• José H. Leal

Perfection from Disorder

We have discussed in the past the fantastic physical features of shell material. The structural properties of mother-of-pearl, also known as nacre, for instance, a material of great strength and resilience, have been tapped for uses in engineering, optics, electronics, and other hi-tech applications. Not all molluscan shells include a nacreous layer, though. When present, nacre is usually the internal shell layer, showing iridescent, always-shifting colors. (the shells of nautiluses and abalones are good examples.)

Cross-section of a shell showing the layered nacre on top of a prismatic shell structure. Credit: © Igor Zlotnikov

The iridescent aspect of nacre is imparted by tens of thousands of super-thin, plate-like calcium carbonate crystals. Light can penetrate those crystals, but only in part, reflecting out as different colors of the visible spectrum. Nacreous crystals are also super regular in their thickness and structure, but new research published in Nature Physics by researchers from the Center for Molecular Bioengineering (Dresden, Germany) and the European Synchrotron Radiation Facility (Grenoble, France) shows that this regularity is not there in the first moments of shell-laying by the mollusk.

Haliotis iris, or Paua, from New Zealand, showing the internal, nacreous layer.

The amazing regularity of nacreous crystals and their perfect arrangement in mother-of-pearl is the result of complex self-correcting processes that fix structural defects that may appear during shell-making. Read about it here, and a cool paper about artificially creating “nacre” here.

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