Nacre is found in some of the most ancient lineages ofbivalves,gastropods, andcephalopods. However, the inner layer in the great majority ofmollusc shells isporcellaneous, not nacreous, and this usually results in a non-iridescent shine, or more rarely in non-nacreous iridescence such asflame structure as is found inconch pearls.
The outer layer of cultured pearls and the inside layer ofpearl oyster andfreshwater pearl mussel shells are made of nacre. Other mollusc families that have a nacreous inner shell layer include marine gastropods such as theHaliotidae, theTrochidae and theTurbinidae.
Nacre is composed of hexagonal platelets, called tablets, ofaragonite (a form ofcalcium carbonate) 10–20 μm wide and 0.5 μm thick arranged in continuous parallellamina.[2] Depending on the species, the shape of the tablets differs; inPinna, 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.[3] These layers are separated by sheets of organic matrix (interfaces) composed ofelasticbiopolymers (such aschitin,lustrin andsilk-likeproteins).
Nacre appearsiridescent because the thickness of the aragonite platelets is close to the wavelength of visiblelight. These structuresinterfere constructively and destructively with different wavelengths of light at different viewing angles, creatingstructural colours.
The crystallographicc-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, theb-axis points in the direction of shell growth, whereas in themonoplacophora it is thea-axis that is inclined this way.[6]
This mixture of brittle platelets and the thin layers of elastic biopolymers makes the material strong and resilient, with aYoung's modulus of 70 GPa and a yield stress of roughly 70 MPa (when dry).[7] 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 itstoughness, making it almost as strong assilicon.[8] The mineral–organic interface results in enhanced resilience and strength of the organic interlayers.[9][10][11] The interlocking of bricks of nacre has large impact on both the deformation mechanism as well as its toughness.[12]Tensile,shear, and compression tests,Weibull analysis,nanoindentation, and other techniques have all been used to probe the mechanical properties of nacre.[13] Theoretical and computational methods have also been developed to explain the experimental observations of nacre's mechanical behavior.[14][15] Nacre is stronger undercompressive loads thantensile ones when the force is applied parallel or perpendicular to the platelets.[13] As an oriented structure, nacre is highlyanisotropic and as such, its mechanical properties are also dependent on the direction.
A variety of toughening mechanisms are responsible for nacre's mechanical behavior. Theadhesive force needed to separate the proteinaceous and the aragonite phases is high, indicating that there are molecular interactions between the components.[13] Inlaminated structures with hard and soft layers, a model system that can be applied to understand nacre, thefracture energy and fracture strength are both larger than those values characteristic of the hard material only.[15] Specifically, this structure facilitates crack deflection, since it is easier for the crack to continue into theviscoelastic and compliant organic matrix than going straight into another aragonite platelet.[13][16] This results in theductile protein phase deforming such that the crack changes directions and avoids thebrittle ceramic phase.[13][17] Based on experiments done on nacre-likesynthetic materials, it is hypothesized that the compliant matrix needs to have a larger fracture energy than theelastic energy at fracture of the hard phase.[17]Fiber pull-out, which occurs in other ceramiccomposite materials, contributes to this phenomenon.[16] Unlike in traditional synthetic composites, the aragonite in nacre forms bridges between individual tablets, so the structure is not only held together by the strongadhesion of the ceramic phase to the organic one, but also by these connectingnanoscale features.[16][13] As plastic deformation starts, themineral bridges may break, creating small asperities that roughen the aragonite-protein interface.[13] The additional friction generated by the asperities helps the material withstand shear stresses.[13] In nacre-like composites, the mineral bridges have also been shown to increase theflexural strength of the material because they can transfer stress in the material.[18] 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 "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.[13] The hardness of this material is also negatively impacted by dehydration.[13] Water acts as aplasticizer for the organic matrix, improving its toughness and reducing its shear modulus.[13] Hydrating the protein layer also decreases itsYoung's modulus, which is expected to improve the fracture energy and strength of a composite with alternating hard and soft layers.[15]
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.[19] However, the negative effects of statistical variations can be offset by interfaces with large strain at failure accompanied by strain hardening.[19] On the other hand, thefracture toughness of nacre increases with moderate statistical variations which creates tough regions where the crack gets pinned.[20] But, higher statistical variations generates very weak regions which allows the crack to propagate without much resistance causing the fracture toughness to decrease.[20] Studies have shown that this weak structural defects act as dissipative topological defects coupled by an elastic distortion.[21]
The process of how nacre is formed is not completely clear. It has been observed inPinna 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.[22] 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.[23]
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.[6] This produces the hexagonal close-packing characteristic of nacre.[6] The growth of these bricks can be initiated in various ways such as from randomly scattered elements within the organic layer,[24] well-defined arrangements of proteins,[2] or they may expand from mineral bridges coming from the layer underneath.[25][26]
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.[27]
A 2021 paper inNature Physics examined nacre fromUnio 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.[21][28]
Fossilnautiloid shell with original iridescent nacre in fossiliferous asphaltic limestone,Oklahoma. Dated to thelate Middle Pennsylvanian, which makes it by far the oldest deposit in the world with aragonitic nacreous shelly fossils.[29]
Nacre is secreted by theepithelialcells of themantle tissue of various molluscs. The nacre is continuously deposited onto the inner surface of the shell, the iridescentnacreous layer, commonly known asmother-of-pearl. The layers of nacre smooth the shell surface and help defend the soft tissues againstparasites and damaging debris by entombing them in successive layers of nacre, forming either a blisterpearl attached to the interior of the shell, or a free pearl within the mantle tissues. The process is calledencystation and it continues as long as the mollusc lives.
The form of nacre varies from group to group. Inbivalves, the nacre layer is formed of single crystals in ahexagonal close packing. Ingastropods, crystals aretwinned, and incephalopods, they are pseudohexagonal monocrystals, which are often twinned.[6]
The main commercial sources of mother-of-pearl have been thepearl oyster,freshwater pearl mussels, and to a lesser extent theabalone, popular for their sturdiness and beauty in the latter half of the 19th century.
An ancient art made by mother of pearl in China. This ancient art dates back to theShang Dynasty. This art is also used on jewelry boxes, decorative items and jewelry. Today this art is one of the Chinese cultural heritage.[citation needed]
Both black and white nacre are used forarchitectural purposes. The natural nacre may be artificially tinted to almost any color. Nacretesserae may be cut into shapes andlaminated to aceramictile ormarble base. The tesserae are hand-placed and closely sandwiched together, creating an irregular mosaic or pattern (such as a weave). The laminated material is typically about 2 millimetres (0.079 in) thick. The tesserae are thenlacquered andpolished creating a durable and glossy surface. Instead of using a marble or tile base, the nacre tesserae can be glued tofiberglass. The result is a lightweight material that offers a seamless installation and there is no limit to the sheet size. Nacre sheets may be used on interior floors, exterior and interior walls, countertops, doors and ceilings. Insertion into architectural elements, such as columns or furniture is easily accomplished.[citation needed]
Jewelry
Mother of pearl is commonly used in jewelry due to its smooth texture and iridescent appearance. It is sourced from the inner layer of mollusk shells, such as oysters andabalones.
Mother of pearl is frequently crafted into earrings, pendants, rings, bracelets, and brooches. It can be carved into various shapes or inlaid into metal settings, often combined with gold, silver, or gemstones. The material is valued for its natural luster and the subtle color variations it displays, which can include white, cream, pink, and green.[31]
Nacre inlay is often used for musickeys and other decorative motifs on musical instruments. Manyaccordion andconcertina bodies are completely covered in nacre, and someguitars have fingerboard or headstock inlays made of nacre (or imitationpearloid plastic inlays). Thebouzouki andbaglamas (Greek plucked string instruments of thelute family) typically feature nacre decorations, as does the related Middle Easternoud (typically around thesound holes and on the back of the instrument).Bows of stringed instruments such as theviolin andcello often have mother-of-pearl inlay at the frog. It is traditionally used onsaxophone keytouches, as well as the valve buttons oftrumpets and other brass instruments. The Middle Easterngoblet drum (darbuka) is commonly decorated by mother-of-pearl.[citation needed]
At the end of 19th century, Anukul Munsi was the first accomplished artist who successfully carved the shells ofoysters to give a shape of human being which led to the invention of new horizon in Indian contemporary art. For theBritish Empire Exhibition in 1924, he received a gold medal.[32][33] His eldest sonAnnada Munshi is credited with drawingIndian Swadesi Movement in the form of Indian advertising.[34] Anukul Charan Munshi's third son Manu Munshi was one of the finest mother-of-pearl artists in the middle of 20th century. As the best example of "Charu and Karu art of Bengal," the formerChief Minister of West Bengal, Dr.Bidhan Chandra Roy, sent Manu's artwork, "Gandhiji's Noakhali Abhiyan", to theUnited States. Numerous illustrious figures, such asSatyajit Ray,Bidhan Chandra Roy, Barrister Subodh Chandra Roy,Subho Tagore,Humayun Kabir,Jehangir Kabir, as well as his elder brother Annada Munshi, were among the patrons of his works of art. "Indira Gandhi" was one of his famous mother of pearl works of art. He is credited with portraying Tagore in various creative stances that were skillfully carved into metallic plates.[35][36] His cousin Pratip Munshi was also a famed mother-of-pearl artist.[37][38]
Mother-of-pearl is sometimes used to makespoon-like utensils forcaviar (i.e. caviar servers[39][40]) so as to not spoil the taste with metallic spoons.
Train photo enhanced with mother of pearl. Made for display in the office of E. J. Gorman, president of the Chicago, Rock Island & Pacific RR. Now at the Western Trails Museum,Knott's Berry Farm, Buena Park, CA.
In 2012, researchers created calcium-based nacre in the laboratory by mimicking its natural growth process.[41]
In 2014, researchers used lasers to create an analogue of nacre by engraving networks of wavy 3D "micro-cracks" in glass. When the slides were subjected to an impact, the micro-cracks absorbed and dispersed the energy, keeping the glass from shattering. Altogether, treated glass was reportedly 200 times tougher than untreated glass.[42]
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^Cuif J. P., Dauphin Y., Sorauf J. E. (2011).Biominerals and fossils through time. Cambridge: Cambridge University Press.ISBN978-0-521-87473-1.OCLC664839176.{{cite book}}: CS1 maint: multiple names: authors list (link)
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^Mohanty, Bedabibhas; Katti, Kalpana S.; Katti, Dinesh R. (2008). "Experimental investigation of nanomechanics of the mineral-protein interface in nacre".Mechanics Research Communications.35 (1–2): 17.doi:10.1016/j.mechrescom.2007.09.006.
^Ghosh, Pijush; Katti, Dinesh R.; Katti, Kalpana S. (2007). "Mineral Proximity Influences Mechanical Response of Proteins in Biological Mineral−Protein Hybrid Systems".Biomacromolecules.8 (3):851–6.doi:10.1021/bm060942h.PMID17315945.
^Katti, Kalpana S.; Katti, Dinesh R.; Pradhan, Shashindra M.; Bhosle, Arundhati (2005). "Platelet interlocks are the key to toughness and strength in nacre".Journal of Materials Research.20 (5): 1097.Bibcode:2005JMatR..20.1097K.doi:10.1557/JMR.2005.0171.S2CID135681723.
^Magrini, Tommaso; Moser, Simon; Fellner, Madeleine; Lauria, Alessandro; Bouville, Florian; Studart, André R. (2020-05-20). "Transparent Nacre-like Composites Toughened through Mineral Bridges".Advanced Functional Materials.30 (27) 2002149.doi:10.1002/adfm.202002149.hdl:20.500.11850/417234.ISSN1616-301X.S2CID219464365.
^abAbid, N.; Mirkhalaf, M.; Barthelat, F. (2018). "Discrete-element modeling of nacre-like materials: effects of random microstructures on strain localization and mechanical performance".Journal of the Mechanics and Physics of Solids.112:385–402.Bibcode:2018JMPSo.112..385A.doi:10.1016/j.jmps.2017.11.003.
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^"Poster by Annada Munshi for ITMEB, 1947".Urban History Documentation Archive, Centre for Studies in Social Sciences, Calcutta. Retrieved24 December 2023 – via ResearchGate.
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Lin, A.; Meyers, M.A. (2005-01-15). "Growth and structure in abalone shell".Materials Science and Engineering A.390 (1–2):27–41.doi:10.1016/j.msea.2004.06.072.
