Volatiles in silicate melts at high pressure and temperature: 2. Water in melts along the join NaAlO 2SiO 2 and a comparison of solubility mechanisms of water and fluorine

Abstract

The solubility mechanisms of water have been determined in melts along the anhydrous composition join SiO 2NaAlO 2 in the Al ( Al + Si) range between 0 and 0.333 as a function of water content. Raman spectroscopy of melts quenched from 1550°C at 15 kbar have led to the following principal conclusions in regard to solubility mechanisms. Water dissolves by forming OH complexes with Si 4+, Al 3+ and Na +. Water also occurs in molecular form. SiOH bonds dominate in water-bearing silica melt, but SiOH bonds cannot be identified in melts with Al ( Al + Si) ≧ 0.125 where, however, AlOH and NaOH complexes exist. The proportion of AlOH relative to NaOH in the hydroxyl complexes ( X OH Al 3+ ) in aluminosilicate melts increases with increasing bulk melt Al ( Al + Si) . With Al ( Al + Si) = 0.125 X OH Al 3+ ) is near 0, but for melts with Al ( Al + Si) = 0.333X OH Al 3+ exceeds 0.8. The proportion of Na-complexed hydroxyl is positively correlated with water content. The structural role of OH resembles that of F in melts along the join SiO 2NaAlO 2 where NaF and AlF complexes are formed. SiF bonds have not been observed in aluminosilicate melts. In both F- and H 2O-bearing systems, nonbridging oxygens are formed in the melts, but F is a more efficient depolymerizer than water. This difference in polymerization reflects the relative stabilities of Al- and Na-bearing fluoride as compared with hydroxyl complexes. An empirical model from which the viscosity of hydrous aluminosilicate melts can be calculated is proposed. In the case of both F and H 2O, the calculated viscosities reproduce the experimentally-determined viscosities within experimental and analytical uncertainty.