![]() ![]() 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 and a yield stress of roughly 70 MPa (when dry). 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. Adjacent tablets have been shown to have dramatically different c-axis orientation, generally randomly oriented within ~20° of vertical. The crystallographic c-axis points approximately perpendicular to the shell wall, but the direction of the other axes varies between groups. These structures interfere constructively and destructively with different wavelengths of light at different viewing angles, creating structural colours. Nacre appears iridescent because the thickness of the aragonite platelets is close to the wavelength of visible light. These layers are separated by sheets of organic matrix (interfaces) composed of elastic biopolymers (such as chitin, lustrin and silk-like proteins). Whatever the shape of the tablets, the smallest units they contain are irregular rounded granules. Depending on the species, the shape of the tablets differs in Pinna, the tablets are rectangular, with symmetric sectors more or less soluble. 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. Physical characteristics Structure and appearance Other mollusc families that have a nacreous inner shell layer include marine gastropods such as the Haliotidae, the Trochidae and the Turbinidae. The outer layer of cultured pearls and the inside layer of pearl oyster and freshwater pearl mussel shells are made of nacre. 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. Nacre is found in some of the most ancient lineages of bivalves, gastropods, and cephalopods. Nacre ( / ˈ n eɪ k ər/ NAY-kər, also / ˈ n æ k r ə/ NAK-rə), also known as mother of pearl, is an organic–inorganic composite material produced by some molluscs as an inner shell layer it is also the material of which pearls are composed. ![]() Nacreous shell worked into a decorative object ![]()
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