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| Titel |
Ion-specific effects on spiral growth of calcite |
| VerfasserIn |
Encarnacion Ruiz-Agudo, Christine V. Putnis, Lijun Wang, Andrew Putnis |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052238
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| Zusammenfassung |
| The mechanisms by which background electrolytes affect the kinetics of non-equivalent step
propagation during calcite growth were investigated using Atomic Force Microscopy (AFM)
at constant driving force and solution stoichiometry. Spiral growth, seen as growth hillocks,
was directly observed and growth steps measured to record the growth in-situ. The results of
this work suggest that the rate of dehydration of carbonate surface sites for cation attachment
and subsequent kink nucleation control the velocity of acute step propagation in
calcite cleavage planes, while the velocity of obtuse step advancement is mainly
determined by hydration of calcium ions in solution. Both velocities can be altered
in the presence of different background ions in solution. Overall calcite growth
rates increase with increasing hydration of calcium in solution (i.e. increasing ion
separation in a solution of sodium-bearing salts) and with decreasing hydration of the
carbonate surface site (i.e. increasing ion-pairing of chloride-bearing salts). The
different sensitivity of obtuse and acute step propagation kinetics to cation and
surface hydration could be the origin of the reversed geometries of calcite growth
hillocks (i.e. rate of obtuse step spreading < rate of acute step spreading) observed in
concentrated (IS = 0.1) KCl and CsCl solutions. At low IS (0.02), ion-specific effects are
mainly associated with changes in the solvation environment of calcium ions in
solution. With increasing electrolyte concentration, the stabilization of surface water by
weakly paired salts becomes increasingly important in determining step spreading
rate. Changes in growth hillock morphology were observed in the presence of F-,
SO42- and Li+ and are ascribed to stabilization of polar (0112) and (0001) faces
due to increased hydration. These forms are commonly found in biomineralized
calcite, suggesting that relatively simple inorganic ions may have similar effects
on mineral growth at the atomic scale as more complex organic molecules. The
conclusions of this study increase our ability to predict crystal reactivity in fluids in
natural systems, which often contain significant and variable amounts of solutes. |
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