Physicists Investigate Remarkable Strength of Mother-of-Pearl

While its iridescent beauty has been appreciated for thousands of years, some scientists are more interested in the structure of nacre, also known as Mother of Pearl. A paper published in the June 29^th issue of Physical Review Letters provides new information about the architecture of nacre, and insight into how it is formed.

Nacre is a biomineral; that is, it is composed of an organic matrix, built by an organism, which functions as mortar to hold together "bricks" of mineral that are acquired by the organism from its environment. Previous examinations of nacre revealed that its mineral content, which consists of a calcium carbonate mineral called aragonite, was organized into columns held together by organic materials.

As reported in the new paper, physicist Pupa Gilbert of the University of Wisconsin-Madison and colleagues used polarized light to examine the structure of the mineral columns within nacre from shells of the mollusk Haliotis rufescens. The polarized light allowed Gilbert to visualize that the columns within the nacre were not uniform. While they were composed entirely of aragonite crystals, the columns were divided into patches, between which the angle of crystal alignment with respect to the observer was different. This gave the columns the appearance of camouflage patches; it also provides an explanation for nacre's remarkable strength. The mis-alignment of adjacent crystals makes it less likely that there could be a single, common cleavage plane for a column, or adjacent columns. In a UW-Madison press release Gilbert states, "It is intuitive that a poly-crystal is mechanically stronger than a single crystal, so perhaps that is an advantage for the animal." In fact, according to Gilbert, nacre is so strong that "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."

In their paper, Gilbert and colleagues also present a model for formation of nacre. In their model, the organic materials are manufactured by the animal first; randomly distributed within the organic matrix are "seeding" sites, where crystals begin to form. Crystals grow vertically within a column until they hit a layer of the organic mortar, and grow horizontally until they encounter another crystal; this would lead to the patchwork pattern observed by Gilbert.

Material scientists are interested in biominerals not only because of their strength, but because they form under ambient conditions. According to a Wikipedia article on biomineralization, artificial synthesis of crystals often requires high temperatures and harsh chemicals. In addition to not requiring extreme treatments for formation, biominerals are often much stronger than pure crystals of the minerals they incorporate. For this reason, the science of biomimetics seeks to learn how to replicate biological processes to create biominerals for human use.

References:

P.U.P.A. Gilbert et al. "Architecture of Columnar Nacre, and Implications for its Formation Mechanism." Abstract at http://scitation.aip.org.ezproxy.library.wisc.edu/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000098000026268102000001&idtype=cvips&gifs=Yes

UW-Madison Press Release. "Mother of Pearl: Classic Beauty and Remarkable Strength." http://www.news.wisc.edu/releases/13849

Wikipedia article, "Biomineralization." http://en.wikipedia.org/wiki/Biomineralisation