COE Seminar
Speaker:Dr. Bjorn O. Mysen
Affiliation:The Geophysical Laboratory of
The Carnegie Institution of Washington
Date & Time: 10:30-11:30 on Tuesday, Sep 19, 2006
Place:#503 in GeologyBuilding
Title(1):Basic Principles of Silicate Melts
Abstract:
The structure of the silicate network in silicate melts and glasses can be described in terms of network-forming cations (Si4+, Al3+, and Fe3+) that form oxygen tetrahedra. These tetrahedra can be linked across bridging oxygen, or linked to different groups of oxygen polyhedra via nonbridging oxygen that are bonded to both a tetrahedrally coordinated, network-forming cation and a network-modifying cation. The tetrahedral may contain 0-4 bridging oxygens (Qn-species), the proportions of which depend on bulk composition of the melts.
The dominant network-forming cation in most silicate glass and melt systems is Si4+. Al3+ may substitute for Si4+ on these structural locations. Ferric iron may also play such a role; whereas, when in tetrahedral coordination, other cations such as P5+ (and possibly Ti4+ and B3+) likely form separate tetrahedral clusters.
Al3+ and Fe3+ in tetrahedral coordination in the silicate network require charge-compensation, generally by alkali metals or by alkaline earths. The type of charge-compensation may govern the extent of SiAl3+ and SiFe3+ ordering.
Alkali metals and alkaline earths serving either as a network-modifying cation or to charge-balance Al3+ are structurally different. The oxygen coordination number of network-modifying cations depends on its electronic properties and tends to increase with ionic radius of metal cation. These oxygen polyhedra tend to become more deformed with increasing ionization potential.
In mixed alkali, alkaline earth (or both) silicate glasses and melts, there is ordering of the metal cations among energetically non-equivalent nonbridging oxygens. The most electronegative network-modifying cation tends to form oxygen polyhedra with nonbridging oxygen in the most depolymerized among coexisting Qn structural units.
Physical and chemical properties of silicate melts are governed by (Si,Al)-O bond strength and the characteristics of Qn-species. Transport properties are controlled by bond strength, whereas the thermodynamic properties arte governed primarily by the nature of the Qn-species.
COE Seminar
Speaker :Dr. Bjorn O. Mysen
Affiliation:The Geophysical Laboratory of
The Carnegie Institution of Washington
Date & Time: 13:30-14:30 on Tuesday, Sep 19, 2006
Place:#503 in GeologyBuilding
Title(2): Properties and Structure of Melts in Silicate-COH Systems at high Pressure - Implications for Volatile-Melt Interaction in reduced and oxidized Upper
Abstract:
Approximately 90 % of the Earth's budget of volatiles is described within the system C-O-H. The remaining volatiles are sulfur containing species.Volatile speciation depends on oxygen fugacity. In the early Earth, lessthan 100 Ga old, the system was reduced and governed by equilibria in thesystem Fe-FeO at high pressure because this is the core forming stage. Thereduced volatiles are H2, CH4, and CO. Significant proportions of H2O werealso present. Once the Earth's core was separated, the oxygen fugacity wascontrolled by silicate equilibria with fO2-values near QFM to MW buffer.
Under these conditions, the volatiles in the C-O-H system were H2O andCO2.
The solubilility and solubility mechanisms in silicate melts of reducedand oxidized C-O-H species in the pressure/temperature range of the uppermantle are quite different. The reduced species cause meltdepolymerization and reduced silica activity, as does H2O, whereasoxidized COH-species such as CO2 have the opposite effect. Experimentalresults and solution modeling pertaining to these issues will be presentedand discussed.