Editing Nacre (section)

==Physical characteristics==

=== Structure and appearance===
{{Further|Structural coloration}}
[[File:Nacre microscopic structure.png|thumb|upright=1.2|Schematic of the microscopic structure of nacre layers]]
[[File:Bruchfläche eines Perlmuttstücks.JPG|thumb|upright=1.2|Electron microscopy image of a fractured surface of nacre]]
{{biomineralization sidebar|exoskeletons}}

Nacre is composed of hexagonal platelets, called tablets, of [[aragonite]] (a form of [[calcium carbonate]]) 10–20&nbsp;[[μm]] wide and 0.5&nbsp;μm thick arranged in a continuous parallel [[wikt:lamina|lamina]].<ref name="doi10.1016/j.jsb.2005.09.009"/> Depending on the species, the shape of the tablets differs; in ''[[Pinna (bivalve)|Pinna]]'', the tablets are rectangular, with symmetric sectors more or less soluble. Whatever the shape of the tablets, the smallest units they contain are irregular rounded granules.<ref>{{Cite book|title=Biominerals and fossils through time|last=Cuif J.P. Dauphin Y., Sorauf J.E.|date=2011|publisher=Cambridge University Press|isbn=9780521874731|location=Cambridge|oclc=664839176}}</ref> These layers are separated by sheets of organic matrix (interfaces) composed of [[Elasticity (physics)|elastic]] [[biopolymers]] (such as [[chitin]], [[Lustrin A|lustrin]] and [[silk]]-like [[protein]]s).

Nacre appears [[Iridescence|iridescent]] because the thickness of the aragonite platelets is close to the wavelength of visible [[light]]. These structures [[interference (wave propagation)|interfere]] constructively and destructively  with different wavelengths of light at different viewing angles, creating [[Structural coloration|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.<ref name='Metzler2007'/><ref name='Olson2012'/>  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 inclined this way.<ref name=Checa2009/>

=== Mechanical properties ===
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&nbsp;[[Pascal (unit)|GPa]] and a yield stress of roughly 70 MPa (when dry).<ref name="Jackson1988" /> 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, spanning multiple length sizes, greatly increases its [[toughness]], making it almost as strong as [[silicon]].<ref name="Gim2019" /> The mineral–organic interface results in enhanced resilience and strength of the organic interlayers.<ref name="r2" /><ref name="r3" /><ref name="r4" /> The interlocking of bricks of nacre has large impact on both the deformation mechanism as well as its toughness.<ref name="r1" />  [[Tensile testing|Tensile]], [[Shear stress|shear]], and compression tests, [[Weibull distribution|Weibull]] analysis, [[nanoindentation]], and other techniques have all been used to probe the mechanical properties of nacre.<ref name=":0">{{Cite journal |last1=Sun |first1=Jiyu |last2=Bhushan |first2=Bharat |date=2012-08-14 |title=Hierarchical structure and mechanical properties of nacre: a review |journal=RSC Advances |language=en |volume=2 |issue=20 |pages=7617–7632 |doi=10.1039/C2RA20218B |bibcode=2012RSCAd...2.7617S |issn=2046-2069|doi-access=free }}</ref> Theoretical and computational methods have also been developed to explain the experimental observations of nacre's mechanical behavior.<ref>{{Cite journal |last1=Ji |first1=Baohua |last2=Gao |first2=Huajian |date=2004-09-01 |title=Mechanical properties of nanostructure of biological materials |url=https://www.sciencedirect.com/science/article/pii/S0022509604000705 |journal=Journal of the Mechanics and Physics of Solids |language=en |volume=52 |issue=9 |pages=1963–1990 |doi=10.1016/j.jmps.2004.03.006 |bibcode=2004JMPSo..52.1963J |issn=0022-5096}}</ref><ref name=":1">{{Cite journal |last1=Okumura |first1=K. |last2=de Gennes |first2=P.-G. |date=2001-01-01 |title=Why is nacre strong? Elastic theory and fracture mechanics for biocomposites with stratified structures |url=https://doi.org/10.1007/s101890170150 |journal=The European Physical Journal E |language=en |volume=4 |issue=1 |pages=121–127 |doi=10.1007/s101890170150 |bibcode=2001EPJE....4..121O |s2cid=55616061 |issn=1292-8941}}</ref> Nacre is stronger under [[Compressive stress|compressive]] loads than [[tensile]] ones when the force is applied parallel or perpendicular to the platelets.<ref name=":0" /> As an oriented structure, nacre is highly [[Anisotropy|anisotropic]] and as such, its mechanical properties are also dependent on the direction.

A variety of toughening mechanisms are responsible for nacre's mechanical behavior. The [[adhesive force]] needed to separate the proteinaceous and the aragonite phases is high, indicating that there are molecular interactions between the components.<ref name=":0" /> In [[laminated]] structures with hard and soft layers, a model system that can be applied to understand nacre, the [[fracture]] energy and fracture strength are both larger than those values characteristic of the hard material only.<ref name=":1" /> Specifically, this structure facilitates crack deflection, since it is easier for the crack to continue into the [[Viscoelasticity|viscoelastic]] and compliant organic matrix than going straight into another aragonite platelet.<ref name=":0" /><ref name=":2">{{Cite journal |last1=Feng |first1=Q. L. |last2=Cui |first2=F. Z. |last3=Pu |first3=G. |last4=Wang |first4=R. Z. |last5=Li |first5=H. D. |date=2000-06-30 |title=Crystal orientation, toughening mechanisms and a mimic of nacre |journal=Materials Science and Engineering: C |language=en |volume=11 |issue=1 |pages=19–25 |doi=10.1016/S0928-4931(00)00138-7 |issn=0928-4931|doi-access=free }}</ref> This results in the [[Ductility|ductile]] protein phase deforming such that the crack changes directions and avoids the [[Brittleness|brittle]] ceramic phase.<ref name=":0" /><ref name=":3">{{Cite journal |last1=Grossman |first1=Madeleine |last2=Pivovarov |first2=Dmitriy |last3=Bouville |first3=Florian |last4=Dransfeld |first4=Clemens |last5=Masania |first5=Kunal |last6=Studart |first6=André R. |date=February 2019 |title=Hierarchical Toughening of Nacre-Like Composites |journal=Advanced Functional Materials |language=en |volume=29 |issue=9 |pages=1806800 |doi=10.1002/adfm.201806800 |s2cid=139307131 |issn=1616-301X|doi-access=free }}</ref> Based on experiments done on nacre-like [[Chemical synthesis|synthetic materials]], it is hypothesized that the compliant matrix needs to have a larger fracture energy than the [[elastic energy]] at fracture of the hard phase.<ref name=":3" /> [[Fiber pull-out]], which occurs in other ceramic [[composite material]]s,  contributes to this phenomenon.<ref name=":2" /> Unlike in traditional synthetic composites, the aragonite in nacre forms bridges between individual tablets, so the structure is not only held together by the strong [[adhesion]] of the ceramic phase to the organic one, but also by these connecting [[nanoscale]] features.<ref name=":2" /><ref name=":0" /> As plastic deformation starts, the [[mineral bridge]]s may break, creating small asperities that roughen the aragonite-protein interface.<ref name=":0" /> The additional friction generated by the asperities helps the material withstand shear stresses.<ref name=":0" /> In nacre-like composites, the mineral bridges have also been shown to increase the [[flexural strength]] of the material because they can transfer stress in the material.<ref>{{Cite journal |last1=Magrini |first1=Tommaso |last2=Moser |first2=Simon |last3=Fellner |first3=Madeleine |last4=Lauria |first4=Alessandro |last5=Bouville |first5=Florian |last6=Studart |first6=André R. |date=2020-05-20 |title=Transparent Nacre-like Composites Toughened through Mineral Bridges |url=http://dx.doi.org/10.1002/adfm.202002149 |journal=Advanced Functional Materials |volume=30 |issue=27 |pages=2002149 |doi=10.1002/adfm.202002149 |s2cid=219464365 |issn=1616-301X|hdl=20.500.11850/417234 |hdl-access=free }}</ref> Developing synthetic composites that exhibit similar mechanical properties as nacre is of interest to scientists working on developing stronger materials. To achieve these effects, researchers take inspiration from nacre and use synthetic ceramics and polymers to mimic the "[[Brickwork|brick-and-mortar]]" structure, mineral bridges, and other hierarchical features.

When dehydrated, nacre loses much of its strength and acts as a brittle material, like pure aragonite.<ref name=":0" /> The hardness of this material is also negatively impacted by dehydration.<ref name=":0" /> Water acts as a [[plasticizer]] for the organic matrix, improving its toughness and reducing its shear modulus.<ref name=":0" /> Hydrating the protein layer also decreases its [[Young's modulus]], which is expected to improve the fracture energy and strength of a composite with alternating hard and soft layers.<ref name=":1" />

The statistical variation of the platelets has a negative effect on the mechanical performance (stiffness, strength, and energy absorption) because statistical variation precipitates localization of deformation.<ref name="Abid2018" /> However, the negative effects of statistical variations can be offset by interfaces with large strain at failure accompanied by strain hardening.<ref name="Abid2018" /> On the other hand, the [[fracture toughness]] of nacre increases with moderate statistical variations which creates tough regions where the crack gets pinned.<ref name="Abid2019" /> But, higher statistical variations generates very weak regions which allows the crack to propagate without much resistance causing the fracture toughness to decrease.<ref name="Abid2019" /> Studies have shown that this weak structural defects act as dissipative topological defects coupled by an elastic distortion.<ref name="Beliaev2021" />

===Formation===
The process of how nacre is formed is not completely clear. It has been observed in ''[[Pinna nobilis]]'', where it starts as tiny particles (~50–80 nm) grouping together inside a natural material. These particles line up in a way that resembles fibers, and they continue to multiply.<ref name='Hovden2015'/> When there are enough particles, they come together to form early stages of nacre. The growth of nacre is regulated by organic substances that determine how and when the nacre crystals start and develop.<ref name=Jackson2010/> 

Each crystal, which can be thought of as a "brick", is thought to rapidly grow to match the full height of the layer of nacre. They continue to grow until they meet the surrounding bricks.<ref name='Checa2009'/> This produces the hexagonal close-packing characteristic of nacre.<ref name='Checa2009'/> The growth of these bricks can be initiated in various ways such as from randomly scattered elements within the organic layer,<ref name=r5/> well-defined arrangements of proteins,<ref name="doi10.1016/j.jsb.2005.09.009"/> or they may expand from mineral bridges coming from the layer underneath.<ref name='Schaffer1997'/><ref name='Checa2011'/> 

What sets nacre apart from fibrous aragonite, a similarly formed but brittle mineral, is the speed at which it grows in a certain direction (roughly perpendicular to the shell). This growth is slow in nacre, but fast in fibrous aragonite.<ref name=b1/>

A 2021 paper in ''[[Nature Physics]]'' examined nacre from ''[[Unio pictorum]]'', noting that in each case the initial layers of nacre laid down by the organism contained spiral defects. Defects that spiralled in opposite directions created distortions in the material that drew them towards each other as the layers built up until they merged and cancelled each other out. Later layers of nacre were found to be uniform and ordered in structure.<ref name=Beliaev2021/><ref>{{cite news |last=Meyers |first=Catherine |date=January 11, 2021 |title=How Mollusks Make Tough, Shimmering Shells |url=https://www.insidescience.org/news/how-mollusks-make-tough-shimmering-shells |work=Inside Science |access-date=June 9, 2021}}</ref>

===Function===
[[File:Fossil nautiloid shell with original iridescent nacre in fossiliferous asphaltic limestone.jpg|thumb|upright=1.2|Fossil [[nautiloid]] shell with original iridescent nacre in fossiliferous asphaltic limestone, [[Oklahoma]]. Dated to the [[Pennsylvanian (geology)|late Middle Pennsylvanian]], which makes it by far the oldest deposit in the world with aragonitic nacreous shelly fossils.<ref>{{Cite AV media|last=John|first=James St|title=Fossil nautiloid shell with original iridescent nacre in fossiliferous asphaltic limestone (Buckhorn Asphalt, Middle Pennsylvanian; Buckhorn Asphalt Quarry, Oklahoma, USA) 1|date=2007-07-31|url=https://www.flickr.com/photos/jsjgeology/15054496278/|access-date=2023-01-09|via=Flickr |type=photo}}</ref>]]
Nacre is secreted by the [[epithelial]] [[cell (biology)|cells]] of the [[Mantle (mollusc)|mantle tissue]] of various molluscs. The nacre is continuously deposited onto the inner surface of the shell, the iridescent ''nacreous layer'', commonly known as ''mother-of-pearl''. The layers of nacre smooth the shell surface and help defend the soft tissues against [[parasite]]s and damaging debris by entombing them in successive layers of nacre, forming either a blister [[pearl]] attached to the interior of the shell, or a free pearl within the mantle tissues. The process is called ''encystation'' and it continues as long as the mollusc lives.

===In different mollusc groups===
{{Further|Mollusc shell#Evolution}}

The form of nacre varies from group to group. In [[bivalves]], the nacre layer is formed of single crystals in a [[hexagonal close packed|hexagonal close packing]].  In [[gastropods]], crystals are [[twinned crystal|twinned]], and in [[cephalopods]], they are pseudohexagonal monocrystals, which are often twinned.<ref name=Checa2009/>