http://www.digitaljournal.com/science/just-why-are-seashells-so-strong/article/454618

How do sea shells get their strength?

Posted Jan 12, 2016 by Tim Sandle
Seashells are very strong, but chalk is easy to break. Both are made from calcium carbonate crystals. So why the difference? Materials scientists have found the answer.
Triton (Cymatiidae) shells at Savannah Botanical Gardens exhibit.  Savannah  GA
Triton (Cymatiidae) shells at Savannah Botanical Gardens exhibit. Savannah, GA
The reason why seashells are tough and lobster claws hard to break, is because of the presence of biological matter situated between the calcium carbonate crystals. These clumps are situated between the crystals and provide the strength. The clumps become incorporated via chemical interactions at the atomic level. This mechanism was previously unknown.
Calcium carbonate is a common, naturally occurring material. It is formed by three main elements: carbon, oxygen and calcium. The way the chemical compound forms crystals, however, leads to different forms and structures. Examples include pearls, seashells, and animal exoskeletons. It is also created when calcium ions in hard water react with carbonate ions creating limescale.
A white pearl necklace.
A white pearl necklace.
Tanka
Despite the ubiquity of the material, scientists do not full understand the geochemical principles that lead to the development of soft and hard materials.
What was understood is that strength is related to impaction. The greater the crystal structure is compressed then the harder it is to break the crystal matrix apart. This is because proteins trapped within calcium carbonate crystals produce a compressive force inside the crystal structure. These forces are a combination of stress and strain.
With shells, it would appear particles of protein land on the crystals as they are forming and become embedded into the structure, forming a matrix. This process also creates microscopic grooves, which also help with the physical forces at play when two edges come together.
It is hoped that the new insight into the way natural minerals form will allow researchers to develop new forms of synthetic materials. One current area of research, within the laboratory where the discovery was made — U.S. Department of Energy's Pacific Northwest National Laboratory — is an examination of materials that will help with energy capture. Here it may be possible to trap carbon dioxide within materials or to add in light-responsive nanoparticles, in a bid to develop new forms of solar power technology.
The research has been published in the journal Nature Communications. The research is titled “Direct observation of mineral–organic composite formation reveals occlusion mechanism.”