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Nacre (pron.: // NAY-kər), also known as mother of pearl, is an organic-inorganic composite material produced by some molluscs as an inner shell layer; it is also what makes up the outer coating of pearls. It is strong, resilient, and iridescent.
Nacre is found in some of the more ancient lineages of bivalves, gastropods, and cephalopods. However, the inner layer in the great majority of mollusc shells is porcellaneous, not nacreous, and this usually results in a non-iridescent shine, or more rarely in non-nacreous iridescence such as flame structure as is found in conch pearls.
The outer layer of pearls and the inside layer of pearl oyster and freshwater pearl mussel shells are made of nacre. Other mollusc families that have a nacreous inner shell layer include marine gastropods such as the Haliotidae, the Trochidae and the Turbinidae.
Physical characteristics 
Appearance and structure 
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.
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. 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. Strength and resilience are also likely to be due to adhesion by the "brickwork" arrangement of the platelets, which inhibits transverse crack propagation. This design at multiple length sizes greatly increases its toughness, making it almost as strong as silicon.
The crystallographic c-axis points perpendicular to the shell wall, but the direction of the other axes varies between groups. 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. The interlocking of bricks of nacre has large impact on both the deformation mechanism as well as its toughness. In addition, the mineral–organic interface results in enhanced resilience and strength of the organic interlayers.
Nacre formation is mediated by the organic matrix, which controls the onset, duration and form of crystal growth. Individual aragonite "bricks" quickly grow to the full height of the nacreous layer, and expand until they abut adjacent bricks. Bricks nucleate on randomly dispersed elements within the organic layer. This produces the hexagonal close-packing characteristic of nacre. Nacre differs from fibrous aragonite – a brittle mineral of the same form – in that the growth in the c-axis (i.e. perpendicular to the shell, in nacre) is slow in nacre, and fast in fibrous aragonite.
Nacre is secreted by the epithelial cells of the 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 parasites 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 
The form of nacre varies from group to group. In bivalves, the nacre layer is formed of single crystals in a hexagonal close packing. In gastropods, crystals are twinned, and in cephalopods, they are pseudohexagonal monocrystals, which are often twinned.
Commercial sources 
The main commercial sources of mother of pearl are the pearl oyster, freshwater pearl mussels, and to a lesser extent the abalone. Widely used for pearl buttons especially during the 1900s, were the shells of the great green turban snail Turbo marmoratus and the large top snail, Tectus niloticus. The international trade in mother of pearl is governed by the Convention on International Trade in Endangered Species of Wild Fauna and Flora, an agreement signed by more than 170 countries.
Decorative uses 
Nacre has been used for centuries for a variety of decorative purposes:
Altarpiece, circa 1520, with extensive use of carved nacre.
Both black and white nacre are used for architectural purposes. The natural nacre may be artificially tinted to almost any color. Nacre tesserae may be cut into shapes and laminated to a ceramic tile or marble 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 mm thick. The tesserae are then lacquered and polished creating a durable and glossy surface.
Instead of using a marble or tile base, the nacre tesserae can be glued to fiberglass. 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.
Mother of pearl buttons are used in clothing either for functional or decorative purposes. The Pearly Kings and Queens are an elaborate example of this.
Nacre is also used to decorate watches, knives, guns and jewellery.
Musical instruments 
Nacre inlay is often used for music keys and other decorative motifs on musical instruments. Many accordion and concertina bodies are completely covered in nacre, and some guitars have fingerboard or headstock inlays made of nacre (as well as some guitars having plastic inlays designed to imitate the appearance of nacre). Greek bouzouki and baglamas instruments typically feature nacre decorations. Bows of stringed instruments such as the violin and cello often have mother of pearl inlay at the frog.
Mother of pearl is sometimes used to make spoon-like utensils for caviar, so as to not spoil the taste with metallic spoons.
Artificially created nacre 
In 2012, researchers at the University of Cambridge created calcium-based nacre in the laboratory by mimicking its natural growth process.
Nacre on its own is inexpensive, but the quality of nacre on a pearl can significantly alter its value.
See also 
- Definition of nacre at dictionary.com.
- Nudelman, Fabio; Gotliv, Bat Ami; Addadi, Lia; Weiner, Steve (2006). "Mollusk shell formation: Mapping the distribution of organic matrix components underlying a single aragonitic tablet in nacre". Journal of Structural Biology 153 (2): 176–87. doi:10.1016/j.jsb.2005.09.009. PMID 16413789.
- Checa, Antonio G.; Ramírez-Rico, Joaquín; González-Segura, Alicia; Sánchez-Navas, Antonio (2008). "Nacre and false nacre (foliated aragonite) in extant monoplacophorans (=Tryblidiida: Mollusca)". Naturwissenschaften 96 (1): 111–22. doi:10.1007/s00114-008-0461-1. PMID 18843476.
- Katti, Kalpana S.; Katti, Dinesh R.; Pradhan, Shashindra M.; Bhosle, Arundhati (2011). "Platelet interlocks are the key to toughness and strength in nacre". Journal of Materials Research 20 (5): 1097. doi:10.1557/JMR.2005.0171.
- Ghosh, Pijush; Katti, Dinesh R.; Katti, Kalpana S. (2008). "Mineral and Protein-Bound Water and Latching Action Control Mechanical Behavior at Protein-Mineral Interfaces in Biological Nanocomposites". Journal of Nanomaterials 2008: 1. doi:10.1155/2008/582973.
- Mohanty, Bedabibhas; Katti, Kalpana S.; Katti, Dinesh R. (2008). "Experimental investigation of nanomechanics of the mineral-protein interface in nacre". Mechanics Research Communications 35: 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. PMID 17315945.
- Jackson, D. J.; McDougall, C.; Woodcroft, B.; Moase, P.; Rose, R. A.; Kube, M.; Reinhardt, R.; Rokhsar, D. S. et al. (2009). "Parallel Evolution of Nacre Building Gene Sets in Molluscs". Molecular Biology and Evolution 27 (3): 591–608. doi:10.1093/molbev/msp278. PMID 19915030.
- Addadi, Lia; Joester, Derk; Nudelman, Fabio; Weiner, Steve (2006). "Mollusk Shell Formation: A Source of New Concepts for Understanding Biomineralization Processes". ChemInform 37 (16). doi:10.1002/chin.200616269.
- Bruce Runnegar & S Bengtson. "1.4 Origin of Hard Parts — Early Skeletal Fossils".
- Jessica Hodin, "Contraband Chic: Mother-of-Pearl Items Sell With Export Restrictions", New York Observer, October 20, 2010
- Aron, Jacob (24 July 2012). "Artifical mother of pearl follows nature's recipe". New Scientist.
Further reading 
- Frýda J., Bandel K. & Frýdová B. (2009). "Crystallographic texture of Late Triassic gastropod nacre: evidence of long-term stability of the mechanism controlling its formation". Bulletin of Geosciences 84(4): 745–754. doi:10.3140/bull.geosci.1169.
- 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.
- Mayer, G. (2005). "Rigid biological systems as models for synthetic composites". Science 310 (5751): 1144–1147. doi:10.1126/science.1116994. PMID 16293751.
- Bruet, B.; Qi, H.J.; Boyce, M.C.; Panas, R.; Tai, K.; Frick, L.; Ortiz, C. (2005). "Nanoscale morphology and indentation of individual nacre tablets from the gastropod mollusc Trochus niloticus". J. Mater. Res. 20 (9): 2400. doi:10.1557/JMR.2005.0273.
- Antonio G. Checa, Julyan H. E. Cartwright, Marc-Georg Willinger and Steven M. Stanley, The Key Role of the Surface Membrane in Why Gastropod Nacre Grows in Towers; Proceedings of the National Academy of Sciences of the United States of America, Vol. 106, No. 1, Jan. 6, 2009
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