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| Titel |
Mg and Sr Incorporation during Calcium Carbonate Precipitation |
| VerfasserIn |
A. Niedermayr, M. Dietzel, S. J. Köhler |
| 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 |
250028814
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| Zusammenfassung |
| Element substitution and fractionation can provide information about the mechanisms of
CaCO3 precipitation, precipitation rates, temperatures and solution chemistry. In the present
study precipitation experiments for the formation of aragonite and calcite were carried out.
Calcium carbonates are formed at various Mg/Ca ratios at temperatures between 5 and 40Ë C
at pH 8.3 by using an advanced CO2-diffusion technique (Tang et al., 2008). XRD,
microprobe and FT-IR analysis were used to qualitatively and quantitatively characterise the
solids.
Different Mg/Ca-ratios in the solution induce different weight fractions of polymorphs at
different temperatures. Calcite crystals are formed at low temperatures and low Mg content,
while aragonite is preferentially formed at higher Mg concentrations or at elevated
temperatures. At higher Mg/Ca ratios rounded Mg-calcite crystals are formed, with higher
Mg content and lower Sr-concentrations in the crystal lattice. Different crystallographic
surfaces differ in their Mg content and a rounded habit is observed when the Mg/Ca ratio is
high, while sharp edged crystals are characteristic for low Mg incorporation. Sr and Mg
incorporation into calcite are inversely related at given Mg/Ca ratios. Rietveld-analyses of
XRD-data indicate a decrease in the cell size even at low Mg incorporation. The position of
the symmetric stretching peak in FTIR spectra decreases from calcite to aragonite with
increasing Mg content in the sample, while the out of plane bending jumps in a discrete
manner between calcite and aragonite independent of Mg-incorporation. This is in
accordance with earlier studies and validates our experimental approach. Mg incorporation
increases with increasing temperatures and lower precipitation rates, which is inverse to
the Sr incorporation behaviour. Respective mechanisms 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 in corporation, Geochimica et Cosmochimica Acta,
72, 3718-3732. |
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