An experimental study on the pressure dependence of viscosity in silicate melts.

The effect of pressure on melt viscosity was investigated for five compositions along the join An(CaAl(2)Si(2)O(8))-Di(CaMgSi(2)O(6)) and four alkali silicates containing lithium, sodium, and potassium in constant ratio of approximately 1:1:1, but alkali-silica ratios are varying. The experiments were performed in an internally heated gas pressure vessel at pressures from 50 to 400 MPa in the viscosity range from 10(8) to 10(11.5) Pas using parallel plate viscometry. The polymerized An composition shows a negative pressure dependence of viscosity while the other, more depolymerized compositions of the join An-Di have neutral to positive pressure coefficients. The alkali silicates display neutral to slightly positive pressure coefficients for melt viscosity. These findings in the high viscosity range of 10(8)-10(11) Pas, where pressure appears to be more efficient than in low viscous melts at high temperature, are consistent with previous results on the viscosity of polymerized to depolymerized melts in the system NaAlSi(3)O(8)-CaMgSi(2)O(6) by Behrens and Schulze [H. Behrens and F. Schulze, Am. Mineral. 88, 1351 (2003)]. Thus we confirm that the sign of the pressure coefficient for viscosity is mainly related to the degree of melt polymerization in silicate and aluminosilicate melts.