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Calcitic vs. Aragonitic shells in molluscs. Information appreciated.


Ludwigia

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In another thread I recently presented a bivalve which I had found whose shell is excellently preserved. All the positive comments prompted me to write a little side note on calcitic and aragonitic preservation of shells. In the course of writing this however, I realized how little I actually know about the subject. Here's what I wrote:

"Apparently almost all molluscs originally developed aragonitic shells, which, although less stable than calcite, provided better protection from predators since it is harder. Some combine both polymorphs with aragonite inside and calcite outside, since it doesn't dissolve as readily as the latter and apparently magnesium plays a role here as well. My observations in the field have however indicated to me that at least in the Jurassic layers I am familiar with it is often the case that in any given layer, bivalves and brachiopod shells are as a whole much better and more often preserved than those of the ammonites.

I do know that at over some stretch in geological history, the aragonite was converted in most cases to the more stable calcite. Perhaps bivalves and brachiopods had thicker shells or a different shell structure to those of the ammonites, which in turn were more susceptible to dissolution before they could be completely converted? It sure would interest me if anyone else could educate me on this subject."

Thinking about this again, I'm not sure if one can generalize in comparing these orders, since I also know that some ammonites have thick shells and some bivalves have thin ones. I was just hoping that someone out there knows a bit more about the subject and could enlighten me a bit.

 

Greetings from the Lake of Constance. Roger

http://www.steinkern.de/

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Most mollusks have aragonitic shells but some are calcitic. Oysters, scallops and some gastropods have calcitic shells. All articulate brachiopods have calcitic shells. I wasn't aware that aragonite is harder than calcite.

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One thing that I've noticed for sure is that the shells of articulate brachiopods seem to survive better than most other molluscs and I think that has to do with the fact that their shells were calcitic right from the very start which helped them survive the heat and pressure of the taphonomic process during the ensueing millenia. Oysters are also quite prolific in the fossil record. Gryphaea comes to mind, which abounds particularly in the lower Jurassic.

 

Greetings from the Lake of Constance. Roger

http://www.steinkern.de/

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Although it is a form of aragonite, nacre may play a role in the preserve or not scenario. 

To wit....

Nacre is composed of hexagonal platelets of aragonite (a form of calcium carbonate) 10–20 µm wide and 0.5 µm thick arranged in a continuous parallel lamina.[2] These layers are separated by sheets of organic matrix composed of elastic biopolymers (such as chitin, lustrin and silk-like proteins). This mixture of brittle platelets and the thin layers of elastic biopolymers makes the material strong and resilient, with a Young's modulus of 70 GPa (when dry).[3] Strength and resilience are also likely to be due to adhesion by the "brickwork" arrangement of the platelets, which inhibits transverse crack propagation. This structure, at multiple length sizes, greatly increases its toughness, making it almost as strong as silicon.
Nacre appears iridescent because the thickness of the aragonite platelets is close to the wavelength of visible light. These structures interfere constructively and destructively with different wavelengths of light at different viewing angles, creating structural colours.
The crystallographic c-axis points approximately perpendicular to the shell wall, but the direction of the other axes varies between groups. Adjacent tablets have been shown to have dramatically different c-axis orientation, generally randomly oriented within ~20° of vertical.[4][5] In bivalves and cephalopods, the b-axis points in the direction of shell growth, whereas in the monoplacophora it is the a-axis that is this way inclined.[6] The interlocking of bricks of nacre has large impact on both the deformation mechanism as well as its toughness.[7] In addition, the mineral–organic interface results in enhanced resilience and strength of the organic interlayers.

 

Tony

Darwin said: " Man sprang from monkeys."

Will Rogers said: " Some of them didn't spring far enough."

 

My Fossil collection - My Mineral collection

My favorite thread on TFF.

 

 

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4 hours ago, Ludwigia said:

Perhaps bivalves and brachiopods had thicker shells or a different shell structure to those of the ammonites, which in turn were more susceptible to dissolution before they could be completely converted?

 

In general (there are exceptions), Brachiopods have thinner shells than marine gastropods, bivalves and cephalopods. .  

Another generality is the deeper the water , the thinner the shells.  

Anyways, its good to have a specimen of each of the existing orders of brachiopods. They provide perspective for the extinct species. The first time I obtained a living terebratulid brachiopod it really helped give perspective to the shell anatomy.
 

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Proteomics,bioinfomatics ,biomimetics,biodiversity ,ocean acidification and sclerochronological studies have pretty much enriched the biomineralization literature....

Some possibly useful input:

 

addadiweinermolluscbiomineraliz62000000.pdf

 

 

cartwrightarxivcheca1211.5576.pdf

nudebiomineralizcollagen1-s0-main.pdf

 

immel

 

 

 

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6 hours ago, Canadawest said:

 

In general (there are exceptions), Brachiopods have thinner shells than marine gastropods, bivalves and cephalopods. .  

Another generality is the deeper the water , the thinner the shells.  

Anyways, its good to have a specimen of each of the existing orders of brachiopods. They provide perspective for the extinct species. The first time I obtained a living terebratulid brachiopod it really helped give perspective to the shell anatomy.
 

 

I didn't realize that brachiopods are so thin-skinned. I just know that the paper-thin layers peel off easily by abrading, which is why I have to keep the pressure low. Maybe it's their size and form that keeps them so well-preserved? Also that with the ratio of thickness to depth is new to me. Why is that? You could  think the other way around due to water pressure.

 

6 hours ago, ynot said:

Although it is a form of aragonite, nacre may play a role in the preserve or not scenario. 

To wit....

Nacre is composed of hexagonal platelets of aragonite (a form of calcium carbonate) 10–20 µm wide and 0.5 µm thick arranged in a continuous parallel lamina.[2] These layers are separated by sheets of organic matrix composed of elastic biopolymers (such as chitin, lustrin and silk-like proteins). This mixture of brittle platelets and the thin layers of elastic biopolymers makes the material strong and resilient, with a Young's modulus of 70 GPa (when dry).[3] Strength and resilience are also likely to be due to adhesion by the "brickwork" arrangement of the platelets, which inhibits transverse crack propagation. This structure, at multiple length sizes, greatly increases its toughness, making it almost as strong as silicon.
Nacre appears iridescent because the thickness of the aragonite platelets is close to the wavelength of visible light. These structures interfere constructively and destructively with different wavelengths of light at different viewing angles, creating structural colours.
The crystallographic c-axis points approximately perpendicular to the shell wall, but the direction of the other axes varies between groups. Adjacent tablets have been shown to have dramatically different c-axis orientation, generally randomly oriented within ~20° of vertical.[4][5] In bivalves and cephalopods, the b-axis points in the direction of shell growth, whereas in the monoplacophora it is the a-axis that is this way inclined.[6] The interlocking of bricks of nacre has large impact on both the deformation mechanism as well as its toughness.[7] In addition, the mineral–organic interface results in enhanced resilience and strength of the organic interlayers.

 

Tony

 

That's really interesting with the nacre. I guess the flexibility plays a major role in surviving under pressure, but what about the heat? And what happens when the organic matter gets devoured by bacteria?

 

1 hour ago, doushantuo said:

Proteomics,bioinfomatics ,biomimetics,biodiversity ,ocean acidification and sclerochronological studies have pretty much enriched the biomineralization literature....

Some possibly useful input:

 

addadiweinermolluscbiomineraliz62000000.pdf

 

 

cartwrightarxivcheca1211.5576.pdf

nudebiomineralizcollagen1-s0-main.pdf

 

immel

 

Thanks for the links. I'll have to check them through this evening since I'm off to work in a minute.

 

Greetings from the Lake of Constance. Roger

http://www.steinkern.de/

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Belemnites show up the preservational differences well. The rostra are (mostly) calcitic and often survive better than any other shells. Phragmocones were aragonitic and you know how scarce a well preserved one is.

Epirostra were probably largely aragonitic, often with calcite layers on the outside. This accounts for many being crushed, or simply lost when there was no calcite layer.

Tarquin

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16 hours ago, Ludwigia said:

That's really interesting with the nacre. I guess the flexibility plays a major role in surviving under pressure, but what about the heat? And what happens when the organic matter gets devoured by bacteria?

Nacre may be one of the reasons for some not being fossilized as well as the critters that do not have any nacre in thier shells.

Darwin said: " Man sprang from monkeys."

Will Rogers said: " Some of them didn't spring far enough."

 

My Fossil collection - My Mineral collection

My favorite thread on TFF.

 

 

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10 hours ago, ynot said:

Nacre may be one of the reasons for some not being fossilized as well as the critters that do not have any nacre in thier shells.

 

I understand that nacre can only survive under anaerobic circumstances. Is that true? Then it would depend on how much heat and pressure it is subject to during diagenesis whether or not it remains fossilized. Do you know which molluscs have no nacre in their shells? Or for that matter which have lost it but have still survived the fossilization process?

 

Greetings from the Lake of Constance. Roger

http://www.steinkern.de/

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I'm so glad this thread was started.I have just collected some Turritellidae and Venericardia fossils and was marvelling at the inside of the Turritellidae inner being so perfect considering its age.My Venericardia all appear similarly fossilized except one which has a partial different coating internally  and some more markings on the outer suggesting different conditions as collected few hundred yards down the beach.I will go back and look for more.

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3 hours ago, Ludwigia said:

 

I understand that nacre can only survive under anaerobic circumstances. Is that true? Then it would depend on how much heat and pressure it is subject to during diagenesis whether or not it remains fossilized. Do you know which molluscs have no nacre in their shells? Or for that matter which have lost it but have still survived the fossilization process?

I do not know.  I think the acidity of the surrounding rock may also play a part in whether it gets fossilized or not.  The appearance of nacre is the opalescence of the shells (mother of pearl), so the shells with nacre are the ones like abalone and nautilus shells.

Darwin said: " Man sprang from monkeys."

Will Rogers said: " Some of them didn't spring far enough."

 

My Fossil collection - My Mineral collection

My favorite thread on TFF.

 

 

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Both Aragonite and Calcite are forms calcium carbonate (CaCO3) which differ only in their crystalline structure.  The biological process which determines which form a mollusk will produce is encoded within it's genes.  Apparently aragonite is more water soluble than calcite as some deposits preserve only calcitic mollusks from such orders as Ostreida, Pectinida, and Limida.  These are primarily bivalve orders whereas gastropods are overwhelmingly aragonite.  Calcitic gastropod exceptions include Pterorytis LINK, Echphora and a few others.

 

Mike

"A problem solved is a problem caused"--Karl Pilkington

"I was dead for millions of years before I was born and it never inconvenienced me a bit." -- Mark Twain

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Higher energy environments require a stronger shell. We often see terms like above or below"wave base" to describe a paleo-environment.If you are above wave base you need to survive strong currents or even the occasional tumbling or rolling from storms or other strong currents.  Back in the Paleozoic brachiopods had a much wider range of shell thickness as they inhabited a greater variety of environments.

 

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Nacre is very strong and has been synthesized using nanotechnology for use in micro ceramics and other applications.Details of synthesis on this website:-

Screenshot_2017-02-04-19-41-20.png

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45 minutes ago, erose said:

Higher energy environments require a stronger shell. We often see terms like above or below"wave base" to describe a paleo-environment.If you are above wave base you need to survive strong currents or even the occasional tumbling or rolling from storms or other strong currents.  Back in the Paleozoic brachiopods had a much wider range of shell thickness as they inhabited a greater variety of environments.

 

 

That could very well explain why many deep sea molluscs are thin and aragonitic, while many coastal ones are thick and calcitic.

 

Greetings from the Lake of Constance. Roger

http://www.steinkern.de/

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Just seen on a geology site about a giant clam pearl weighing 75lbs 34 kg just imagine finding a fossil of one like that.They say its worth $100 million!

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There are many variables that go into shell preservation. The most fundamental is the molecular stability of a chemical regardless of environmental factors. Its not my field  but calcium carbonate shells have about a 55 million year potential lifespan. In theory, under perfect conditions, a modern shell and a fossil specimen up to that age would have the same shell chemistry.

 

Of course there are never perfect preservation conditions.

 

Freshwater shells are supposedly thinner because there is much less mineral concentrate in the water to grow thick shells.  Deep water for similar reasons and the lack of turbulence.  However, like many reasons, these are likely oft  repeated educated speculation. 

 

We need be aware that that shell chemistry is one of the variables that can create a real bias in the species that are preserved,  we collect and how we see the outward appearance.  Because a species is common at a site, doesn't mean it was dominant in that ecosystem.

 

 

 

 

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23 hours ago, Yvie said:

Just seen on a geology site about a giant clam pearl weighing 75lbs 34 kg just imagine finding a fossil of one like that.They say its worth $100 million!

Thanks for the tip. Very interesting!

 

19 hours ago, Canadawest said:

There are many variables that go into shell preservation. The most fundamental is the molecular stability of a chemical regardless of environmental factors. Its not my field  but calcium carbonate shells have about a 55 million year potential lifespan. In theory, under perfect conditions, a modern shell and a fossil specimen up to that age would have the same shell chemistry.

 

Of course there are never perfect preservation conditions.

 

Freshwater shells are supposedly thinner because there is much less mineral concentrate in the water to grow thick shells.  Deep water for similar reasons and the lack of turbulence.  However, like many reasons, these are likely oft  repeated educated speculation. 

 

We need be aware that that shell chemistry is one of the variables that can create a real bias in the species that are preserved,  we collect and how we see the outward appearance.  Because a species is common at a site, doesn't mean it was dominant in that ecosystem.

 

 

 

 

So what happens to the chemistry of shells which are older than 55 million years? Are you talking about the aragonite ones which are then transformed into calcite? So there is more calcium carbonate in the upper part of the water column than the lower. Your last comment is also intriguing.

 

Greetings from the Lake of Constance. Roger

http://www.steinkern.de/

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2 hours ago, ynot said:

What about the bearpaw formation in Montana where the nacre is still present in the cretaceous deposit.

 

 

It would depend on the specifics of the chemical recipe of the locale and species.  How much calcite, silica and trace elements of  magnesium, copper, iron, etc. Some calcite would have been in shell formation with the aragonite...some after deposition.   

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3 hours ago, Ludwigia said:

So what happens to the chemistry of shells which are older than 55 million years? Are you talking about the aragonite ones which are then transformed into calcite? So there is more calcium carbonate in the upper part of the water column than the lower. Your last comment is also intriguing.

 

 

 

A whole bunch of stuff can happen. What minerals are washing through the deposits, etc.  Carbonates, calcite...?  One weird thing that happens when there are distinct sedimentary layers is that minerals of young layers of fossils can seep into underlying older layers. This can 'mess up' trying to age material unless the actual index foseils are known.  There can be a bit of a tension between old paleontologists that study fossils and geologists that use chemical analysis in aging deposits.

 

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32 minutes ago, Canadawest said:

 

 

It would depend on the specifics of the chemical recipe of the locale and species.  How much calcite, silica and trace elements of  magnesium, copper, iron, etc. Some calcite would have been in shell formation with the aragonite...some after deposition.   

What I meant is that the nacre of the shells is still present in the fossils from this cretaceous deposit.

Darwin said: " Man sprang from monkeys."

Will Rogers said: " Some of them didn't spring far enough."

 

My Fossil collection - My Mineral collection

My favorite thread on TFF.

 

 

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