resumo
The crossed lamellar microstructure of mollusk shells shows a very complex hierarchical architecture constituted of long rod-shaped aragonite crystals stacked parallel to each other inside each first order lamella, which are almost perpendicular to the ones contained in parallel neighboring lamellae. To better understand the construction and properties of the crossed lamellar microstructure we have performed a detailed study to determine the crystallographic characteristics and their evolution during shell growth using scanning electron microscopy, transmission electron microscopy and X-ray diffraction texture analysis. The arrangement of crystals is rationalized by a set of twin law relationships between aragonite crystals. Specifically, the aragonite rods, or third order lamellae within each first order lamella, internally consist of polysynthetic twins bounded by {1 1 0} mirror planes. In turn, the polysynthetically twinned aragonite crystals also show a constant crystallographic orientation with respect to aragonite crystals in adjacent first order lamellae. It can be seen as another twin law in which crystals from adjacent lamellae are bounded by (1 1 0) planes but with their c-axes rotated within this plane by 30 degrees. Thus there are two sets of twin laws that relate crystal units at lower (third order lamellae) and higher (first order lamellae) length scales. These hierarchical relationships play a crucial role in the construction, organization and properties of this complex microstructure. The later orientational relationships have never been described in geological aragonite and are only found in biogenic materials with a crossed lamellar microstructure. Their occurrence is probably determined by the presence of shell organic components which regulate crystal growth and may favor unusual crystallographic relationships. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
palavras-chave
STROMBUS-GIGAS; CONCH; DESIGN
categoria
Engineering; Materials Science
autores
Rodriguez-Navarro, AB; Checa, A; Willinger, MG; Bolmaro, R; Bonarski, J
nossos autores
Marc-Georg Willinger
InvestigadorGrupos
Projectos
RMNE-UA-National Network of Electron Microscopy (REDE/1509/RME/2005 )
Structural and chemical characterization at the nanometer scale (PTDC/CTM/100468/2008)
agradecimentos
We thank Achim Klein (Fritz-Haber Institut, Berlin) for sample preparation. A.R.N. and A.C. are thankful for funding from research groups RNM-179 and RNM-190, Projects P08-RNM-04169 and RNM-6433 (Junta de Andalucia) and CGL2010-20748-CO2-01 (Ministerio de Ciencia e Innovacion). Funding to M.G.W. was provided by Projects REDE/1509/RME/2005 and PTDC/CTM/100468/2008 of the Portuguese Foundation for Science and Technology. We also thank two anonymous referees for the constructive comments which have helped to substantially improve this paper.