Debunking another widely held myth. Nacreous v Non-Nacreous

[SteveM]

Moderately or lowly/slightly are definitely acceptable. My usage of highly was for example, not exclusivity.

Quahog (Mercenaria mercenaria) are moderately nacreous. Geoduck (Panope generosa) are slightly nacreous.

Indeed, scallops are a better example of foliated calcite structures. I stand corrected as to conch pearls, those are crossed-lamellar, thus nacreous. To what degree, I'm not certain. Moderate to high perhaps? Thanks for pointing that out.

Once again we are dealing with ambiguity that defies understanding. Terraced or columnar aragonite platelets, carefully organized and secreted by the mantle, comprise nacre. It either is, or isn't, nacre. Crossed-lamellar is the anti-nacre, perhaps even the prototypical 'porcelainous' microstructure.

Are we two Titanics crossing in the night? I welcome all input towards a better understanding of these issues.

 

[CortezPearls]

Yes. Terms like porcelaineous describe appearance of pearls without implying structure. It's common knowledge that ceramics are created in a kiln at a high temperature. Prefixing "non" to a word should only be used in the context of it's opposite. Even in a lay context, it's redundant to suggest non-Martians like us use a term like that in any context other than living on Earth.

This thread actually applies to the "irritant" thing too, which I also find irritating. Irritation whether rashes or scratches are musculoskeletal, not exoskeletal factors. Molluscs don't have movement in the same way humans or other animals with locomotion. In humans et al, these may give rise to infections, where myostracial pearls are sterile in almost every case. In the case of periostracial pearls, there is a grain of truth in the grain of sand thing, but only in a minority of cases. Parasitic pearls certainly are not sterile. Encapsulation is likened to bridging when shells are cracked. It's purpose is to create a water tight barrier to prevent pathogens or parasites from infiltrating soft tissues, then to minimize stresses caused by friction and pressure. In those cases, irritation is clearly a secondary if not tertiary factor. The only time it may be primary is when objects are lodged in the extrapallial space (between the shell and the mantle) as is the case with cultured "mabe" pearls. These almost never give rise to loose pearls, instead ones fused to the shell in the same manner objects can be spray painted to a wall. Hence the reason no graft tissue is used in the aquaculture process. Smooth objects are used as nuclei. Angular objects don't cause irritation insomuch as perforation of adjacent soft tissues.

The gem labs really need to revisit both of these terms.

I agree. The issue is that most people have a tendency to "humanize" other life forms (anthropormorfism) because they cannot gauge the experience of another living being, except under their human experience, thus "irritation" seems obvious, when it is simply not.
It will requiere a special gemologist to succeed in this space.

 

[CortezPearls]

Moderately or lowly/slightly are definitely acceptable. My usage of highly was for example, not exclusivity.

Quahog (Mercenaria mercenaria) are moderately nacreous. Geoduck (Panope generosa) are slightly nacreous.

Indeed, scallops are a better example of foliated calcite structures. I stand corrected as to conch pearls, those are crossed-lamellar, thus nacreous. To what degree, I'm not certain. Moderate to high perhaps? Thanks for pointing that out.

Perhaps it will be easier if words are associated with percentages?
Slightly - Less than 5%
Lowly - From 5 to 10%
Moderately - 11 to 25%
Not really suggesting these should be the numbers...just an idea.

 

[SteveM]

Perhaps it will be easier if words are associated with percentages?
Slightly - Less than 5%
Lowly - From 5 to 10%
Moderately - 11 to 25%
Not really suggesting these should be the numbers...just an idea.

Are these percentages / degrees of nacreousness referring to the distribution of microstructures throughout the different regions and layers of a shell? If not, can we clarify how nacre, crossed lamellar, prismatic, etc can be secreted together as a homogenous amalgam?

 

[Lagoon Island Pearls]

Are these percentages / degrees of nacreousness referring to the distribution of microstructures throughout the different regions and layers of a shell? If not, can we clarify how nacre, crossed lamellar, prismatic, etc can all be mixed together as a homogenous matrix?

A very important point. Nacreous is a growth phase. It occurs between the prismatic and quiescent phases nine to eleven times a year according to lunar periods, hence layered. This phase involves elegant termination of calcium carbonate, whatever that may be. Although one could, there is no reason to describe it for purposes of this discussion.

As an analogy in fine carpentry (though not necessarily multi-layered), it does not matter whether it's urethane, shellac, lacquer, varnish, wax or spit. It's all a "finish".

Nacre is a finish and all shelled molluscs have it to varying degrees (even some annelids and corals). It's what we actually see when looking at pearls or shell linings. This is where the confusion stems, because the term non-nacreous was erroneously applied to pearls that do not present with rainbow iridescence and orient. Nacre can be almost perfectly clear and as such, we must not disqualify it.

 

[SteveM]

A very important point. Nacreous is a growth phase. It occurs between the prismatic and quiescent phases nine to eleven times a year according to lunar periods, hence layered. This phase involves elegant termination of calcium carbonate, whatever that may be. Although one could, there is no reason to describe it for purposes of this discussion.

As analogy in fine carpentry (though not necessarily multi-layered), it does not matter whether whether it's urethane, shellac, lacquer, varnish, wax or spit. It's all a "finish".

Nacre is a finish and all shelled molluscs have it to varying degrees (even some annelids and corals). It's what we actually see when looking at pearls or shell linings. This is where the confusion stems, because the term non-nacreous was erroneously applied to pearls that do not present with rainbow iridescence and orient. Nacre can be almost perfectly clear and as such, we must not disqualify it.

This is a very helpful observation. It is still not clear how we can speak of percentages or degrees of nacreousnous. Whether expressing as iridescence or not, nacre is nonetheless nacre.

"It either is, or isn't, nacre."

My conclusion here is that a crossed-lamellar pearl such as Conch that is known not to have an invisible nacreous layer may certainly be referred to as 'non-nacreous.'

Trusting that this precipice on the cliff of knowledge will entice further enlightenment!

 

[Lagoon Island Pearls]

My conclusion here is that a crossed-lamellar pearl such as Conch that is known not to have an invisible nacreous layer may certainly be referred to as 'non-nacreous.'

Crossed-lamellar is an elegant termination of aragonite between the Myo/periostracial and quiescent phases, thus interfacing the prismatic and nacreous phases to a great degree. The movement between these phases are gradual, not abrupt. In fact, this interlocking is the basis for exceptional shell strength. If anything it's more nacreous than mere calcitic lathes at the prismatic level.

Crossed-lamellar structure constitutes the bulk of the shells in Gastropods (the largest molluscan group) and in Bivalves (the second largest group), and it is the only microstructure found in Scaphopoda (Carter, 1990, Almagro et al., 2016). Therefore, more than 90% of the species in Mollusca contain this structure within their shells (Almagro et al., 2016). Another interesting phenomenon that needs to be noted is that crossed-lamellar structure contains a fairly low organic content, e.g., only ~1% in S. gigas (Rodriguez-Navarro et al., 2012), compared to ~5% in nacre (Addadi and Weiner, 1992). It is probably metabolically cheap to produce; even so, it still exhibits the highest nominal fracture toughness among all Molluscan microstructures (Kuhn-Spearing et al., 1996). For example, the fracture resistance of dry shell with a crossed-lamellar structure can reach 4 kJ/m2 in the flexural tests (Kuhn-Spearing et al., 1996), while it is only ~0.4 kJ/m2 for the dry samples and ~1.8 kJ/m2 for the wet samples of nacreous shells (Jackson et al., 1990).

 

[SteveM]

Crossed-lamellar is an elegant termination of aragonite between the Myo/periostracial and quiescent phases, thus interfacing the prismatic and nacreous phases to a great degree. The movement between these phases are gradual, not abrupt. In fact, this interlocking is the basis for exceptional shell strength. If anything it's more nacreous than mere calcitic lathes at the prismatic level.

Crossed-lamellar structure constitutes the bulk of the shells in Gastropods (the largest molluscan group) and in Bivalves (the second largest group), and it is the only microstructure found in Scaphopoda (Carter, 1990, Almagro et al., 2016). Therefore, more than 90% of the species in Mollusca contain this structure within their shells (Almagro et al., 2016). Another interesting phenomenon that needs to be noted is that crossed-lamellar structure contains a fairly low organic content, e.g., only ~1% in S. gigas (Rodriguez-Navarro et al., 2012), compared to ~5% in nacre (Addadi and Weiner, 1992). It is probably metabolically cheap to produce; even so, it still exhibits the highest nominal fracture toughness among all Molluscan microstructures (Kuhn-Spearing et al., 1996). For example, the fracture resistance of dry shell with a crossed-lamellar structure can reach 4 kJ/m2 in the flexural tests (Kuhn-Spearing et al., 1996), while it is only ~0.4 kJ/m2 for the dry samples and ~1.8 kJ/m2 for the wet samples of nacreous shells (Jackson et al., 1990).

Yes the low organic component of crossed-lamellar shell material has frustrated all our attempts at genetic extraction for pearl species confirmation.

But the issue of 'degree of nacreousness' is still eluding us. What percentage should be assigned to crossed-lamellar shell material, assuming that the aragonite-platelet model is not the only one for nacre?

 

[Lagoon Island Pearls]

Yes the low organic component of crossed-lamellar shell material has frustrated all our attempts at genetic extraction for pearl species confirmation.

But the issue of 'degree of nacreousness' is still eluding us. What percentage should be assigned to crossed-lamellar shell material, assuming that the aragonite-platelet model is not the only one for nacre?

Yes, absolutely. Interestingly enough, the aformentioned quote suggests a difference between cross-lamellar and nacreous, but only to the degree of structure, not finish.

I would still assert the final step in the third phase is nacreous. The aragonitic platelet model is not exclusive to nacre.

 

[SteveM]

Yes, absolutely. Interestingly enough, the aformentioned quote suggests a difference between cross-lamellar and nacreous, but only to the degree of structure, not finish.

I would still assert the final step in the third phase is nacreous. The aragonitic platelet model is not exclusive to nacre.

OK so per Douglas' suggestion above, what would be the percentage?

The aragonite platelet model of nacre would be differentiated by its organic component. If only our unidentified porcelainous pearls had such a layer!

 

[Lagoon Island Pearls]

OK so per Douglas' suggestion above, what would be the percentage?

The aragonite platelet model of nacre would be differentiated by its organic component. If only our unidentified porcelainous pearls had such a layer!

That would be a huge range over the phyla of the animal kingdom. At the end of the day it doesn't matter because lowly nacreous is still nacreous.

I agree we can split hair when it comes to the degree or arrangement of aragonite present in fewer species. I still assert cross-lamellar is nacre albeit non-tabular. Both are still orthorhombic nonetheless. Calcium carbonate is commonly dendritic or pseudo-hexagonal, but can also be acicular, tabular or prismatic.

However, when we look at the bigger picture, most species which were once lumped into a non-nacreous category are unquestionably nacreous.

 

[SteveM]

That would be a huge range over the phyla of the animal kingdom. At the end of the day it doesn't matter because lowly nacreous is still nacreous.

I agree we can split hair when it comes to the degree or arrangement of aragonite present in fewer species. I still assert cross-lamellar is nacre albeit non-tabular. Both are still orthorhombic nonetheless. Calcium carbonate is commonly dendritic or pseudo-hexagonal, but can also be acicular, tabular or prismatic.

However, when we look at the bigger picture, most species which were once lumped into a non-nacreous category are unquestionably nacreous.

At this point we must agree to disagree. I will be sticking with the terraced/columnar organic secretion models that result in iridescent phenomenon. But will happily campaign for the true-pearl status of 'porcelanous/porcellanous/porcelainous' specimens.

 

[Lagoon Island Pearls]

At this point we must agree to disagree. I will be sticking with the terraced/columnar organic secretion models that result in iridescent phenomenon. But will happily compaign on behalf of the true-pearl status of 'porcelanous/porcellanous/porcelainous' specimens.

And that's a fair point, which would imply nacre which is non-terraced/columnar.

I would still submit all are orthorhombic terminations in a proteinaceous matrix to the same end... nacre.

 

[SteveM]

And that's a fair point, which would imply nacre which is non-terraced/columnar.

I would still submit all are orthorhombic terminations in a proteinaceous matrix to the same end... nacre.

If there is anything in the scientific literature that concisely and transparently defines nacre in such a manner I would want to be the first to see it.

 

[Lagoon Island Pearls]

If there is anything in the scientific literature that concisely and transparently defines nacre in such a manner I would want to be the first to see it.

Nacre is primarily an immediate barrier between hard and soft tissues. Secondarily, once laid up by successive layers it's becomes part of a structure.

Two markedly different roles in biology, where one morphs into another. We ought not ignore the root cause to suit the overall effect.

 

[SteveM]

Nacre is primarily an immediate barrier between hard and soft tissues. Secondarily, once laid up by successive layers it's becomes part of a structure.

Two markedly different roles in biology, where one morphs into another. We ought not ignore the root cause to suit the overall effect.

The equivalencies you are drawing make common usage of the term 'nacreous' as obsolete as the term 'non-nacreous.' Who's going to rewrite the book?

 

[Lagoon Island Pearls]

The equivalencies you are drawing make common usage of the term 'nacreous' as obsolete as the term 'non-nacreous.' Who's going to rewrite the book?

Perhaps, but if nacreous goes poof, so does non-nacreous by default.

We build dams from concrete and steel. Beavers build dams from sticks and mud. Nature builds dams from rocks. Let's say I built a dam from a random material... lets say glass, does this mean it's no longer a dam but something else? No, that would be absurd.

We do agree on the main premise. Largely being nacre is aragonitic. To suggest nacre is exclusive to other characteristics of aragonite except one, is equally absurd.

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Natural arrangement of fiber-like aragonites and its impact on mechanical behavior of mollusk shells​

The crossed-lamellar microstructure in mollusk shells, originally identified by Bøgild in 1930, is one of the most common types in shells, which can be found in more than 90% of the species; for example, it is the majority both in gastropods and bivalves, and the only microstructure in scaphopoda (Carter, 1990; Almagro et al., 2016). Such a crossed-lamellar structure can be classified as platelet-like or fiber-like structure based on the shapes of building blocks (Li et al., 2017). The microstructures and the related mechanical properties of Stombus gigas or Busycon carica shell with three fiber-like crossed-lamellar macrolayers have drawn considerable attention of investigators (Li et al., 2013, 2015, 2019; Romana et al., 2013; Osuna-Mascaró et al., 2014; Shin et al., 2016; Salinas et al., 2017; Li and Li, 2019), where three macrolayers are arranged in a 0°/90°/0° mode. The existing research findings have demonstrated that the mechanical properties of such a 0°/90°/0° mode for crossed-lamellar structure are dependent upon the existing of organic matrix (Menig et al., 2001; Osuna-Mascaró et al., 2014; Li et al., 2015)

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Clearly the author draws a conclusion similar, not exclusive to other aragonitic structures. They also define it's presence in 90% of molluscan species.

 

[SteveM]

I like it. I also see that the bulk of the cited research post-dates my time at the SEM in Granada, 2011-2013. It still seems a stretch to apply the term 'nacre', so broadly. Neither do you see such reference in the literature. You would be better than me at finding a more generic term for the soft tissue/shell barrier, leaving the predominant understanding of nacre intact.

 

[Lagoon Island Pearls]

I like it. I also see that the bulk of the cited research post-dates my time at the SEM in Granada, 2011-2013. It still seems a stretch to apply the term 'nacre', so broadly. Neither do you see such reference in the literature. You would be better than me at finding a more generic term for the soft tissue/shell barrier, leaving the predominant understanding of nacre intact.

Again agreed mainly because of the conflation between the two terms. The burning question being, which came first, the chicken or the egg.

We also agree on the majority of cases where once mythical non-nacreous objects are indeed nacreous in terms of structure. Clams pearls for certain. Even most gastropod pearls are nacreous. I'm not closing the book on conch though, because I'm not certain they are absent of the tabular platelet model in a minor way... especially at the myostracum as the OP suggests. I will revisit this after this post.

However I strongly disagree the nuances of cross-lamellar structures being an exception to nacre. A brick wall is a vertically offset structure bound together by mortar. If I were to build a structure using the cross-lamellar model 00-90-00, it would be exponentially stronger, but it's still a brick wall.

The burden to change something which is common knowledge in all molluscs (a barrier) for a detail that may or may not be nuanced (a structure) in the alternative is misplaced, especially knowing cross-lamellar construction occurs in 90% of otherwise nacreous molluscs, it is infinitely simpler to agree it's nacre.

 

[Lagoon Island Pearls]

Ok, revisiting the OP, this:
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Similar to Bivalvia, shell structures underlying the myostracum layer in the Polyplacophora, Gastropoda, Scaphopoda, and Cephalopoda exhibited a diverse display. Columnar nacreous were present in both Gastropoda and Cephalopoda, while composite prismatic, crossed lamellar, and simple lamellar were also found as the bulk microstructure of tested shells. Moreover, both calcite and aragonite were found to be the composition of the main part of the shell layers.
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They're all nacreous to some degree.