Stacked layers of aragonite tiles         Regularly spaced surface grooves
The green bars represent 10 micron.     Images ©T L Tan, D Wong & P Lee, Iridescence of a shell of mollusk Haliotis Glabra, Optics Express, Vol. 12, Issue 20, pp. 4847-4854 (2004)

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Shining Shells

I came across these gem-like mollusc shells a few days ago in the Museum of Zoology in Cambridge (UK). This freshly reopened museum is delightful, brain stirring and a must. The image does little justice to the shells, they shone colours brighter than the gems of a jeweller�s window. Their secret lies in the structure of their lining of nacre, a miracle of bio-engineering.

Image ©Les Cowley

The tile structure�s regularity is remarkable. Protein secretions control its formation.

Initially there is random nucleation of aragonite on a thin protein layer. The fastest crystal growth would normally be perpendicular to the layer along the �c� axis but this is rather neatly prevented by the selective absorption of proteins onto the top crystal faces. Horizontal growth proceeds instead and contact with adjacent tiles eventually forces a regular spacing.

Periodic injections of an inhibitor protein is hypothesised to limit the tile thickness to 0.5 micron. An upper terrace of crystal then starts to form on the face of each tile and eventually grows into a second layer.

And so on.

The colours come mainly from the nacre surface structure. It has uniform microscopic ridges - A naturally made diffraction grating. Some light penetrates into the nacre and that adds to the surface generated colour by multilayer interference.

Where come the ridges?
Nacre is made up of polygonal (usually hexagonal) tiles of a very hard crystalline form of calcium carbonate, aragonite. Each tile is a single crystal some 10 microns across. A thin matrix of conchiolin, polysaccharide and protein fibres, cements them together into tessellated stacked layers to form a very strong structure.

The edges of the layer stacks form the surface ridges. That the ridges produce the colours was first proved in the 19th Century by David Brewster of Edinburgh University. He made a wax cast of a shell surface. The wax cast also showed iridescent colours!