At present, nanostructured and amorphous CaCO 3 are believed to play an
important role in the forming of nacre, which seems to contradict the epitaxial
match between the structural organic matrix and the formed mineral. The
growth mechanism of vaterite crystals in lackluster pearls is in demand for further
study.
The investigation provided a similar hierarchy to nacre. It is well worth to
mention that two kinds of contact modes between aragonite and vaterite tablets
(side-side and front-back) were observed, which highlights the coexisting state of
aragonite and vaterite in one pearl. It is reported that strong texture of [010], [101],
and [102] was found in different scale with various methods. Vaterite tablets have a
high degree of oriented arrangement in three dimensions from several neighboring
tablets to macroscopic scale. The distribution of misorientation angles showed the
domain structure and the cluster character in vaterite tablets.
In conclusion, the formation of vaterite crystals in lackluster pearls has typical
biomineralization characteristics: (1) the size and morphology of inorganic crystals
are regular; (2) the crystals are oriented in arrays; (3) the transition from vaterite to
aragonite tablets is abrupt. Thus, both aragonite and vaterite in pearls have the
same growth mechanism according to the semblable morphology and structure.
The study of the aragonite–vaterite switch is noteworthy as the remarkable
calcite–aragonite switch in shell. The (010) plane in vaterite layers is a significant
crystalline surface, just as the (001) plane in aragonite tablets of nacre.
6.2.4 Crab (Meyers et al. 2008)
Arthropods are the largest animal phylum. They include the trilobites, chelicerates,
myriapods, hexapods, and crustaceans. All arthropods are covered by an exoskeleton, which is periodically shed as the animal grows. The exoskeleton of arthropods
consists mainly of chitin. In the case of crustaceans, there is a high degree of
mineralization, typically calcium carbonate, which gives mechanical rigidity.
The arthropod exoskeleton is multifunctional: it supports the body, resists
mechanical loads, and provides environmental protection and resistance to desiccation. The outermost region is the epicuticle, a thin, waxy layer which is the main
waterproofing barrier. Beneath the epicuticle is the procuticle, the main structural
part, which is primarily designed to resist mechanical loads. The procuticle is further
divided into two parts, the exocuticle (outer) and the endocuticle (inner), which have
similar composition and structure. The endocuticle makes up around 90 vol.% of the
exoskeleton. The exocuticle is stacked more densely than the endocuticle. The
spacing between layers varies from species to species. Generally, the layer spacing
in the endocuticle is about three times larger than that in the exocuticle. The
exoskeleton is highly anisotropic, both in structure and mechanical properties.
A striking feature of arthropod exoskeletons is their well-defined hierarchical
organization, which reveals different structural levels. At the molecular level, there
are long-chain polysaccharide chitins that form fibrils, 3 nm in diameter and 300 nm
6 Principles of Calcium-Based Biomineralization
163
important role in the forming of nacre, which seems to contradict the epitaxial
match between the structural organic matrix and the formed mineral. The
growth mechanism of vaterite crystals in lackluster pearls is in demand for further
study.
The investigation provided a similar hierarchy to nacre. It is well worth to
mention that two kinds of contact modes between aragonite and vaterite tablets
(side-side and front-back) were observed, which highlights the coexisting state of
aragonite and vaterite in one pearl. It is reported that strong texture of [010], [101],
and [102] was found in different scale with various methods. Vaterite tablets have a
high degree of oriented arrangement in three dimensions from several neighboring
tablets to macroscopic scale. The distribution of misorientation angles showed the
domain structure and the cluster character in vaterite tablets.
In conclusion, the formation of vaterite crystals in lackluster pearls has typical
biomineralization characteristics: (1) the size and morphology of inorganic crystals
are regular; (2) the crystals are oriented in arrays; (3) the transition from vaterite to
aragonite tablets is abrupt. Thus, both aragonite and vaterite in pearls have the
same growth mechanism according to the semblable morphology and structure.
The study of the aragonite–vaterite switch is noteworthy as the remarkable
calcite–aragonite switch in shell. The (010) plane in vaterite layers is a significant
crystalline surface, just as the (001) plane in aragonite tablets of nacre.
6.2.4 Crab (Meyers et al. 2008)
Arthropods are the largest animal phylum. They include the trilobites, chelicerates,
myriapods, hexapods, and crustaceans. All arthropods are covered by an exoskeleton, which is periodically shed as the animal grows. The exoskeleton of arthropods
consists mainly of chitin. In the case of crustaceans, there is a high degree of
mineralization, typically calcium carbonate, which gives mechanical rigidity.
The arthropod exoskeleton is multifunctional: it supports the body, resists
mechanical loads, and provides environmental protection and resistance to desiccation. The outermost region is the epicuticle, a thin, waxy layer which is the main
waterproofing barrier. Beneath the epicuticle is the procuticle, the main structural
part, which is primarily designed to resist mechanical loads. The procuticle is further
divided into two parts, the exocuticle (outer) and the endocuticle (inner), which have
similar composition and structure. The endocuticle makes up around 90 vol.% of the
exoskeleton. The exocuticle is stacked more densely than the endocuticle. The
spacing between layers varies from species to species. Generally, the layer spacing
in the endocuticle is about three times larger than that in the exocuticle. The
exoskeleton is highly anisotropic, both in structure and mechanical properties.
A striking feature of arthropod exoskeletons is their well-defined hierarchical
organization, which reveals different structural levels. At the molecular level, there
are long-chain polysaccharide chitins that form fibrils, 3 nm in diameter and 300 nm
6 Principles of Calcium-Based Biomineralization
163