Fast Flow Dynamics in Alumino-Silicate Melts

The heat capacity, shear modulus, shear viscosity and density of Na2O-Al2O3-SiO2 melts indicate that there is a change in melt structure at Al3+~Na+ - where there are no longer enough Na+ to charge balance the Al3+ in tetrahedal co-ordination. Such a change in structure with composition also requires a change in flow mechanism with composition, and thus a change in the rates at which parts of the melt strcuture move. The rate of motion of structural units in silicate melts can be determined via forced oscillation methods. A low-frequency forced oscillation technique has been used to measure the frequency and temperature dependence (to 1000 °C) of the shear modulus and viscosity of a range of Na2O-Al2O3- SiO2 melts. The frequency range is between 1 and 0.001 Hz, and thus the viscosity range is from 108-1015 Pa s. The frequency-dependence of the shear modulus can be described by simple structural realaxation theory. The measured relaxation times (τ) for the peralkaline compositions agree with the Maxwell relaxation time (τM); but with increasing Al2O3 content, the measured relaxation times of the peraluminous melt structure gradually become shorter than τM. The relaxation time for the most peraluminous melt is 100 times faster than τM. Such a decrease in τ/τM has not been seen before. This shortening of relaxation times indicates that the large amount of Al3+ in these melts changes not only the structure, but also the flow mechanism of the melt. The Al3+ appears to take the slower moving Si and O with it as it moves, thus reducing the structural relaxation time and creating a fast flow mechanism in Al3+-rich melts. A second relaxation peak is seen at timescales 7.5 orders of magnitude faster than the slowest relaxation time. This very fast relaxation is assumed to be associated with the movement of Na+ in the melt. The addition of iron to these melts results in a broadening of the slowest relaxation peak. This indicates that Fe3+ moves slightly faster than Al3+ in these melts; and will also produce fast flow mechanisms in Fe-rich melts.