Nacre
(mother of pearl)
From Wikipedia, the free encyclopedia
Nacre, also known as mother of pearl, is an organic-inorganic composite material produced by some mollusks. It is strong, resilient, and iridescent. Pearls and the inside layer of the pearl oyster shells and the freshwater pearl mussel shells are made of nacre. Many other families of mollusks also have a shell layer which is nacreous, including the Trochidae and the Turbinidae.
Nacre is composed of hexagonal platelets of aragonite (calcium carbonate (CaCO3) crystals) 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. 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 increases its toughness enormously, making it almost equivalent to that of silicon.
Nacre appears iridescent because the thickness of the aragonite platelets is comparable to the wavelength of visible light. This results in constructive and destructive interference of different wavelengths of light, resulting in different colors of light being reflected at different viewing angles.
Nacre is secreted by the epithelial cells of the mantle tissue of some species of mollusk. The nacre is deposited onto the inner surface of the shell, the iridescent nacreous layer, commonly known as mother of pearl. This smooths the shell and defends against parasites and damaging detritus by entombing them in successive layers of nacre, forming a pearl. The process is called is encystation and continues while the mollusk lives.
Chief sources are the pearl oyster, found in warm and tropical seas, primarily in Asia; freshwater pearl mussels, which live in many rivers of the United States, Europe, and Asia; and the abalone of California, Japan, and other Pacific regions.
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