The Secret Strength Of Mollusk Shell

The windowpane oyster, known to scientists as the bivalve Placuna placenta, gets its name from its remarkable shell, which is transparent enough to read through. The shell is 99% calcite, the most stable polymorph of CaCO₃. One might think that this high mineral content would make P. placenta’s shell brittle, but the shell is about 10 times more likely to resist penetration than single-crystal calcite from a geological source. MIT researchers Christine Ortiz and Ling Li decided to find out why. They made small indentations in P. placenta shell and geological calcite and compared the two using various types of microscopy (Nat. Mater. 2014, DOI: 10.1038/nmat3920 ). Compared with geological calcite, Ortiz and Li note, “P. placenta fractures in a more graceful way.” When indented, geological calcite tends to crack, but the mollusk shell dissipates energy via deformation twinning, a process by which a crystal’s atoms move ever so slightly such that the solid forms crystal twins. Twins are pairs of adjacent crystalline domains that are slightly misoriented with respect to one another. Ortiz and Li propose that the phenomenon might help design lightweight armor that resists penetration.