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| Titel |
Direct observations of the role of solution composition in magnesite dissolution |
| VerfasserIn |
H. E. King, C. V. Putnis |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250058593
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| Zusammenfassung |
| Magnesite, MgCO3, occurs in association with the alteration of ultramafic rocks and
serpentine during metamorphic and metasomatic events. Its formation from the carbonation
of olivine is an indication of natural CO2 sequestration. Both magnesite dissolution and
precipitation are controlled by the strongly hydrated nature of the Mg2+ ion in solution.
Flow-through experiments have shown that increased dissolution results from an increase in
ionic strength of the solution (Pokrovsky and Schott, 1999). To test whether this observation
is related to changes in Mg hydration, we have compared the dissolution of the {1014}
magnesite surface in solutions of different electrolytes (NaCl, NaNO3 and Na2SO4) using
atomic force microscopy (AFM). As the dissolution of magnesite is slow at ambient
conditions, experimental solutions (water, 10, 100 and 500 mM) were acidified (pH
2). In all electrolyte solutions, dissolution of the magnesite surface began by the
nucleation of randomly spaced etch pits followed by the sudden nucleation of many pits
across the entire surface. Coalescence of etch pits produced islands of remnant
surface which then dissolved until a single layer was removed and a new flat surface
generated. The process was then repeated via the further nucleation of random
etch pits one unit cell deep (~ 0.3 nm). Dissolution rates were obtained from the
frequency of this dissolution cycle using measurements of surface roughness and
etch pit spreading rates. The reactivity of magnesite in the presence of added ions
varied in the order Na2SO4 < NaCl < NaNO3. Increased dissolution rates in
the presence of NO3 compared to Cl is consistent with changes in Mg hydration
(Ruiz-Agudo et al., 2010). However, although the ionic strength of the equivalent
Na2SO4 concentrations was higher than that of NaCl (expected to increase magnesite
dissolution), we observed a decrease in dissolution. The effect of sulfate is most likely
dependent on the formation of Mg-SO4 contact ion pairs (Rudolph et al., 2003) that
are not expected to form in Cl- or NO3- solutions. In all experiments, despite
the undersaturation of original solutions with respect to Mg-carbonate phases, a
precipitate formed on the surface indicating the formation of a fluid boundary layer
supersaturated with respect to a new phase (probably hydrated magnesite). This is
also consistent with Mg concentrations in experimental outflow solutions. At high
ionic strength, despite the direct observations of an increase in dissolution, the Mg
concentrations decreased. The precipitated phase was effectively passivating the mineral
surface.
References
Pokrovsky O.S. and Schott J. (1999) Geochim. Cosmochim. Acta 63, 881-897.
Rudolph W.W. et al. (2003) Phys Chem Chem Phys 5, 5253-5261.
Ruiz-Agudo E. et al. (2010) Chem. Geol. 281, 364-371. |
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