ABSTRACT. Many biominerals contain micro-or nano-crystalline mineral components, organized accurately into architectures that confer the material with improved mechanical performance at the macroscopic scale. We present here a new effect, which enables to observe the relative orientation of individual crystals at the sub-micron scale. We call it polarization-dependent imaging contrast (PIC), as it is an imaging development of the well-known x-ray linear dichroism. Most importantly PIC is obtained in situ, in pristine biominerals. We present here PIC in the prismatic and nacreous layers of Haliotis rufescens (red abalone), confirm it in geologic calcite and aragonite, and corroborate the experimental data with theoretical simulated spectra. PIC reveals new and unexpected aspects of nacre architecture that have inspired theoretical models for nacre formation.2 INTRODUCTION. Nacre, or mother-of-pearl, is intensely studied by materials scientists, mineralogists, physicists as well as chemists, because of its remarkable mechanical properties and its fascinating and poorly understood formation (1,2). Nacre is a composite of layered 400-nm thick aragonite tablets (3), and 30-nm thick organic matrix layers (4,5). Aragonite, an orthorhombic CaCO 3 polymorph, is hard but brittle. Aragonite accounts for 95% of nacre's mass, leading one to expect the mechanical characteristics of nacre to be similar to those of aragonite, yet nacre is 3000 times more resistant to fracture than aragonite (6). Materials scientists have only recently began to learn how to prepare synthetic composites outperforming their components by such large factors, and do so inspired by nacre (7,8,9,10), although not as efficiently and orderly organized as natural nacre. It is therefore of extreme interest to understand and possibly harness the mechanisms of nacre formation. Here we report unprecedented observations on the structure and architecture of nacre enabled by the use of x-ray absorption near edge (XANES) spectroscopy (11), combined with PhotoElectron Emission spectroMicroscopy (X-PEEM)(12). These observations inform and inspire new theoretical models for nacre formation mechanisms (13).

We analyze the structure of Haliotis rufescens nacre, or mother-of-pearl, using synchrotron spectromicroscopy and x-ray absorption near-edge structure spectroscopy. We observe imaging contrast between adjacent individual nacre tablets, arising because different tablets have different crystal orientations with respect to the radiation's polarization vector. Comparing previous data and our new data with models for columnar nacre growth, we find the data are most consistent with a model in which nacre tablets are nucleated by randomly distributed sites in the organic matrix layers.

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We have used laser ablation to grow series of PbTiO 3 /BaTiO 3 ͑PTO/BTO͒ multilayers with a modulation wavelength ⌳ that varies between 50 ÅϽ⌳Ͻ360 Å. Modeling the x-ray-diffraction patterns of the multilayers indicates that the PTO layers are a-axis oriented. The Raman measurements reinforce this interpretation. The soft-mode Raman line shifts abruptly in frequency above ⌳ϭ240 Å due to possible strain relaxation of the multilayer, and the ⌳ dependent position of an asymmetric line in the vicinity of 200 cm Ϫ1 is modeled as a confined mode, a signature of a modulated structure. RAPID COMMUNICATIONS R6450PRB 61 F. Le MARREC et al.