rather than on heteroepitaxial nucleation (Schaffer et al. 1997). The growth through
mineral bridges may ensure perfect alignment of the crystal axes along a stack
without disrupting the basic brick and mortar alternation which provides the
material with elasticity and increased resistance to fracture (Addadi and Weiner
1997). The multiple nucleation sites on any given layer are assumed discrete with
random distribution which may lead to the nano-polycrystalline structure of individual layers (Bezares et al. 2008). Although the mineral bridges are mainly located
in the nacre layers of gastropods (Song et al. 2002; Lin et al. 2008), and absent in
mature nacreous layers of bivalves (Rousseau et al. 2005), the mineral bridge model
is still suitable to explain the microstructure of incipient nacre in pearl oyster shell
(Saruwatari et al. 2009).
The different growth mechanisms of gastropods and bivalves may be related to
their different nacreous microstructures.
12.5.2 The Orientation of Crystal Growth
According to template theory, calcium carbonate crystal is formed by
heteroepitaxial nucleation on the organic matrix. X-ray and electron diffraction
observations show that the chitin fibers and the protein polypeptide chains (they are
oriented perpendicular to each other) are aligned with the a and b aragonite
crystallographic axes, respectively, which means that the organic matrix determines
the orientation of crystal growth (Weiner and Traub 1984). Aragonite crystals
preferably grow along their c axis until they reach the upper layer of chitin and
then expand laterally in the plane of the sheets where the crystal growth is less
restricted unless they make contact with other aragonite tablets in the same layer
(Mann 2001; Nudelman et al. 2007). However, the crystal orientation in the a-b
plane of the incipient nacre is a result of competition between adjacent nacre
crystals within the growth lamellae, which favors selection of crystals whose
fastest growth axis (b-axis) is oriented parallel to the direction of propagation of
the lamella (Checa and Rodriguez-Navarro 2005; Checa et al. 2006). Both
heteroepitaxy and selection by competition possibly superimpose in bivalve
nacre, with the latter probably having a prevalent effect (Checa et al. 2006;
Saruwatari et al. 2009).
Gastropod nacre formation may involve the spiral growth model rather than the
competition model mentioned above. In gastropods, the aragonite platelets grow
vertically via helices that surround numerous screw dislocation cores, and extend
horizontally simultaneously (Yao et al. 2009). These new findings may aid in
creating novel organic–inorganic micro/nano composites through synthetic or
biomineralization pathways (Yao et al. 2009)
Further nanoscopic and cross-sectional investigation shows that the aragonitic
crystal growth of incipient nacre is not only a biotic process, but also an inorganic
one, such as geometrical selection and mineral bridges (Saruwatari et al. 2009).
346
L.-p. Xie et al.
mineral bridges may ensure perfect alignment of the crystal axes along a stack
without disrupting the basic brick and mortar alternation which provides the
material with elasticity and increased resistance to fracture (Addadi and Weiner
1997). The multiple nucleation sites on any given layer are assumed discrete with
random distribution which may lead to the nano-polycrystalline structure of individual layers (Bezares et al. 2008). Although the mineral bridges are mainly located
in the nacre layers of gastropods (Song et al. 2002; Lin et al. 2008), and absent in
mature nacreous layers of bivalves (Rousseau et al. 2005), the mineral bridge model
is still suitable to explain the microstructure of incipient nacre in pearl oyster shell
(Saruwatari et al. 2009).
The different growth mechanisms of gastropods and bivalves may be related to
their different nacreous microstructures.
12.5.2 The Orientation of Crystal Growth
According to template theory, calcium carbonate crystal is formed by
heteroepitaxial nucleation on the organic matrix. X-ray and electron diffraction
observations show that the chitin fibers and the protein polypeptide chains (they are
oriented perpendicular to each other) are aligned with the a and b aragonite
crystallographic axes, respectively, which means that the organic matrix determines
the orientation of crystal growth (Weiner and Traub 1984). Aragonite crystals
preferably grow along their c axis until they reach the upper layer of chitin and
then expand laterally in the plane of the sheets where the crystal growth is less
restricted unless they make contact with other aragonite tablets in the same layer
(Mann 2001; Nudelman et al. 2007). However, the crystal orientation in the a-b
plane of the incipient nacre is a result of competition between adjacent nacre
crystals within the growth lamellae, which favors selection of crystals whose
fastest growth axis (b-axis) is oriented parallel to the direction of propagation of
the lamella (Checa and Rodriguez-Navarro 2005; Checa et al. 2006). Both
heteroepitaxy and selection by competition possibly superimpose in bivalve
nacre, with the latter probably having a prevalent effect (Checa et al. 2006;
Saruwatari et al. 2009).
Gastropod nacre formation may involve the spiral growth model rather than the
competition model mentioned above. In gastropods, the aragonite platelets grow
vertically via helices that surround numerous screw dislocation cores, and extend
horizontally simultaneously (Yao et al. 2009). These new findings may aid in
creating novel organic–inorganic micro/nano composites through synthetic or
biomineralization pathways (Yao et al. 2009)
Further nanoscopic and cross-sectional investigation shows that the aragonitic
crystal growth of incipient nacre is not only a biotic process, but also an inorganic
one, such as geometrical selection and mineral bridges (Saruwatari et al. 2009).
346
L.-p. Xie et al.