and mineral bridges keep neighboring crystals maintaining the same crystal orientation in three dimensions as a domain structure. In recent years, some research
results seem to be different from classical theories, for instance, independent
nucleation sites for each crystal tablet, nanostructure in single aragonite tablets,
and a continuous layer of disordered amorphous CaCO 3 covering aragonite
platelets. Therefore, understanding how nacre is formed has represented a major
challenge in the field of nacre biomineralization.
In south China, freshwater cultured pearls are generally gestated in Hyriopsis
cumingii Lea. Vaterite is found in these freshwater cultured lackluster pearls. This
kind of vaterite generally coexists in a half-lackluster pearl with aragonite, the main
component of the lustrous part of the pearl, or presents individually in a complete
lackluster pearl. Vaterite often acts as a precursor in the formation of aragonite or
calcite and is a very unstable phase of calcium carbonate rarely occurring in nature.
Different from normal biomineralization, it is difficult to obtain simple and pure
vaterite phase in abnormal mineralization, often presented with calcite or aragonite.
Although vaterite in lackluster pearl is deposited abnormally under the control of
environment, this deposit is often pure, and its dimension can extend to the
macroscopic scale. Its aggregation is also one of the biggest minerals precipitated
with vaterite completely in a biological system.
The existence of vaterite is one of the key factors influencing the quality of
freshwater cultured pearls. Compared to the number of elegant studies on the
calcite–aragonite switch, little is known about the aragonite–vaterite interface in
biomineralization. The phase, morphology, structure, and orientation of vaterite
crystals deposited in lackluster pearls were studied to control the quality of freshwater pearls.
Pearl powder is very similar to the patterns of vaterite samples referring to
the standard PDF card 72–0506, without any peak of other calcium carbonate
polymorphs. The only mineral phase in a complete lackluster pearl is vaterite.
Powder X-ray diffraction (XRD) patterns of the half-lackluster pearls revealed
that they are composed of both aragonite and vaterite. Figure 6.10 shows the
microarchitecture of lackluster pearls, which reveals that the deposition of vaterite
is in the form of a “brick and mortar” hierarchy similar to nacre. Figure 6.10a shows
that the vaterite tablet is an elongated rectangle with dimensions of approximately
8 Â 2 mm
2 ; they align tightly with parallel lengths in two dimensions and assemble
layer by layer. The side elevation of vaterite tablets in Fig. 6.10b shows that the
vaterite layer is a lamella of 0.4 mm thick tablets, thinner and more irregular than
nacre, which may result in the lackluster characteristic in the pearls. Figure 6.10c
depicts that another section showing width and thickness is a rhombus. Interestingly, the crystals between successive sheets appear to nucleate close to the center
of preexisting tablets located in the layer below, as shown in Fig. 6.10c, which is a
remarkable “stack-of-coins” structure of gastropod shells and quite different from
that observed in nacre. Here we give a three-dimensional structure sketch of both
vaterite tablets and layers in lackluster pearls, as shown in Fig. 6.10d. Planes x, y, and
z stand for the observed surfaces in Fig. 6.10a, b, c, respectively (Qiao et al. 2007).
6 Principles of Calcium-Based Biomineralization
161
results seem to be different from classical theories, for instance, independent
nucleation sites for each crystal tablet, nanostructure in single aragonite tablets,
and a continuous layer of disordered amorphous CaCO 3 covering aragonite
platelets. Therefore, understanding how nacre is formed has represented a major
challenge in the field of nacre biomineralization.
In south China, freshwater cultured pearls are generally gestated in Hyriopsis
cumingii Lea. Vaterite is found in these freshwater cultured lackluster pearls. This
kind of vaterite generally coexists in a half-lackluster pearl with aragonite, the main
component of the lustrous part of the pearl, or presents individually in a complete
lackluster pearl. Vaterite often acts as a precursor in the formation of aragonite or
calcite and is a very unstable phase of calcium carbonate rarely occurring in nature.
Different from normal biomineralization, it is difficult to obtain simple and pure
vaterite phase in abnormal mineralization, often presented with calcite or aragonite.
Although vaterite in lackluster pearl is deposited abnormally under the control of
environment, this deposit is often pure, and its dimension can extend to the
macroscopic scale. Its aggregation is also one of the biggest minerals precipitated
with vaterite completely in a biological system.
The existence of vaterite is one of the key factors influencing the quality of
freshwater cultured pearls. Compared to the number of elegant studies on the
calcite–aragonite switch, little is known about the aragonite–vaterite interface in
biomineralization. The phase, morphology, structure, and orientation of vaterite
crystals deposited in lackluster pearls were studied to control the quality of freshwater pearls.
Pearl powder is very similar to the patterns of vaterite samples referring to
the standard PDF card 72–0506, without any peak of other calcium carbonate
polymorphs. The only mineral phase in a complete lackluster pearl is vaterite.
Powder X-ray diffraction (XRD) patterns of the half-lackluster pearls revealed
that they are composed of both aragonite and vaterite. Figure 6.10 shows the
microarchitecture of lackluster pearls, which reveals that the deposition of vaterite
is in the form of a “brick and mortar” hierarchy similar to nacre. Figure 6.10a shows
that the vaterite tablet is an elongated rectangle with dimensions of approximately
8 Â 2 mm
2 ; they align tightly with parallel lengths in two dimensions and assemble
layer by layer. The side elevation of vaterite tablets in Fig. 6.10b shows that the
vaterite layer is a lamella of 0.4 mm thick tablets, thinner and more irregular than
nacre, which may result in the lackluster characteristic in the pearls. Figure 6.10c
depicts that another section showing width and thickness is a rhombus. Interestingly, the crystals between successive sheets appear to nucleate close to the center
of preexisting tablets located in the layer below, as shown in Fig. 6.10c, which is a
remarkable “stack-of-coins” structure of gastropod shells and quite different from
that observed in nacre. Here we give a three-dimensional structure sketch of both
vaterite tablets and layers in lackluster pearls, as shown in Fig. 6.10d. Planes x, y, and
z stand for the observed surfaces in Fig. 6.10a, b, c, respectively (Qiao et al. 2007).
6 Principles of Calcium-Based Biomineralization
161