Peralkalinity, Al{rightleftharpoons}Si Substitution, and Solubility Mechanisms of H2O in Aluminosilicate Melts |
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Authors: | MYSEN BJORN |
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Institution: | Geophysical Laboratory 5251 Broad Branch Rd., N.W., Washington, DC 20015 |
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Abstract: | The solubility mechanisms of H2O in peralkaline sodium aluminosilicatequenched melts (anhydrous NBO/T = 0.5) have been studied withRaman spectroscopy as a function of Al/(Al + Si) (003)and H2O content (07.5 wt.%). The coexisting structuralunits in the anhydrous quenched melts are TO2 (Q4), T2O5(Q3),and TO3 (Q2). In Al-free Na2Si4O9 (NS4) melt, H2O forms complexes with Na+(NaOH bonds) and with Si4+ (SiOH bonds). MolecularH2O is also detected. TO3 structural units are not detectedin this composition. In the H2O concentration range between0 and 4 wt.%, there is an approximately 20% increase in NBO/Tresulting from the increased abundance ratio, T2O5/TO2. Withfurther increments in water activity, the NBO/T of hydrous NS4melt is reduced. The depolymerization results from hydroxylationof the silica tetrahedra, whereas polymerization is due to formationof complexes with NaOH bonding. In Al-bearing compositions on the Na2Si4O9Na2(NaAl)4O9join, there is evidence for AlOH bonding in additionto NaOH and SiOH bonds. Among these complexes,the relative abundance of those with SiOH bonds diminisheswith increasing Al/(A1 + Si), whereas complexes with AlOHand NaOH bonds become more important. Complexes withNaOH bonds dominate for H2O4 wt.%, whereas complexeswith AlOH dominate at higher water content. The threestructural units, TO3, T2O5, and TO2, were observed in bothanhydrous and hydrous peralkaline sodium aluminosilicate melts.Their abundance varies, however, with the H2O concentrationin the melts. The NBO/T decreases to a minimum (a 3050%lowering of NBO/T relative to anhydrous materials) for low H2Ocontents (34 wt.% H2O), and increases as the H2O contentis increased further. |
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