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| Titel |
Spectroscopic characterisation of biological vaterite: relations to synthetic and geological vaterites |
| VerfasserIn |
D. E. Jacob, U. Wehrmeister, A. L. Soldati, W. Hofmeister |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250029828
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| Zusammenfassung |
| The pair aragonite and calcite are some of the most intensively studied polymorphous
minerals. These CaCO3 polymorphs are most commonly observed in biological minerals
produced by marine molluscs, whereas in freshwater molluscs mostly aragonite and vaterite,
the third CaCO3 polymorph is identified (e.g. Wehrmeister et al., 2007). Vaterite is the
thermodynamically most unstable CaCO3 polymorph and is often discussed as a precursor
phase in the mineralization of aragonite or calcite by organisms. Apart from these biological
parageneses, vaterite is also known as rare small polycrystalline aggregates from geological
occurrences. In laboratory crystallisation experiments, vaterite can be stabilized
either kinetically or with the help of organic macromolecules (e.g. Falini et al.,
2005).
Despite considerable research on vaterite, it is less well known that the crystal structure of
vaterite is not unobjectionably determined. Due to the overall small crystal sizes,
single crystal XRD analysis of vaterite is very difficult and this could be one of
the reasons for the lack of a conclusive determination of the crystal structure. At
least four different crystal structure proposals have to be considered: One proposed
vaterite to be pseudo hexagonal and to crystallise in the orthorhombic space group
Pnma (Meyer 1959). In addition, three crystal structures with hexagonal unit cells
are proposed: Kamhi (1963) and Meyer (1969) proposed the same crystal space
group: P63/mmc, whereas the site symmetry of the carbonate ion is proposed to
be different with 2mm and m, respectively. Lastly, Lippmann (1973) proposed a
structure based on the high-temperature modification of YbBO3 with space group
6322.
Here, we present new and complete Raman spectra for biological, geological and
synthetic vaterite. The spectroscopic results are evaluated in light of all published crystal
structures for vaterite and are aimed at gaining more detailed information about the
crystallographic features of vaterite. Additionally, the influence of magnesium on the widths
of the Raman bands (FWHM = full width at half maximum) is studied in order to detect a
potential correlation between the magnesium content and the FWHMs in analogy to similar
studies on aragonite and calcite.
The results imply that, although a hexagonal symmetry and the space group P63/mmc are
commonly used in the literature, it is equally possible that there are at least three structurally
non-equivalent carbonate groups in the vaterite structure. It could be speculated that the
(CO3) layers are generally similar, whereas the stacking sequence of these layers may lead to
a lower symmetry.
Non-beaded freshwater cultured pearls from China and Japan, grown in mussels of the
genus Hyriopsis were studied for their different CaCO3- polymorphs and chemical
composition. Vaterite was identified by Micro-Raman spectroscopy in polished
cross-sections. Vaterite forms relatively small areas (1-1.5 mm diameter) which are spherical
to irregular and always in close proximity to the centre of the pearl. Trace elements in
different CaCO3 polymorphs were measured using LA-ICP-MS (New Wave Research 213
nm Laser/Agilent 7500ce quadrupole ICP-MS).
Geological samples come from the Bellerberg (Eifel Mountains, Germany). The rocks
contain a mixture of minerals (e.g. ettringite, calcite, aragonite and tobermorite)
including white to dark yellow microcrystalline mineral aggregates of vaterite.
Synthetic vaterite samples were produced in collaboration with the Department of
Inorganic and Analytical Chemistry at the University of Mainz by precipitation from a
solution of CaCl2 and poly(aspartic acid), in the presence of (NH4)2CO3 (Loges et al.,
2006).
The Raman band position and FWHM of all samples are in relative good agreement,
independent of their origin. The characteristic features of the Raman spectrum of vaterite are:
at least eight relative broad bands in the region of the external lattice modes, splitting of the
most intense band ν1 into three distinct bands, activation and splitting of ν2 and splitting
of both, ν3 and ν4 into six distinct Raman bands. FWHMs of the lattice modes
are large (8 cm-1– 44 cm-1) compared to those of aragonite, implying that the
structure is not well ordered and possibly affected by stacking faults, layer shifts or
syntactic intergrowth; irregularities that further complicate the crystal structure
determination.
Correlation of Mg-contents in vaterite areas in freshwater cultured pearls with
Raman spectra obtained at the same spots show that the FWHMs are influenced by
the magnesium content. Similarly, the spectra for the vaterite samples synthesized
with differing Mg contents show successively increasing FWHM with increasing
magnesium content both for the lattice modes and the ν1 – symmetric stretching
mode.
Wehrmeister et al., 2007. J. Gemmology; 31: 269-276, Falini et al., 2005. Eur. J. Inorg.
Chem. 2005(1): 162 -167, Meyer, H. 1969. Angew. Chem. 21: 678-679, Kamhi, S., 1963.
Acta Cryst. (1963).16: 770 – 772, Meyer, H., 1969. Z. Kristallographie 128: 183 – 212,
Lippmann, F., 1973. New York, Springer, Soldati et al., 2008. Min. Mag. 72: 577- 590, Jacob
et al., 2008. Geochim. Cosmochim. Acta 72: 5401- 5415, Loges et al., 2006. Langmuir 22:
3073-3080. |
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