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| Titel |
Evidence for variable crystallinity in bivalve shells |
| VerfasserIn |
D. E. Jacob, U. Wehrmeister |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250066901
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| Zusammenfassung |
| Bivalve shells are used as important palaeoclimate proxy archives and monitor regional
climate variations. The shells mostly exist of two crystalline polymorphic phases of calcium
carbonate calcite (rombohedric) and aragonite (orthorhombic). Calcite is the most stable
polymorph at standard conditions, whereas vaterite (hexagonal) is the least stable and only
rarely found in these structures.
Shells are characterized by organized structures and several micro architectures of mollusc
shell structures have been identified: Nacre shows different types: columnar and bricked
forms and consists of composite inorganic- organic at the nano-scale. They are well known to
display a “brick and mortar” structure. By AFM and FIB/TEM methods it could be shown,
that its nanostructure consists of the structures in the range of 50 – 100 nm [1, 2].
These structures are vesicles, consisting of CaCO3 and are individually coated by a
membrane. Most probably, the mantle epithelian cells of the bivalve extrude CaCO3
vesicles.
By Raman spectroscopic investigations the crystalline CaCO3 polymorphs calcite, aragonite
and vaterite, as well as ACC were determined. For some species (Diplodon chilensis
patagonicus, Hyriopsis cumingii) pure ACC (i.e. not intermingled with a crystalline phase)
could be identified. The presence of an amorphous phase is generally deduced from the
lack of definite lattice modes, whereas a broad Raman band in this region is to
observe.
In most of the cultured pearls (Pinctada maxima and genus Hyriopsis) the ν1–Raman band of
ACC clearly displays an asymmetric shape and splits into two different bands according to a
nanocrystalline and an amorphous fraction. The FWHMs of most of the crystalline fractions
are too high for well crystallized materials and support the assumption of nanocrystalline
calcium carbonate polymorph clusters in ACC. They are primarily composed of amorphous
calcium carbonate (ACC) which is later transformed into a crystalline modification [3].
Remnants of ACC can be traced by Raman spectroscopy and allow insight into the
processes of biomineralization in bivalves. The crystallinity of solid materials can be
quantified Raman spectroscopy by comparison of the FWHM of the ν1– Raman
peak.
We developed a Raman crystallinity index in order to get a measure of the degree of
crystallinity [4]. Results presented for shells from several different marine and freshwater
bivalve species and compare these with synthetically ACC and stable ACC from
Porcellio scaber. The crystalline fraction in the different ACC-areas in shells and pearls
supports the hypothesis that ACC acts as a transient precursor to crystalline CaCO3,
in which the polymorph is already moulded into nanoclusters. The distribution
and the state of crystallization of the calcium carbonate polymorphs in most of the
ACC areas are not homogenous and the term ACC includes structurally different
material.
[1] Jacob et al., 2008, Geochim. Cosmochim. Acta 72, 229, [2] Jacob et al., 2011; J. Struct.
Biol. 173 (2), 241, [3] Addadi et al., Adv. Mater. 2003; (15), 959, [4] Wehrmeister et al.,
2011. J. Raman Spectrosc. 42, (5), 926. |
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