Crystal growth via spiral motion in abalone shell nacre

We present a structural feature of nacre in the red abalone shell: micrometer-scale screw dislocations in the aragonite layers and resultant growth via spiral motion. Compared to typical ionic or covalent crystals, nacre contains 10⁶ screw dislocations per square centimeter, a difference of three orders of magnitude. Using electron microscopy, ion microscopy, and an in situ nano-manipulator, we studied the structure of screw dislocation cores in detail. We considered that these screw dislocations contribute significantly to the strengthening mechanisms that lead to nacre's extraordinary work of fracture, which is three orders of magnitude greater than that of aragonite and other monolithic crystals. This work suggests that the lamellar layers of aragonite propagate via a large number of continuous spiral growth domains as the "stacks of coins" become confluent. This model may provide a basis for creating new comparable micro/nanocomposites through synthetic or biomineralization means.