Pyrolysis experiments performed on single prisms showed that they could be
entirely dissociated in minute flat crystals (Fre ´rotte 1987). In an oversimplified
view, one could consider that each single prism is constituted of a pile of numerous
flat crystallites – intercalated with layers of organic material – that are deposited
layer per layer. However, as noted by Cuif and coworkers (1983a, b), this view is
completely deceptive, for at least two reasons: (1) The relationships between the
organic and the mineral phases are much more complex than a simple alternation
between organic and mineral layers. (2) The so-called “elementary” crystals produced by pyrolysis are not homogeneous, but they have a micro-granular texture;
this suggests that they are themselves constituted of nanocrystals, which exhibit
the same crystallographic orientation. Such an organization of “single crystals” in
elementary nanoblocks has been observed for nacre tablets (Oaki and Imai 2005),
and may represent a universal rule in biomineralization.
Whatever the organization of the intraprismatic matrix, whatever the size and
shape of the elementary units that constitute each prism, all these units exhibit
exactly the same optical orientation of their three axes, which is, by itself, absolutely
remarkable. This explains the “monocrystal” appearance of each single prism.
The set of experiments performed by Cuif and coworkers, by using different
surface treatments (proteolytic digestion, bacterial treatment, slight decalcification,
and fixation) showed different ultrastructural aspects of the distribution of the
intracrystalline matrix (Cuif et al. 1981; Cuif and Raguideau 1982; Cuif et al.
1983a, b, 1985, 1987a). First, in accordance with the striation observed on the
surface of each prism, the intracrystalline organic network exhibits a periodicity,
which is perpendicular to the prism axis. In cross section, although periprismatic
sheaths separate contiguous prisms, this periodicity, which corresponds to cycles of
mineral deposition, is found from prisms to prisms. More surprising was the
visualization by Cuif and coworkers of a longitudinal organization of the prisms.
By using appropriate treatments with glutaraldehyde/acetic acid/alcian blue, they
observed a longitudinal periodicity of the organic network. This periodicity is not
strictly parallel to the axis of the prisms, but slightly oblique. This longitudinal
organization of the matrix is correlated to the presence of ridges (that look like
“ripple-marks”) in the internal side of the prisms observed in section perpendicular
to the prisms axis (Fig. 13.2b). It is striking to observe that these ridges are parallel
in a single prism, but also parallel from prism to prism. Cuif suggested that these
ridges result from the intersection of the periodic plans of the intracrystalline
network and of the plan perpendicular to the prism axis. So far, the significance
of the longitudinal organization of the intracrystalline matrix remains obscure.
13.3.5 The Prism/Nacre Transition and the Nacreous Layer
The transition zone between the prismatic and the nacreous layer is rather abrupt
(Cuif et al. 1985). It is marked by the formation of an intermediate layer, about
50 mm thick (Fig. 13.3a, b). We sometimes observed that the thickness of this layer
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F. Marin et al.
entirely dissociated in minute flat crystals (Fre ´rotte 1987). In an oversimplified
view, one could consider that each single prism is constituted of a pile of numerous
flat crystallites – intercalated with layers of organic material – that are deposited
layer per layer. However, as noted by Cuif and coworkers (1983a, b), this view is
completely deceptive, for at least two reasons: (1) The relationships between the
organic and the mineral phases are much more complex than a simple alternation
between organic and mineral layers. (2) The so-called “elementary” crystals produced by pyrolysis are not homogeneous, but they have a micro-granular texture;
this suggests that they are themselves constituted of nanocrystals, which exhibit
the same crystallographic orientation. Such an organization of “single crystals” in
elementary nanoblocks has been observed for nacre tablets (Oaki and Imai 2005),
and may represent a universal rule in biomineralization.
Whatever the organization of the intraprismatic matrix, whatever the size and
shape of the elementary units that constitute each prism, all these units exhibit
exactly the same optical orientation of their three axes, which is, by itself, absolutely
remarkable. This explains the “monocrystal” appearance of each single prism.
The set of experiments performed by Cuif and coworkers, by using different
surface treatments (proteolytic digestion, bacterial treatment, slight decalcification,
and fixation) showed different ultrastructural aspects of the distribution of the
intracrystalline matrix (Cuif et al. 1981; Cuif and Raguideau 1982; Cuif et al.
1983a, b, 1985, 1987a). First, in accordance with the striation observed on the
surface of each prism, the intracrystalline organic network exhibits a periodicity,
which is perpendicular to the prism axis. In cross section, although periprismatic
sheaths separate contiguous prisms, this periodicity, which corresponds to cycles of
mineral deposition, is found from prisms to prisms. More surprising was the
visualization by Cuif and coworkers of a longitudinal organization of the prisms.
By using appropriate treatments with glutaraldehyde/acetic acid/alcian blue, they
observed a longitudinal periodicity of the organic network. This periodicity is not
strictly parallel to the axis of the prisms, but slightly oblique. This longitudinal
organization of the matrix is correlated to the presence of ridges (that look like
“ripple-marks”) in the internal side of the prisms observed in section perpendicular
to the prisms axis (Fig. 13.2b). It is striking to observe that these ridges are parallel
in a single prism, but also parallel from prism to prism. Cuif suggested that these
ridges result from the intersection of the periodic plans of the intracrystalline
network and of the plan perpendicular to the prism axis. So far, the significance
of the longitudinal organization of the intracrystalline matrix remains obscure.
13.3.5 The Prism/Nacre Transition and the Nacreous Layer
The transition zone between the prismatic and the nacreous layer is rather abrupt
(Cuif et al. 1985). It is marked by the formation of an intermediate layer, about
50 mm thick (Fig. 13.3a, b). We sometimes observed that the thickness of this layer
364
F. Marin et al.