HR: 0830h
AN: V51H-0388    [PDF]
TI: Framboidal Vaterite and its Transformation to Calcite
AU: * Nehrke, G
EM: gnehrke@awi-bremerhaven.de
AF: Carbon Group Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, Bremerhaven, 27570 Germany
AU: Van Cappellen, P
EM: pvc@geo.uu.nl
AF: Department of Geochemistry Faculty of Earth Science University Utrecht, Budapestlaan 4, Utrecht, 3584 CD Netherlands
AU: van der Wijden, C H
EM: chvdw@geo.uu.nl
AF: Department of Geochemistry Faculty of Earth Science University Utrecht, Budapestlaan 4, Utrecht, 3584 CD Netherlands
AB: Vaterite, the most soluble of the three anhydrous calcium carbonate polymorphs, is observed to form in laboratory experiments performed at high degrees of supersaturation. Under standard condition it rapidly transforms into calcite and (or) aragonite. Experimental studies on vaterite are generally based on bubbling carbon dioxide through a calcium carbonate solution. The morphology of the crystals formed is often described to be "of some spherical shape", with a typical diameter of about 5æm. A detailed investigation of the morphology by means of a high resolution scanning electron microscope (SFEG-SEM) revealed that these spheres in turn are build up of smaller spherical particles with diameters typically 100 nm. Here we propose the following mechanism for the formation of these "framboidal" structures. Nucleation of the first particles occurs on the carbon dioxide to solution interface of the bubble. The fast growth of the individual crystals leads to a depletion in solution around the crystal, which stops crystal growth at a size of about 100 nm. The collapsing gas bubble concentrates the particles within its former volume. Additional evidence for such a mechanism comes from the observation that terminating the experiment after formation of the first particles does not result in any "framboidal" aggregates. Using CO2 bubbling sometimes results in a vaterite powder containing additional calcite. A pure vaterite powder and a powder containing calcite were compared to show different transformation pathways. The morphology of the solids is identical, i.e. the calcite present is not visible under the SFEG-SEM. Calcite formation is observed immediately after a pure vaterite is added to water. Synchronous nucleation at many different sites was observed. The observation that crystal growth was not effected by changes in stirring speed confirms that diffusion did not limit crystal growth. On the hand the powder initially already containing calcite transforms by growth of a few bigger sized crystals. The impact of differences in stirring speed on the transformation rate can be explained by diffusion limited growth of a restricted number of bigger crystals, i.e. less nucleation sites available. Due to Oswald Ripening smaller crystals will eventually disappear and the final morphology of both solids will look similar. However, the different kinetics be explained only by following the change in morphology during the complete transformation process. All evidence of different transformation pathways gets lost as the morphology of the solids before and after transformation is nearly identical.
DE: 1045 Low-temperature geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting