Structure of Natural Melts

In natural melts, the presence of high field strength ions such as Fe , TF, and P, which, like silicon, preferentially assume a tetrahedral coordination with oxygen, complicates the structure, and the constitution of the anion matrix may not be deduced on the basis of the equations in section 6.1.2. Structural parameters valid for compositionally complex melts were proposed by Mysen et al. (1980) and Virgo et al. (1980) on the basis of the results of Raman spectroscopy. These parameters are NBO/Si and NBO/T (NBO = Non-Bridging Oxygen T groups all tetrahedrally coordinated cations—i.e., Fe , TF, P ,  

The following disproportionation reactions dominate, respectively, in the three ranges  

These equations do not imply any progression in the polymerization process and reduce to identities in the Fincham-Richardson formalism (cf table 6.3). Nevertheless, the experimental evidence of Mysen et al. (1980) and Virgo et al. (1980) may be explained by the progressive polymerization steps listed in table 6.3. These equilibria are consistent with the Fincham-Richardson formalism (of which they constitute simple multiples) and obey the proportionality rules of figure 6.4. 

Whiteway et al. (1970) derived an equation defining the fractional amount Nx of the various polymer units of extension X as a function of the ratio between bridging oxygen (0°) and singly bonded oxygen in an initially totally depolymer-ized melt (0 ) composed entirely of SiO monomers (i.e., a = 0°/0 )  

Another group of elements attentively investigated for their coordination states in silicate glasses and melts is the rare earth elements (REE). Existing stud- 

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