MADISON Some of the most common minerals in biology, including those in bones and shells, have a mysterious structure: Their crystals are positioned in the same orientation, making them behave as one giant crystal, even though they do not look like a faceted crystal.
It's as if grains of salt were spilled on a rug, yet instead of landing randomly, all wound up with exactly the same angle and rotation.
In a new study in the Journal of the American Chemical Society, Pupa Gilbert, a professor of physics at the University of Wisconsin-Madison, has explained this seemingly miraculous feat of nature.
Gilbert and colleagues, including postdoctoral researcher Christopher Killian and biomineral expert Joanna Aizenberg of Harvard University, studied the tooth of the purple sea urchin. Gilbert adds that the same process may occur in mollusk shells and in the bones and teeth of vertebrate animals.
"Many of these biominerals are highly co-oriented," Gilbert says. "If you look at the nanometer scale, the crystals share a single orientation. But they are separate from each other, which raises an interesting question: How do they orient, how do they come to behave as if they were single crystals?"
The sea urchin is a common marine animal that eats algae and small animals and lives in tide pools or shallow water. Urchins use their five long teeth to grind away rock to make a shallow pocket where they can live protected behind a wreath of sharp, bony spines.
Gilbert's fascination with the urchin's tooth follows a long tradition: Aristotle, the Greek naturalist and philosopher, wrote about the urchin in about 343 BC. Indeed, the teeth are held in a "jaw" called "Aristotle's lantern."
The crystal in an urchin's tooth originates as a powdery, wet, "amorphous" form of the common mineral calcium carbonate, the major constituent of limestone.
As Gilbert studied the teeth using a high-resolution X-ray mi
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University of Wisconsin-Madison