Calcium Carbonate, Mollusks, and Body Armor

The following is a summary of the results of the MIT research outlined in this article: http://newsoffice.mit.edu/2014/tough-nails-yet-clear-enough-read-through

This article by David L. Chandler outlines the results of work done by student Ling Li, and Professor Christine Oritz. The author begins by describing the characteristics of the Placuna placenta shell, revealing that despite being made of a substance called calcite, that is usually weak, brittle, and crumbly, it is in fact extremely resistant to puncture damage and is optically clear. Calcite is a structural derivative of calcium carbonate, with the molecular formula of CaCO3. The research revealed that the hardness and optical clarity are brought about by an unusual nanostructure that has great potential for energy dissipation. 

To test this, the scientists inflicted multiple punctures with a diamond needle point, and then utilized 

Colorized SEM of the punctures.
(Ling Li)

Electron Scanning to examine what exactly happens to the structure. These tests revealed that the damage resistance of the armor is actually the result of the atomic-level physical structure called twinning. This structure occurs when the atoms in the boundary of a crystalline lattice “cube” are shared by two crystal growths, producing a mirror image. The author states that a repetition of this pattern provides an exceptionally strong barrier to damage when a physical puncture takes place, more efficiently keeping the wave of damage from spreading outward like a crystalline “domino action.” The author fails to explain this in great detail, but does add that this is the action that also preserves the optical clarity of the surrounding structure, and that it results in a structure that is 10-times more efficient in energy dissipation than pure minerals. 

The macroscopic results of twinning. This one is Pyrite.
(Public domain) 

It is stated in the article that the applications of these findings are mostly found in military body armor projects. Ceramic armor plates in recent use have proven successful only in one-shot circumstances, as the first impact greatly weakens the physical integrity of the entire system. These research findings are particularly significant in this aspect, as the mollusk’s armor can resist multiple punctures with little resistance lost. 

References

David, L., Chandler. (2014, March 30). Tough as nails, yet clear enough to read through. Retrieved from http://newsoffice.mit.edu/2014/tough-nails-yet-clear-enough-read-through

Ling Li and James C. Weaver. (2014). The Effects of Multiple Indentations [Scanning Electron Micrograph] retrieved April 10, 2014, from http://newsoffice.mit.edu/2014/tough-nails-yet-clear-enough-read-through