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| Titel |
Fluid mediated transformation of aragonitic cuttlebone to calcite |
| VerfasserIn |
C. Perdikouri, A. Kasioptas, A. Putnis |
| 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 |
250025914
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| Zusammenfassung |
| The aragonite to calcite transition has been studied extensively over the years because of its
wide spectra of applications and of its significant geochemical interest. While studies of
kinetics (e.g. Topor et al., 1981), thermodynamics (e.g. Wolf et al., 1996) and behavior of
ions such as Sr and Mg (e.g. Yoshioka et al., 1986) have been made there are still unanswered
questions regarding this reaction especially in the cases where the effects of fluid composition
are considered.
It is well known that when heated in air, aragonite transforms by a solid state reaction to
calcite. The aragonite cuttlebone of the sepia officinalis that was used for our experiments
undergoes a phase transition at ~370-390Ë C, measured by in situ heating experiments in
a Philips X’pert X-ray powder diffractometer equipped with a HTK 1200 High
temperature oven. Successive X-ray scans were taken at isothermal temperatures at 200C
intervals. A similar temperature range was found by Vongsavat et al. 2006, who studied
this transition in Acropora corals. It is possible however to promote this transition
at considerably lower temperatures by means of a fluid mediated reaction where
the replacement takes place by a dissolution-precipitation mechanism (Putnis &
Putnis, 2007). We have successfully carried out hydrothermal experiments where
cuttlebone has been converted to calcite at 200Ë C. Using the PhreeqC program we
calculated the required composition of a solution that would be undersaturated with
respect to aragonite and saturated with respect to calcite leading to dissolution of the
aragonite and to a consequent precipitation of the new calcite phase, similar to the
experiments described in an earlier study (Perdikouri et al, 2008). This reaction is not
pseudomorphic and results in the destruction of the morphology, presumably due to the molar
volume increase. A total transformation of the cuttlebone produced a fine calcite
powder.
The cuttlebone exhibits a unique microstructure, made up of interconnected chambers.
The aragonite grown during biomineralization of the cuttlebone is interlaced with
a β-chitin organic phase that provides the framework for the morphology that is
observed. Experiments carried out with the same constant conditions but for different
periods of time have revealed the evolution of the transformation to calcite. At shorter
reaction times the product was made up of calcite powder and of well preserved
aragonite septa, as was confirmed by powder X-ray diffraction. In other words, the
vertical pillars appear to react at faster rates than the horizontal septa. It has been
reported by Florek et al. 2008 that the septa contain higher quantities of β-chitin. The
aim of this study is the investigation of these observations and the determination
of the effect of the organic component on the kinetics of the aragonite to calcite
transformation.
Florek M., Fornal E., Gómez-Romero P., Zieba E., Paszkowicz W., Lekki J.,Nowak J.,
Kuczumow A. Materials Science and Engineering C, In Press (2008)
Perdikouri C., Kasioptas A., Putnis C.V., Putnis A. Mineralogical Magazine 72, 111-114
(2008)
Putnis A., Putnis C.V. Solid State Chemistry 180, 1783-1786 (2007)
Topor N. D., Tolokonnikova L. I., Kadenatsi B. M. Journal of Thermal Analysis 20,
169-174 (1981)
Vongsavat V., Winotai P., Meejoo S. Nuclear Instruments and Methods in Physics
Research B 243, 167–173 (2006)
Wolf G., Lerchner J., Schmidt H., Gamsjäger H., Königsberger E., Schmidt P. Journal of
Thermal Analysis 46, 353-359 (1996)
Yoshioka S., Ohde S., Kitano Y., Kanamori N. Marine Chemistry 18, 35-48 (1986) |
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