HR: 0800h
AN: T41B-1193    [Abstracts]
TI: How well do we understand the dissolution of water in minerals?
AU: * Freund, F
EM: ffreund@core2.gsfc.nasa.gov
AF: NASA Goddard Space Flight Center, Geodynamics MS 921, Greenbelt, MD 20771 United States
AU: * Freund, F
EM: ffreund@core2.gsfc.nasa.gov
AF: San Jose State University, Department of Physics, San Jose, CA 95192-0106 United States
AU: * Freund, F
EM: ffreund@core2.gsfc.nasa.gov
AF: NASA Ames Research Center, Earth System Science and Technology, Moffett Field, CA 94035-1000 United States
AB: Dissolution of H$_{2}$O in nominally anhydrous minerals is a multi-step process, which is still insufficiently understood in spite of its far-reaching consequences for understanding the deep Earth's water cycle. To study this process on a fundamental level we use the structurally simplest model mineral, MgO, which crystallizes in a densely packed, face-centered cubic structure and is predominantly ionic. When H$_{2}$O dissolves in MgO, two H$^{+}$ substitute for one Mg$^{2+}$ introducing OH$^{-}$ anions and Mg$^{2+}$ vacancies. The energetically most favorable, and hence, most likely defect is that of OH$^{-}$ pairs next to Mg$^{2+}$ vacancies. However, these OH$^{-}$ pairs are not stable. Upon cooling below ~600øC they rearrange electronically: they split off an H$_{2}$ molecule and their two oxygen anions change their valency from 2- to 1- undergoing spin pairing to form a peroxy anion, O$_{2}$$^{2-}$. We present experimental evidence (i) for the appearance of the H-H stretching band in the infrared spectrum of large MgO single crystals grown from an H$_{2}$O-laden melt, due to lattice-bound H$_{2}$ molecules, and (ii) for the copious H$_{2}$ evolution from nanocrystalline OH$^{-}$-doped, ultrahigh purity MgO. We further show that the peroxy anions control the dielectric polarization of the MgO, i.e. effective dielectric constant, and the electrical conductivity. Recognizing that the dissolution of water does not stop at the stage of forming OH$^{-}$, that OH$^{-}$ can "disappear" from the IR spectrum through their conversion to H$_{2}$, and that oxygen can adopt another valency than the usual 2- is likely to profoundly affect the way how we look at and should study the Earth's deep water cycle.
DE: 8045 Role of fluids
DE: 3904 Defects
DE: 3914 Electrical properties
DE: 1655 Water cycles (1836)
DE: 1645 Solid Earth
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting