Microscopy of Mother-of-Pearl Suggests Novel Formation Mechanisms

Daniel S. Burgess

The iridescent substance called nacre, or mother-of-pearl, is of particular interest to materials scientists because this layered arrangement of CaCO₃ crystal platelets is some 3000 times more resistant to fracture than pure crystal. An understanding of nacre thus promises to enable the development of tougher and/or lighter building materials.

Nacre, or mother-of-pearl, is a material of interest to researchers because it is thousands of times more resistant to fracture than pure CaCO₃ crystal. A study has discovered the presence of an amorphous CaCO₃ layer in nacre, contradicting epitaxial explanations for the formation of the material. Courtesy of Nadine Nassif.

The presumption has been that nacre takes the form of ordered layers of aragonite platelets grown by an epitaxial process in a soft organic matrix of chitin and proteins. The results of a study by researchers in Germany suggest otherwise.

Nadine Nassif of Helmut Cölfen’s team at Max Planck Institut für Kolloid- und Grenzflächenforschung in Potsdam explained that the observations of abalone nacre by high-resolution transmission electron microscopy and nuclear magnetic resonance (NMR) spectroscopy revealed the presence of amorphous CaCO₃. This 3- to 5-nm-thick continuous layer coating the single-crystalline aragonite platelets is incompatible with the epitaxial explanations.

Ongoing NMR research seeks to characterize the interface of the amorphous CaCO₃ with the organic matrix, Nassif said. And the doctoral work of a colleague at the institute suggests that general physicochemical processes may drive the formation of nacre more than had been assumed.

The team also included scientists from Martin Luther Universität and Max Planck Institut für Mikrostrukturphysik, both in Halle, and Bundesanstalt für Materialforschung und -prüfung in Berlin.

PNAS, Sept. 6, 2005, pp. 12,653-12,655.