google.com/search?hl=en&tbm=isch&source=hp&biw=1366&bih=664&ei=aZctXPrmBemGggfY-ZbABw&q=septarian+nodule&oq=septarian+nodule&gs_l=img.12..0l8.3094.3094..4453...0.0..0.87.87.1......0....1j2..gws-wiz-img.....0.qpPGdYTnAnE

+1 I agree as well. Get that thing cut and polished and you’ll have a nice display piece, hopefully. 

[KimTexan]

My first question is does the Elk River area produce septarians?

Have you tried placing a drop of vinegar on it? Most septarians are made of a form of calcium carbonate. If it is calcite it will fizz when acid or vinegar is placed on it.

 

I assume septarians can form in almost any geological period. The majority of what I have are Cretaceous from Texas and Utah. I do have at least 3 from the Pennsylvanian, but they’re not what I’d consider a true septarian. But my definition of a true septarian may not be the most accurate. Here in Texas they generally arise from mollusks. 

The dominant period there is Ordovician.

 

[Rockwood]

4 hours ago, KimTexan said:

My first question is does the Elk River area produce septarians?

Have you tried placing a drop of vinegar on it? Most septarians are made of a form of calcium carbonate. If it is calcite it will fizz when acid or vinegar is placed on it.

 

I assume septarians can form in almost any geological period. The majority of what I have are Cretaceous from Texas and Utah. I do have at least 3 from the Pennsylvanian, but they’re not what I’d consider a true septarian. But my definition of a true septarian may not be the most accurate. Here in Texas they generally arise from mollusks. 

The dominant period there is Ordovician.

 

I was about to give up on my first thought, boxwork, until I got to this.

Good points.

[fossilnut]

6 hours ago, KimTexan said:

But my definition of a true septarian may not be the most accurate. Here in Texas they generally arise from mollusks. 

Could you say more about about "they arise from mollusks". That is a very interesting comment. Thanks

[caldigger]

You sure some mad scientist didn't try to cross hybrid a walnut and a peanut together? 

[Plax]

Google images doesn't show anything like this as a septarian nodule but I'll defer to your expertise.

[doushantuo]

reminiscent(superficially) of Paleodictyon.

Which is NOT AT ALL the same as saying it could be one

DO NOT regard this as a possible ID!!!

[Harry Pristis]

 

This reminds me of a Mississippian geodized blastoid not unlike this brachiopod, except separated along calyx plates.  Just a guess.

 

 

[KimTexan]

3 hours ago, fossilnut said:

Could you say more about about "they arise from mollusks". That is a very interesting comment. Thanks

Septarian nodules in my region at least start by a module forming around mollusk shells. Mollusk shells that have nacre are composed of crystalline aragonite plates. Aragonite crystalline plates are what produce the mother of pearl appearance. As I understand it, mother of pearl is not the presence of the pearly color, but rather the manipulation of light to make it appear to have color.

I believe most mollusk shells are comprised of calcite. Both calcite and aragonite are 2 of the 3 crystalline forms of calcium carbonate.

Back to the nodule:

Over time as the shell begins to decay in a moist environment the crystalline aragonite  plates will go into solutions and be reprecipitated as aragonite crystals. Over time you may have the minerals go into solution again and then be reprecipitated as calcite. Calcite is the most stable crystalline form of calcium carbonate. 

If I have misrepresented the facts or process please feel free to correct me or set me straight. I’ve stated it as I have come to understand it.

Here is an example of a septarian nodule with I believe both aragonite and calcite. 

 

I believe the brown is the aragonite that arose from shells in the gray layer and the tan to yellow is calcite overlaying the aragonite.

 

This pic is a bit dark, but there is a brown layer around the dark gray area where my pinky is and by my thumb.

 

These are examples of the shell material in the gray clay in the midst of decomposition from shell into aragonite, but have not made the calcite transition yet. You can see a white shell layer, dark brown shell or aragonite and then where the shell has made the transition to aragonite crystals.

The 2nd and 3rd pic show it best. The shell is brown, but still looks smooth brown. Needless to say sea shells are not naturally brown. I find the whole process fascinating. It’s the biologist and chemist in me. Seeing what you’ve only read about in books actually in the process of happening before your eyes makes it so vivid and captivating.

 

 

The example above is what I consider a true septarian. However, crystalline veins developing inside of a nodule is not limited to this process. I have examples of others that developed, but the crystals are not calcite or aragonite in the examples I have.

I also have some that I believe arose from non-nacre containing mollusks. They look different.

Now that I’ve totally hijacked @Bobstreasures Sorry about that Bob. Let’s get back to this curious object.

 

It could very well have arisen from a snail, brachiopod or cephalopod. With all the intricate pattern I’m inclined to think it was from an organism with a complex structure. Not just a bivalve or brachiopod.

It is clearly mineralized and it seems only logical to assume the minerals arose from within whatever was inside the nodule. 

Have you done a harness test? Calcite is a bit soft, but chalcedony or other silica based minerals are more in the 6.5 to 7 range. Not easily scratched with a knife. Calcite would be scratched by a knife. 

[fossilnut]

@KimTexan Thanks for that info and great pictures.

@Harry Pristis Very interesting never seen any like that. Thanks for posting

[Innocentx]

7 hours ago, Rockwood said:

I was about to give up on my first thought, boxwork

Yes, I think boxwork may be in the running here.

[Innocentx]

1 hour ago, Harry Pristis said:

This reminds me of a Mississippian geodized blastoid not unlike this brachiopod, except separated along calyx plates.  Just a guess.

That's something I've never seen and it's very very interesting.

[FranzBernhard]

Very thought provoking hypothesis of sepatarian nodule formation, Kim!

 

I can clearly see some shell in some of your specimens. Could it be, that this is just coincidence?

 

1 hour ago, KimTexan said:

it seems only logical to assume the minerals arose from within whatever was inside the nodule.

Thats true and logical. But isn´t the matrix of a septarian nodule composed of a material that is rich in finely divided calcium carbonate? Which can give way to the bizarr veining due to a process called pressure solution? Thats just a guess, I haven´t googled it up, and we will not solve the mystry here .

Franz Bernhard

 

 

[Plax]

2 hours ago, Harry Pristis said:

 

This reminds me of a Mississippian geodized blastoid not unlike this brachiopod, except separated along calyx plates.  Just a guess.

that explains the biological look as opposed to a geological look of the septarian nodules. Thanks Harry!

Edited January 3, 2019 by Plax
changed a word

[ynot]

3 hours ago, KimTexan said:

seems only logical to assume the minerals arose from within whatever was inside the nodule. 

Sorry, but I have to disagree with this as there is way more calcium carbonates in the rock than what would be available from such a small quantity of shell. Most (if not all) of it had to come from somewhere else in the formation or overlying rock.

[abyssunder]

It's a little too strange for a septarian nodule, but geodized fossil might be a good guess. Please make the acid test to see if the crystal walls are fizzing or not. If not, they are made of silica.

 

[KimTexan]

5 hours ago, ynot said:

Sorry, but I have to disagree with this as there is way more calcium carbonates in the rock than what would be available from such a small quantity of shell. Most (if not all) of it had to come from somewhere else in the formation or overlying rock.

I think what you say is largely true. I believe calcite also can come from an external source. It is highly likely that some of the yellow layer of calcite is from an external source. How much I have no clue. However, I believe it unlikely that the aragonite is coming from an external source since the shells obviously contain aragonite rather than calcite. Also, it is the metastable form of calcium carbonate unless it formed under high pressure or in a magnesium rich environment. I assume that the dominant form of calcium carbonate in the marine environment and here in my part of Texas and in sedimentary material is calcite. So I wouldn’t expect an aragonite solution to be infilling these nodules. Of course that is making a lot of assumptions that could be wrong.

From my own observation it appears evident, to me at least, that aragonite Is arising from the shell itself in the example provided where it is dark brown.

I do have one example where calcite appears to be arising from a shell, but the example isn’t as robust or overtly evident.

 

If someone can provide a better explanation I’m open to it. I’m a scientist so proven, peer review research carries a lot of weight with me.  

 

I know calcite can also occur in the brown form. I also know aragonite can occur in the yellow form. It isn’t always easy to distinguish between the two in the field. Granted aragonite is slightly harder than calcite. In the field I occasionally observe septarians to have the acicular crystalline form on the surface of the brown crystals, which I believe is not found in calcite. I’m not a mineralogist so I could be wrong about calcite not taking the acicular form, but I don’t believe it does.

[ynot]

1 hour ago, KimTexan said:

that aragonite Is arising from the shell itself in the example provided where it is dark brown.

The minerals in the shell will act as a "seed" - propagating the growth of like mineral.

 

My earlier comment was not including the minerals of the shell, but meant as a comment on the crack filling minerals.

To further those statements- the original minerals of the shell are still in the shell, hence the opalescence.

 

Calcite is not the only calcium carbonate mineral. There are no less than three minerals or phases of CaCO3. Aragonite and vaterite are polymorphs (latin for "many shapes") with calcite, meaning they all have the same chemistry, but different crystal structures and symmetries.

Also, calcite has the most crystal habits (shapes) of all the known minerals, somewhere around 120.

 

Unless You have done extensive tests it is difficult to determine whether you are looking at one calcium carbonate or the other.

[FranzBernhard]

2 hours ago, ynot said:

Unless You have done extensive tests it is difficult to determine whether you are looking at one calcium carbonate or the other.

Thats true! Often they are very hard to distinguish, sometimes more easy, especially if they occur in well-formed, free-growing crystals.

 

3 hours ago, KimTexan said:

Also, it is the metastable form of calcium carbonate unless it formed under high pressure or in a magnesium rich environment. I assume that the dominant form of calcium carbonate in the marine environment and here in my part of Texas and in sedimentary material is calcite. So I wouldn’t expect an aragonite solution to be infilling these nodules

Completely true! And the magnesium part is especially important for near-surface formation of aragonite.

 

Franz Bernhard

[supertramp]

Could you add a lateral pic of the specimen?

geodized fossil seems to be a good option, thought the fossil itself is not recongnizable in this case 

https://www.google.com/search?q=geodized+fossil&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiRk5Ok3dPfAhVNxoUKHSXDAhsQ_AUIDigB&biw=1517&bih=695#imgrc=q4LcQyZBto_mDM:

 

ciao