MADISON -- While the shiny material of pearls and abalone shells has long been prized for its iridescence and aesthetic value in jewelry and decorations, scientists admire mother-of-pearl for other physical properties as well.

Also called nacre ("NAY-ker"), mother-of-pearl is 3,000 times more fracture-resistant than the mineral it is made of, aragonite, says Pupa Gilbert, a physicist at the University of Wisconsin-Madison. "You can go over it with a truck and not break it - you will crumble the outside [of the shell] but not the [nacre] inside. And we don't understand how it forms - that's why it's so fun to study."

Understanding the mechanism by which nacre forms would be the first step toward harnessing its strength and simplicity, she says. "We don't know how to synthesize materials that are better than the sum of their parts."

Writing in the June 29 issue of Physical Review Letters, Gilbert and her colleagues in the UW-Madison department of physics and School of Veterinary Medicine, the Institute for the Physics of Complex Matter in Switzerland and the UW-Madison Synchrotron Radiation Center, now describe unexpected elements of nacre architecture that may underlie its strength and offer clues into how this remarkable material forms.

Like our bones and teeth, nacre is a biomineral, a combination of organic molecules - made by living organisms - and mineral components that organisms ingest or collect from their environment. The aragonite mineral in nacre is made of calcium carbonate, which marine animals form from elements abundant in seawater.

Though a mere 5 percent of abalone nacre is organic, this small fraction somehow lays enough foundation for the mineral components to assemble spontaneously, Gilbert says.

"Ninety-five percent of the mass of this biomineral is self-assembled, while only 5 percent is actively formed by the organism," she says. "It is one of the most efficient mechanisms yo
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Contact: Pupa Gilbert
pupa@physics.wisc.edu
608-262-5829
University of Wisconsin-Madison
2-Jul-2007