HR: 15:25h
AN: V23B-08    [Abstracts]
TI: The Valence State of Silicon and Redox Dynamics in Aluminosilicate Melts
AU: * Cooper, R F
EM: reid_cooper@brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912-1846 United States
AU: Pettersen, C
EM: claire.pettersen@icecube.wisc.edu
AF: University of Wisconsin-Madison, Materials Science Program, 1509 University Ave., Madison, WI 53706 United States
AU: Everman, R L
EM: reverman@gmail.com
AF: University of Wisconsin-Madison, Materials Science Program, 1509 University Ave., Madison, WI 53706 United States
AB: Physicists have long been aware of the many valence states of Si and the roles these play in the kinetics of thermal oxidation of Si single crystals and the molecular structure of the amorphous oxide film (e.g., Borman et al., 1991). Similarly, the dynamics of oxidation and of vaporization of SiC are also affected by the presence of Si²⁺ in the amorphous silica surface film (e.g., Dunham et al., 1998; Mendybaev et al., 2002). Nevertheless, Si^2+,4+ heterovalency is little considered in redox studies of silicate melts as reported in the petrology literature. We have performed experiments in which a liquid bronze (Cu,Sn) alloy was reacted with (1) a magnesium aluminosilicate melt and (2) a Zn²⁺-doped magnesium aluminosilicate melt, all done at a low oxygen fugacity (sufficient to keep the metal alloy from oxidizing in reaction with the gas environment). The driving potential for metal melt-silicate melt reaction has two components: (a) reduction of the silicate melt and oxidation of the metal alloy; (b) formation of a homogeneous silicate solution that incorporates ionic Cu and Sn. The reaction morphologies present compelling evidence that Si⁴⁺ in the silicate melt is reduced in part to Si²⁺, initially so as to incorporate Cu^+,2+ into the melt; as the reaction proceeds, however, the Si²⁺ mobility becomes important in charge-compensation of the "inward" flux of Sn²⁺. Addition of Zn²⁺ to the starting silicate melt forces a spatially periodic variation in the silicate melt structure (as suggested by the chemistry) as the reaction proceeds. In separate experiments, reduction of an FeO-bearing calcium-magnesium aluminosilicate melt in a CO-rich environment creates a reaction morphology suggestive of reduction of Si⁴⁺ to facilitate the incorporation of carbonate ions into the melt. These experiments are perhaps exotic; nevertheless, they provoke the consideration of the potential role(s) played by silicon valence in any "self-buffering" process associated with the evolution of planetary interiors. Borman et al., Phys. Rev. Lett. 67:2387-2390 (1991). Dunham et al., Mater. Sci. Forum 264-2:391-394 (1998). Mendybaev et al., Geochim. Cosmochim. Acta 66:661-682 (2002).
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3640 Igneous petrology
DE: 3939 Physical thermodynamics
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005