Mother-of-pearl, also known as nacre, is not just aniridescent substance whose optical characteristics impress the observerand which is often used for jewellery. It is also an excellent materialfor working with. Nacre consists of 97 percent lime, but has a thousandtimes higher breaking strength. The reason has to do with the layercomposition of mother-of-pearl. Now, Max Planck and BAM scientists havediscovered that the surface of the lime platelets in mother-of-pearl isnot at all ordered in layers, as had been previously assumed. Becauseof this fact, it can be ruled out that the crystals are controlledthrough ordered layers on the organic matrix. This understanding ofnacre, and the mechanism by which it is built, is essential foremulating the same refined principle in building new materials.
Fora long time, mother-of-pearl has been considered as an interestingbiogenous material. During this time, researchers have been trying tounderstand its astounding characteristics. Its unusual breakingstrength is due to a structure based on soft organic layers and hardlime platelets.
If we could only begin to copy this buildingprinciple, it would lead to a revolution in the construction industry.Possible goals for this kind of biomimetic materials research could befirmer gypsum plasterboard, or pieces of concrete with a lower weightbut with the same strength. The lime platelets in nacre crystalliseinto aragonite - a crystal form which is normally not stable underambient conditions. Until now, researchers had assumed that thiscrystallization of the lime platelets was determined by ordered layersof protein which lie on a pre-formed layer of chitin. Chitin can befound in nature, as for example a scaffolding material in the shells ofinsects.
But the latest findings of the Max Planck scientistshave found these assumptions to be false. Instead of an orderedcrystalline layer, which would be in contact with the organic matrix,the scientists found tiny - five nanometres thick - layers of amorphous- that, is disordered - calcium carbonate on the surface of themonocrystalline platelets in nacre.
This disordered and wavysurface provides evidence against the postulated specific interactionbetween the mineral material and the organic matrix. The finding couldbe clearly supported by 13C and 1H solid state nuclear magneticresonance (NMR) spectroscopy. Furthermore, in NMR experiments theresearchers detected the amorphous character of the surface layer andruled out any interaction between it and the organic scaffolding.
Thereason for the existence and development of the disordered upper layeron the crystal could be based upon the fact that impurities accumulatein the surface layer. In crystallization, these are not built into theordered crystal lattice - similar to what happens in the process ofzone melting in metallurgy.
The amorphous layer (ACC) couldindeed have another function. It replaces the previously assumed directinteraction of the high energy (001) aragonite layer through a gradientlayer made of aragonite, ACC, and organic matrix. The energies of theboundary layer could be significantly lower here, and thus athermodynamic force could exist for the development of an amorphousupper layer. It is still not clear in which direction thecrystallographic orientation of the platelet eventually moves. In thecurrent study, the scientists have acted on the assumption that anelectrostatic attraction exists between the inorganic platelets and theorganic matrix.
Nadine Nassif, Nicola Pinna, Nicole Gehrke, Markus Antonietti, Christian Jäger, and Helmut Cölfen
Amorphous layer around aragonite platelets in nacre
PNAS 2005 102: 12653-12655; published online before print: August 29 2005, print: September 6, 2005, Vol. 102, No. 36
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