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| Titel |
Phase transitions of CaCO3 at high P and T determined by in-situ Vibrational Spectroscopy in Diamond-Anvil-Cells |
| VerfasserIn |
Monika Koch-Müller, Sandro Jahn, Natalie Birkholz, Ulrich Schade |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250087587
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| Publikation (Nr.) |
 EGU/EGU2014-1644.pdf |
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| Zusammenfassung |
| Carbonates are the most abundant carbon-bearing minerals on Earth. They can be transported
into the upper and lower mantle via subduction processes. Knowledge of the stability of solid
carbonates adapting different structures with increasing pressure and temperature is therefore
of great importance to understand the structure and dynamics of the Earth. Even for the very
simple system CaCO3 the phase relations of at high pressure and temperature are still not
fully understood. It has been known for many years that calcite (cc) can adopt different
structures with increasing pressure (e.g. Bridgman, 1939: cc-I to III; Tyburczy and Ahrens,
1986: cc-VI). But only recently Merlini et al. (2012) were able to solve the crystal
structures of some of these high-pressure polymorphs namely cc-III, cc-IIIb and
cc-VI. They report that cc-VI has a higher density then aragonite under the same
conditions.
To study the stability of the CaCO3-polymorphs, experiments were performed in
conventional diamond anvil cells (DAC) at ambient temperatures as a function of
pressure up to 30 GPa as well as in internally heated diamond anvil cells (DAC-HT) in
the pressure range 9 to 20 GPa and temperatures up to 800 K. As probe for the
structural changes we used conventional mid-infrared-, synchrotron far-infrared- and
Raman-spectroscopy. Within the cc-III stability field (3 to 15 GPa at room temperature, e.g.
Catalli and Williams, 2005) we observed in all types of experiments consistently
two different spectral patterns: one at lower P < 5 GPa and another at P > 5 < 15
GPa independent on the starting material and the pressure- and time-path of the
experiments. Whether these P-induced structural changes may be linked to the
above mentioned different structures of cc-III is not yet clear. Also, in all types
of experiments we confirmed the transition of cc-III to cc-VI at about 15 GPa at
room temperature. Merlini et al. (2012) speculated that temperature may stabilize
the structures of cc-III to lower pressure and surprisingly we found the same for
the cc-III to cc-VI transition. The reaction has a negative slope of about -0.007
GPa/K. However, our density-functional theory calculations indicate that cc-VI is still
metastable in respect to aragonite at least at zero K. We will prove if temperature
and the incorporation of smaller cations than Ca have an influence on the stability
relations.
References:
Bridgman P.W. (1939) Am J Sci, 237, 7 – 18.
Catalli K. and Williams Q. (2005) Am Mineral, 90, 1679 – 1682.
Merlini M. et al. (2012) EPSL, 333-334, 265 – 271.
Tyburczy J. A. and Ahrens T. J. (1986) J Geophysical Research, 91, 4730 – 4744. |
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