Pearls and abalone shells have long been prized for their iridescent good looks.
But scientists admire mother-of-pearl (also called nacre) for other physical properties including a fracture resistance 3,000 times better than the mineral aragonite from which it’s made, says Pupa Gilbert, a physicist at the University of Wisconsin-Madison. "You can drive a truck over an abalone shell and it won’t break. You’ll crumble its shell but not the nacre inside.” Understanding the mechanisms that form nacre is the first step toward harnessing its strength and simplicity, says Gilbert. "We don't know how to synthesize materials that are better than the sum of their parts."
Gilbert and her colleagues in the UW-Madison dept. of Physics and School of Veterinary Medicine, the Institute for the Physics of Complex Matter in Switzerland, and the UW-Madison Synchrotron Radiation Center recently reported on unexpected elements of nacre architecture that may underlie its strength and offer clues about how this remarkable material forms.
Similar to 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.
Only 5% of the organism's mass is organic. But this small fraction lays enough foundation for the mineral components to assemble spontaneously, Gilbert says. To gain insight into this self-assembly process, Gilbert and graduate student Rebecca Metzler examined the structure of abalone nacre using synchrotron radiation light emitted by electrons speeding around a curved track.
The structure of abalone shells had previously been likened to a brick wall with layers of organic "mortar" separating individual crystalline "bricks". But the polarized light from the synchrotron revealed that the nacre wall was not uniform. Instead, it contained distinct clumps of bricks, each an irregular column of crystals with identical composition but a crystal orientation different than neighboring columns. Orientation affects how crystals emit electrons. Some of the columns of bricks in the sample appear white and others appear black and more appear gray, depending on their crystal orientation, Gilbert explains. The overall effect resembles a camouflage pattern, each roughly columnar cluster a slightly different shade.
Gilbert suggests that this mosaic architecture of nacre, with numerous non-aligned crystals, could lead to a stronger material by preventing the formation of natural cleavage planes like those that form the facets of a cut diamond where a single crystal can easily break. "It is intuitive that a poly-crystal is mechanically stronger than a single crystal, so perhaps that is an advantage for the animal," Gilbert says. With this new information about nacre structure and the help of UW-Madison theoretical physicist Susan Coppersmith, the group used modeling to help determine how nacre structures form. The team developed a model that suggests that the animal creates the organic "mortar" layers first, peppered with randomly distributed crystal nucleation, or seeding sites.
They predict that mineral crystals start growing inside the shell and extend horizontally until they touch another growing crystal and vertically until they hit the overlying mortar. If that crystal touches another of the scattered crystal formation sites on the next tier up, it triggers growth of a new crystal with the same crystal orientation, gradually building a rough column of irregular width.
Future tests will help refine the model as well as examine other biominerals, such as human teeth and the nacre of other species such as pearl oysters, mussels, or nautiluses. The work is funded by grants from the UW-Madison Graduate School and the National Science Foundation.