Modeling distribution of polymerized anions on
the liquidus of the Na_{2}O-SiO_{2}
system

In previous studies, we have presented a STRUCTON computer program
designed for statistical simulations of molecular-size distribution of
Si-anions in polymerized silicate melts (Polyakov, Ariskin, 2008). The INPUT file of this model includes
proportions of five *Q ^{n}*- structons– “SiO

*Q*^{n-1} + *Q*^{n+1} = 2*Q*^{n },š

*k _{n}*=[

These reactions cause fractions of *Q*^{1},* Q*^{2} and *Q*^{3}-structons
to increase with respect to fractions of “end-member” *Q*^{0}- andš *Q*^{4}-structons,
The distribution of *Q*^{n}-structons
observed from Raman and NMR spectroscopy is never coincides with the random
distribution for a given melt composition (Mysen,
Richet, 2005) Such deviation of the real *Q*^{4}-structon
distribution fromš the random one affects
the distribution of polymer species and should be taken into account in correct
models. For
this purpose, a special subroutine of the STRUCTON program was developed. It
calculates the distributionš
of
*Q*^{n}-structons
for a given composition using values of constants of the disproportionation
reactions (1). The spectroscopic data on these constants were rationalized
using the following Arrhenius temperature dependence:

_{}, (2)

where *s _{n}*
is the entropic factors;

As an example, we have carried out a set of testing calculations for the
Na_{2}O-SiO_{2} system in the range of 33.3-50.0 mol% of SiO_{2}.
Results of these calculations along the liquidus of
the binary system have demonstrated a systematic decrease in the proportion of *Q*^{0}-species with respect to
that following from stochastic modeling. Na_{6}Si_{2}O_{7}
(40 mol% SiO_{2}) and Na_{2}SiO_{3} (50 mol% SiO_{2})
melts are enriched in *Q*^{1}-
and *Q*^{2}-structons,
respectively (see Shildt et al., 2009). This results in the predominance of Si_{2}O_{7}^{6-} dimers in the Na_{6}Si_{2}O_{7} melt and “flat” rings
Si_{n}O_{3n}^{2n-}(Si_{3}O_{9}^{6-},
Si_{4}O_{12}^{8-}, Si_{5}O_{15}^{10-},
*etc*.) in the Na_{2}SiO_{3} melt. Such
predominance of stoichiometric polymeric compounds in
the melts follows directly from our modeling.

**References:**

Ariskin
A.A., Polyakov V.B. Simulation of molecular mass
distributions and evaluation of O^{2- }concentrations in polymerized
silicate melts // Geochemistry International. 2008. Vol. 46. P. 429-447.

*Mysen** B., Richet P.* Silicate glasses and melts:
properties and structure.

Polyakov V.B., Ariskin A.A. Simulation of the
composition and proportions of anions in polymerized silicate melts // Glass Physics
and Chemistry. 2008. Vol. 34. P. 50-62.

Shildt A.V., Polyakov V.B., Ariskin
A.A. Calculation of distributions of Q-species on the liquidus of the Na_{2}O-SiO_{2} system:
Updating the STRUCTON model // Abs. of Annual seminar on experimental
mineralogy, petrology, and geochemistry (Moscow, Vernadsky
Institute, 2009).š

*This study was
supported by the RFBR grant 08-05-00194-a.*