HR: 17:20h
AN: V24A-06    [Abstracts]
TI: A Theoretical Study on the Dissolution Mechanisms of Forsterite
AU: * Liu, Y
EM: lyun2003@yahoo.com
AF: Dept. of Geosciences, Virginia Polytechnic Institute (Vtech), Blacksburg, VA 24060 United States
AU: Rimstidt, J
EM: jdr02@vt.edu
AF: Dept. of Geosciences, Virginia Polytechnic Institute (Vtech), Blacksburg, VA 24060 United States
AU: Gibbs, G V
EM: gvgibbs@vt.edu
AF: Dept. of Geosciences, Virginia Polytechnic Institute (Vtech), Blacksburg, VA 24060 United States
AB: {\it Ab initio} quantum chemistry and molecular dynamics simulation methods (including both classic and {\it ab initio} MD) have been used to study the dissolution progress of forsterite and other olivine series minerals at the molecule-scale. Several very interesting points were found from this study: (1) Protonation of bridging oxygen (BO) of a forsterite surface will lengthen the Mg-O bond but not necessarily break it. Mg-O bond breaking will take place as a result of more dramatic thermal fluctuations. However, at the edges or kinks of the surface, protonation is capable of directly making the Mg-O bond break. Hence the dissolution rate can be much faster at such kink or edge sites. (2) The Mg-O breaking does not take place on the first Mg-O layer. It happens on the second or third layers in the mineral bulk structure. Similar results were reported recently (e.g. Rustad, et al. 2004). This means the cations far from ($>$ 4 \AA) the surface may have strong effects on the dissolution process. (3) The slowest step of the dissolution is the dissociation of Si(OH)$_{4}$ -Mg(OH$_{2}$)$_{5}$$^{2+}$. Hence, it is the controlling step. (4) During dissolution, Mg dissociates from forsterite by forming Mg(OH$_{2}$)$_{6}$$^{2+}$ and Si dissociates from forsterite by forming neutral Si(OH)$_{4}$(aq). Every time the Si-O(H)-Mg linkage breaks, Si gets the OH while Mg combines with a water molecule from the solution. This way is more energetically favorable to further dissolution. It reduces the positive charge on the surface and hence helps to the further protonation process. (5) Different dissolution rates of the olivine series minerals can be explained by differences in the Si(OH)$_{4}$-M$^{2+}$(H$_{2}$O)$_{5}$ bond strengths.
DE: 1030 Geochemical cycles (0330)
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1099 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting