The nacre layer, or mother of pearl, at the inner surface of the abalone shell and of other mollusk shells and pearls (see images below) has a fracture resistance 3000 times greater than that of aragonite, the pure mineral of which it is composed. The toughening effect is due to well-defined nanolayers of organics at the interfaces between micro-tablets of aragonite^1,2.
          Nacre formation is poorly understood at the molecular level, and this piqued our interest in this material, joining the interdisciplinary community of scientists around the world tackling this problem. It is our opinion that the key to nacre formation mechanisms lies at the organic-mineral interface. Understanding the role of that interface is thus pivotal to the development of new biomimetic materials. We desire to understand the fundamental mechanisms leading to the formation of these beautiful, extraordinarily efficient, self-assembling natural structures. We have proven that spectromicroscopy can analyze organic and mineral structures, therefore we are now eager to analyze the organic-mineral interface in nacre³. We recently discovered a new polarization-dependent imaging contrast mechanism, also known as linear dichroism. This contrast revealed that immediately adjacent stacks of co-oriented tablets are finite in height, have very different crystallographic c-axis orientations, and consequently the nacre growth direction cannot be identified with the c-axis. The appearance of the columns also inspired new models for the formation mechanism of nacre.

Polarization-dependent imaging contrast in a side view of red abalone nacre. Different gray levels correspond to different c-axis orientations with respect to the illuminating x-ray polarization vector. Immediately adjacent columns are mis-aligned in c-axis direction4.

1. JD Currey. Mechanical properties of mother of pearl in tension. Proc. R. Soc. Lond. B 196, 443-463 (1977).
   
2. JD Currey. Hierarchies in biomineral structures. Science 309, 253-254 (2005).
   
3. PUPA Gilbert, BH Frazer, M Abrecht. The organic-mineral interface in biominerals. Reviews in Mineralogy and Geochemistry. In: Molecular Geomicrobiology. Vol 59. JF Banfield, KH Nealson, J. Cervini-Silva (eds), Mineralogical Society of America, Washington DC, p 157-185 (2005).
   
4. RA Metzler, M Abrecht, RM Olabisi, D Ariosa, CJ Johnson, BH Frazer, SN Coppersmith, PUPA Gilbert. Columnar Nacre Architecture and Possible Formation Mechanism. Phys. Rev. Lett. 98, 268102-1/4 (2007). Also featured in Science 317, 175 (2007).
   

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