December 9, 2015

How mother-of-pearl forms

Die Muschel Pinna nobilis.
(Bild: Biodiversity Heritage Library, CC BY 2.0, via Wikimedia Commons*)

(December 9, 2015)  FAU researchers show how mother-of-pearl is formed from nanoparticles

Materials scientists at FAU have shown for the first time that the mother-of-pearl in clam shells does not form in a crystallisation process but is a result of the aggregation of nanoparticles within an organic matrix. This could lead to a better understanding of the structure of biomaterials which may be useful in the development of new high-performance ceramics. The findings of the research group led by Prof. Dr. Stephan E. Wolf have been published in the latest issue of the renowned journal Nature Communications (doi: 10.1038/ncomms10097).

Prof. Wolf and his team used a special technique to investigate the structure of mother-of-pearl. Using a diamond wire saw, they cut a 60-centimetre wedge out of the shell of a large Pinna nobilis – a type of clam found in the Mediterranean – which they then polished using a novel method before examining it under a scanning transmission electron microscope. ‘We borrowed the wedge-polishing technique from the semiconductor industry,’ Stephan Wolf explains. ‘This method makes it possible to look at extremely large areas, something that was very difficult to do before.’

Traditional model disproved
The high-resolution images from the scanning transmission electron microscope showed that the structure of the shell is very heterogeneous – from irregular calcite prisms on the outside to the smooth mother-of-pearl on the inside of the shell, with an organic layer in the middle. ‘The transition from the organic to the mother-of-pearl layer is particularly interesting,’ Stephan Wolf says. ‘Here we find the first nanoparticles of between 50 and 80 nanometres in size that aggregate more and more as they get closer to the inside of the shell and merge to form mother-of-pearl platelets, finally forming the highly structured mother-of-pearl that we all know.’

Prefabrication in nature
With their findings the Erlangen-based researchers have shown for the first time that mother-of-pearl does not form through a crystallisation process in which atoms or ions in a saturated solution are deposited successively – as previously thought – but instead forms through the aggregation of prefabricated nanocrystals. ‘If we compare the growth process of mother-of-pearl to building a house, the clam uses a kind of prefabricated construction method, while crystallisation is like building a wall out of individual bricks,’ Stephan Wolf explains.

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