Aragonite vs Calcite

[CDarwin]

Throughout the fossil record there is a tendency for shelled invertebrates to replace the calcite with the mineral aragonite (orthorhombic calcium carbonate). This is something of a bane for paleontologists because calcite is much less likely to dissolve in water and so thus is more likely to fossilize. That is why the fossil record for Paleozoic corals is better than the fossil record for Cenozoic corrals, the older coral is all made of calcite where the younger corals are aragonite. There are similar tendencies among mollusks.

 

Why was this? The only reason I can think of is that the aragonite would be easier to precipitate out of water. Are there some properties of aragonite of which I'm not aware that makes it better as a shell constituent?

[geoguy]

I'm a paleontologist and need clarification of your opening statement. what do you mean by replace calcite? among the phyla, echinodermata have cacite skeletons but just about everything else from brachiopoda to mollusca to chordata have calcium in the form of aragogonite, c. phospahte, etc. there is no replaced calcite. Sometimes the reverse is the case and calcite crystals are the replacement infill mineral in fossilization.

 

Re Paleozoic vs Cenozoic (and Mesozoic) corals. although these corals are often lumped together, many of the Coral researcher s (Bamber, Pedder,etc.) don't tfind any relation between the two groups. Mesozoic and Cenozoic corals are not thought to be descendents of Paleozoic corals any more than any other member of the coelenterata phyla.

 

Shell material from brachiopods, corals, mollusca, cephalopoda is not all that unstable that it is easily dissoved (in the right conditions), aragonite is stable for around 55 million years ...and calcium phosphate is extremely stable. Mollusca fosils from the Paleocene (55 million) and younger are usually 'the real thing' and not mineral replacements. Calcium phosphate fossils are even older before breakdown.

 

as for your question, I don't know why calcium in the form of aragonate is so commonly used in shell structure. I'm a paleontologist coming to the field via geology (like most who study paleozoic fossils) and not from the biology side of the science. The answer to your question is more likely found not in paleontology studies but mallacological papers.

[John Cuthber]

One form (calcite) is stable at low temperatures and the other (aragonite) at high temperatures. Whichever form is more stable is less soluble at any given temperature. Since the transition temperature isn't excessive you can get both forms in shells. The proteins that the organisms add also affect the crystalisation.

[Ophiolite]

This may partially answer your question:

 

Falini, G. et al

Control of Aragonite or Calcite Polymorphism by Mollusk Shell Macromolecules Science 5 January 1996: Vol. 271. no. 5245, pp. 67 - 69

 

Many mineralizing organisms selectively form either calcite or aragonite, two polymorphs of calcium carbonate with very similar crystalline structures. Understanding how these organisms achieve this control has represented a major challenge in the field of biomineralization. Macromolecules extracted from the aragonitic shell layers of some mollusks induced aragonite formation in vitro when preadsorbed on a substrate of -chitin and silk fibroin. Macromolecules from calcitic shell layers induced mainly calcite formation under the same conditions. The results suggest that these macromolecules are responsible for the precipitation of either aragonite or calcite in vivo.

Source: http://www.sciencemag.org/cgi/content/short/271/5245/67

[CDarwin]

I'm a paleontologist and need clarification of your opening statement. what do you mean by replace calcite? among the phyla, echinodermata have cacite skeletons but just about everything else from brachiopoda to mollusca to chordata have calcium in the form of aragogonite, c. phospahte, etc. there is no replaced calcite. Sometimes the reverse is the case and calcite crystals are the replacement infill mineral in fossilization.[/Quote]

 

I mean older groups have calcite while younger groups have aragonite.

 

This is just a trend I noticed while I was reading through a book I got on fossil invertebrates. If I'm wrong about it (which is more than possible), just tell me. I only know vaguely what I'm talking about with primate evolution.

 

This may partially answer your question:

 

Falini, G. et al

Control of Aragonite or Calcite Polymorphism by Mollusk Shell Macromolecules Science 5 January 1996: Vol. 271. no. 5245, pp. 67 - 69

 

Many mineralizing organisms selectively form either calcite or aragonite, two polymorphs of calcium carbonate with very similar crystalline structures. Understanding how these organisms achieve this control has represented a major challenge in the field of biomineralization. Macromolecules extracted from the aragonitic shell layers of some mollusks induced aragonite formation in vitro when preadsorbed on a substrate of -chitin and silk fibroin. Macromolecules from calcitic shell layers induced mainly calcite formation under the same conditions. The results suggest that these macromolecules are responsible for the precipitation of either aragonite or calcite in vivo.

Source: http://www.sciencemag.org/cgi/content/short/271/5245/67

 

Thank you for finding that, it is interesting from a mechanical standpoint, but it still doesn't really tell me the selective why.

[geoguy]

I'll completely defer to you on primate evolution.

 

Here in Alberta I've collected Latest Cretaceous and Paleocene mammal teeth. Unlike the rich deposits of dinos, champsosaurs, crocs, etc. here, 'mammal' fossils are more or less just teeth...and teeth that don't look like much to the untrained eye (my eyes are untrained). I can 'find' the teeth, but after that defer to whomever. I've found primate teeth but but can't say I ever recognized them from all the other orders that are found. The teeth are usually not much bigger than heads of pins. Except at particular sites where there is sieving for mammal teeth,one might not find more than one or two mammal teeth a collecting season (in contrast to hundreds of teeth from other vertebrate classes). Even when 'probable' primate teeth are found there's not always agreement to the order.

 

Unfortunately a lot of the research that went into early mammal orders was done prior to the 1970's or so and, like most areas of paleontolgy, is more of a past pursuit than a current one. The expertise of a lot of the seasoned researchers is gone.

[lucaspa]

I'll completely defer to you on primate evolution.

 

Here in Alberta I've collected Latest Cretaceous and Paleocene mammal teeth.

 

The mineral in teeth (and bones) is hydroxyapetite. Ca10(PO4)6OH2. Very different from any form of calcium carbonate.

[geoguy]

The mineral in teeth (and bones) is hydroxyapetite. Ca10(PO4)6OH2. Very different from any form of calcium carbonate.

 

??? In response to my saying what?

 

Enamel, dentin are all composite materials composed of a hard mineral - hydroxyapatite (HA) and protein + water HA is Calcium Phosphate (apatite) where some of the phosphates are replaced by hydroxyls. . We use acetic acid to separate teeth such as those from paleozoic sharks from limestone matrix. The calcium phosphate is not dissolved by this acid. Other material other thatn teeth, such as those of the class inarticulate brachiopods are also calcium phosphate.

[CDarwin]

I don't think everyone is talking about the same thing.

[lucaspa]

??? In response to my saying what?

 

It was rigth there in my quote of your post:

"Here in Alberta I've collected Latest Cretaceous and Paleocene mammal teeth. "

 

The OP is talking shells and you are talking teeth. Separate things with different types of mineral.

 

We use acetic acid to separate teeth such as those from paleozoic sharks from limestone matrix. The calcium phosphate is not dissolved by this acid.

 

That will work, but be advised that ANY acid will dissolve HA. What you are relying on is that the limestone will dissolve in weak acid MUCH faster than will the HA. However, some HA will also be dissolved. The typical way to demineralize bones and teeth is 0.6 N HCl. However, a slower but gentler (to the proteins in the organic matrix) is citric acid (or acetic acid) + EDTA.

 

Other material other thatn teeth, such as those of the class inarticulate brachiopods are also calcium phosphate.

 

I'm not sure what you are trying to say here. The information I have seen does say that inarticular brachiopods have shells of calcium phosphate (not hydroxyapetite, yet a 3rd mineral) while all other brachiopods have shell of calcium carbonate.

 

"All of the previously viewed brachiopods are called articulate brachiopods---that is, they open their shells with a system of teeth and sockets that serve as a hinge that is controlled by adductor and diductor muscles that close and open the valves when the need arose. Inarticulate brachiopods, however, have no teeth or sockets and open their shells by an entirely muscular and ligament process. Articulate brachiopods have calcium carbonate shells whereas inarticulate brachiopods have calcium phosphate shells. " http://csd.unl.edu/fossils/nebrinvertstull.asp

 

It appears that the "teeth and sockets" of articulate brachiopod shells are also calcium carbonate. If you have different information I would, of course, be interested in hearing it.

[CDarwin]

It was rigth there in my quote of your post:

The OP is talking shells and you are talking teeth. Separate things with different types of mineral.

 

Geoguy was just saying as an aside that he doesn't know much about teeth. It didn't really have anything to do with my original post. I think that's what he meant, at least.

[lucaspa]

Geoguy was just saying as an aside that he doesn't know much about teeth. It didn't really have anything to do with my original post. I think that's what he meant, at least.

 

As I read Geoguy, he was saying he didn't know much about shells but instead collected dino and mammalian teeth: "Here in Alberta I've collected Latest Cretaceous and Paleocene mammal teeth. ... I can 'find' the teeth, but after that defer to whomever."

 

I'm not sure why he posted on this thread, since the post started with "I'll completely defer to you on primate evolution." and this thread has nothing to do with primate evolution. Perhaps he got the wrong thread and wanted your thread on the origin of primates?

[CDarwin]

I'm not sure why he posted on this thread, since the post started with "I'll completely defer to you on primate evolution." and this thread has nothing to do with primate evolution. Perhaps he got the wrong thread and wanted your thread on the origin of primates?

 

I said something about primate evolution.

 

I only know vaguely what I'm talking about with primate evolution.

[lucaspa]

I said something about primate evolution.

 

AH! Yes, I missed that sentence. Now I'm even more confused about what Geoguy was trying to communicate. Just have to wait until he clarifies.

[McCrunchy]

I'm a physical chemist, I was wondering about the evolutionary advantage of calcite over aragonite as well. From what I've understood :

 

A) shells can either be constituted

1) entirely of calcite

2) entirely of aragonite

3) aragonite with calcite on top (exposed to the environment), with a sharp separation line

4) multiply-layered calcite/aragonite laminate structure

 

(see e.g. http://222.aslo.org/lo/toc/vol_18/issue_2/0326.pdf)

 

B) in an experiment (Falini et al., see above) conducted on one mixed shell species (type 3) above), it was demonstrated that proteins secreted by the animal specify what polymorph will be grown in what region of the shell.

 

C) small temperature/Ca2+/ pH/carbonate content changes can, in artificially grown CaCO3, have one of the polymorph being nucleated preferentially over the other.

 

From B), we might infer that, for the particular specimen on which the effect was demonstrated, aragonite doesn't simply transform to calcite as a result of aging, or environmental parameters © [ Of course the proteins that nucleated calcite in this experiment could have "aged" as well, and thus possibly just lost their aragonite-nucleating properties - but let's ignore this possibility for the time being]. Hence there does seem to be an evolutionary advantage in building aragonite shell, and also in building type 3) shells.

 

 

What I don't know :

 

- what is the geographical distribution of the polymorphs in different oceans ?

 

- what is the time distribution of calcite over aragonite ? You were saying that aragonite seems to be more present in younger groups - we need to be sure about that.

 

- in mixed aragonite/calcite type shells, how does the ratio of the polymorphs evolve from birth on ?

 

- many other things ... (I'm only a PhD, no white beard yet !)

 

 

In any case, if I were a mollusk, I'd be a type 3) shell, cause:

 

- aragonite has 3.5 to 4 hardness on Mohs scale, which could give me better chances of surviving if crushed by shark's teeth (HA : hardness 5) than calcite (hardness : 3).

 

- calcite doesn't dissolve as readily as aragonite, so I'll put it on my outer shell, to have better chances of withstanding ocean carbonate content fluctuations.

 

- calcite is maybe more attractive to female oysters than aragonite ? Only they know the difference ....

 

 

I'd be glad to hear opinions on the topic ( especially from anyone more knowledgeable than I am).

 

Nicolas, alias

McCrunchy

[nathan3011]

I'm a physical chemist, I was wondering about the evolutionary advantage of calcite over aragonite as well. From what I've understood :

 

A) shells can either be constituted

1) entirely of calcite

2) entirely of aragonite

3) aragonite with calcite on top (exposed to the environment), with a sharp separation line

4) multiply-layered calcite/aragonite laminate structure

 

(see e.g. http://222.aslo.org/lo/toc/vol_18/issue_2/0326.pdf)

 

B) in an experiment (Falini et al., see above) conducted on one mixed shell species (type 3) above), it was demonstrated that proteins secreted by the animal specify what polymorph will be grown in what region of the shell.

 

C) small temperature/Ca2+/ pH/carbonate content changes can, in artificially grown CaCO3, have one of the polymorph being nucleated preferentially over the other.

 

From B), we might infer that, for the particular specimen on which the effect was demonstrated, aragonite doesn't simply transform to calcite as a result of aging, or environmental parameters © [ Of course the proteins that nucleated calcite in this experiment could have "aged" as well, and thus possibly just lost their aragonite-nucleating properties - but let's ignore this possibility for the time being]. Hence there does seem to be an evolutionary advantage in building aragonite shell, and also in building type 3) shells.

 

 

What I don't know :

 

- what is the geographical distribution of the polymorphs in different oceans ?

 

- what is the time distribution of calcite over aragonite ? You were saying that aragonite seems to be more present in younger groups - we need to be sure about that.

 

- in mixed aragonite/calcite type shells, how does the ratio of the polymorphs evolve from birth on ?

 

- many other things ... (I'm only a PhD, no white beard yet !)

 

 

In any case, if I were a mollusk, I'd be a type 3) shell, cause:

 

- aragonite has 3.5 to 4 hardness on Mohs scale, which could give me better chances of surviving if crushed by shark's teeth (HA : hardness 5) than calcite (hardness : 3).

 

- calcite doesn't dissolve as readily as aragonite, so I'll put it on my outer shell, to have better chances of withstanding ocean carbonate content fluctuations.

 

- calcite is maybe more attractive to female oysters than aragonite ? Only they know the difference ....

 

 

I'd be glad to hear opinions on the topic ( especially from anyone more knowledgeable than I am).

 

Nicolas, alias

McCrunchy

 

I was going to reply with something but clearly you are far more knowledgable than myself in this area so i thought that there isn't anything more that i could tell you that you don't already know!!

[Theophrastus]

Having read only a sparse amount of content initially, I'm actually very interested in this thread, given a previous interest in mallacology that hasn't quite receeded. I hope that if anyone finds an answer, that they post it, please.

I really doubt this is a whole answer, but I know that aragonite over time, deteriorates in ordinary conditions to form calcite, and as such may be a valid reason for why there are less older aragonite- based shells. This deterioration would occur following the mollusk's death, or the mollusk's rejection of the shell, as there would be no living epithelial mantle to upkeep (is that even a word?) shell growth. I'm not sure however whether this is a solution, as I have heard that the material, retains its initial (aragonite's) needle- like (or pseudohexagonal as it is in some cases) crystal structure. Similarly, the time required for such a process, I am unsure of. My greatest hopes that we achieve our goal, of an answer, and a thanks to Cdarwin for the interesting question!

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I've done some further research into shell structure, and found that McCrunchy's initial guess seems to be correct, as most shells, while bearing a calcite base, generally formed in crystals growing perpendicular to the mantle, the outer layer, consisting of conchiolin, an organic matrix, is generally structured parallel to the mantle, preparing the necessary conditions, to allow for aragonite, not calcite growth, on the outside surface. Together, they form the periostracum, the strong outer layer, of the shell.

 

This of course, can be seen as optimum, as McCrunchy said, due to aragonite's greater strength, and protection is of course one of the main functions of the shell.

In terms of strength, the conchiolin also allows for strong bonding with nacre, or mother- of- pearl, on the inside of the shell, and regardless, the result is one of a strong matrix for the shell. The iridescent colour of the final mother- of- pearl, is in fact due to optical interference between the layers, a testament to the structure.

 

I'm quite sure that it has been proven that calcite, due to the oceanic conditions, 100's of millions of years ago was the primary material for the shell but it seems that it was at some point, replaced in key regions by aragonite, due to its more optimum qualities. I'm not sure about whether this is a complete trend, or whether the change is dependent upon geographic region, and other key factors, which it probably is.

 

In terms of an evolutionary perspective, I've found that many purely calcite based shells, result from the lack of this outer layer, the periostracum, for various reasons, which as we have discussed, is based mostly of conchiolin and crystalised aragonite. However, in my research, while it seems that a calcite/ aragonite shell is more beneficial, the periostracum is not rather common amongst mollusc shells. This I think I can attribute to a similar fact to why calcite develops more readily in nature: it is simply easier.

 

While aragonite is more conservative in the conditions it requires for formation, generally formed geologically as a product of rare conditions, as it is unstable at ordinary temperatures and pressures.

Similarly, the mollusc must recreate these circumstances, for aragonite to form, whereas calcite, is a more common polymorph, as it is the one that most regularly forms, under ordinary conditions.

 

Purely aragonite- based shells also exist, such as in the class, polyplacophora, which is particularly well- known for its hard interlinked shell plates, which protect its otherwise fragile body. As McCrunchy said, such can be seen as evolution on the mollusc's part, due to the greater strength of aragonite, however, just as before, due to the greater simplicity of the production of a calcite layer, the calcite- based shell structure remains more common. Any additional detail to what I've found is most welcome, particularly from the perspective of reasoning for this evolutionary process. Any more information, in regards to the reasoning underlying the general majority of species, utilising a calcite based shell in spite of non- optimum conditions is also welcome, as are any corrections to the above. Hope this help answer your question!

 

ps:In regards to my initial guess, I've realised that the problem is that while the calcite morphs into aragonite, various methods of analysis of the crystal structure can easily be used to attain what the initial calcium carbonate polymorph was, which of course yields my initial idea, null and void.

 

pps: For your bemusement and interest, I have several links to show where I'm coming from...

 

http://en.wikipedia.org/wiki/Mollusc_shell

 

http://en.wikipedia.org/wiki/Conchiolin

 

http://www.ncbi.nlm.nih.gov/pubmed/17540895

 

ppps: While you may hear that magnesium carbonate, and calcium and magnesium phosphate minerals also exist in shells, with incredibly rare exceptions, they are only found in trace amounts, and thus irrelevant to the question, as they do not compose the base structure. Only include these if they are vital in particular biochemical reactions (thanks)

Edited May 31, 2009 by Theophrastus
Consecutive posts merged.