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| Titel |
Magnesium and Strontium Incorporation into Calcium Carbonate Polymorphs and ACC - Experimental Study |
| VerfasserIn |
A. Niedermayr, M. Dietzel, S. J. Köhler, S. Petautschnig |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042827
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| Zusammenfassung |
| Incorporation of magnesium and strontium into aragonite and calcite is well known and has
been extensively used for temperature reconstruction. Moreover, knowledge on
element incorporation behaviour provides fundamental insight into calcium carbonate
growth mechanisms. Element and isotope distributions are also used for monitoring
aspects to evaluate ongoing sinter formation and inhibition effects in man made
environments. However, the precipitation rate impact on magnesium and strontium
incorporation in aragonite as well as the effect of magnesium on strontium incorporation
in calcite is still a matter of debate, and little is known about the incorporation
behaviour of magnesium and strontium in amorphous calcium carbonate (ACC) and
vaterite.
In this study, magnesium and strontium incorporation into the three calcium carbonate
polymorphs and ACC are experimentally investigated at low temperature. Spontaneous
calcium carbonate precipitation experiments without seeds were carried out using a
CO2-diffusion technique (TANG et al., 2008) and batch mixing. Experiments are conducted
(i) at various Mg/Ca ratios (up to 10), and/or (ii) in presence of polyaspartic acid at
temperatures from 5Ë to 50Ë C and various CO2-uptake rates. In the mixing experiments
a metal chloride solution is rapidly mixed with a sodium (hydrogen-) carbonate
solution.
Calcite is preferentially formed at lower temperatures, at lower supersaturation degrees
(Ω) and low Mg/Ca ratios, while aragonite is preferentially formed at high Mg/Ca ratios
and/or at elevated temperatures. Vaterite is formed at relatively high Ω values and in presence
of polyaspartic acid. ACC is obtained at the highest supersaturation in mixing experiments
and its formation is stimulated by magnesium. If magnesium is present, calcite crystals have a
steep rhombic or pseudo dodecahedronal shape with a rougher surface and acute sides are
rounded. This affects strontium incorporation. Generally, strontium incorporation increases
with increasing precipitation rate for calcite and vaterite, while the inverse is observed for
aragonite. Incorporation of magnesium is similar for calcite, vaterite and ACC, and smallest
for aragonite. Preliminary results indicate that strontium incorporation in ACC is correlated
with magnesium incorporation. Respective processes and proposed models are
discussed.
Reference:
Tang J., Köhler S.J. and Dietzel M. (2008) Sr2+/Ca2+ and 44Ca/40Ca fractionation during
inorganic calcite formation: I. Sr incorporation, Geochimica et Cosmochimica Acta, 72,
3718-3732. |
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