Entropy dependence of viscosity and the glass-transition temperature of melts in the system diopside-anorthite

Viscosities of diopside-anorthite melts were measured over the wide range of temperature (near the glass-transition temperature–1580°C/1bar) and pressure (5–20 kb/above the liquidus temperature). The measurements were carried out by the fibre-elongation method for low temperature and the counter-balanced sphere method for high temperature at 1 bar, and the sinking and floating spheres method for high temperature at high pressure. Some of the values obtained deviated slightly from those in the literature. The data on viscosity and the glasstransition temperature have been interpreted on the basis of the configurational entropy theory, by which temperature and compositional effects on viscosity were explained well. The configurational entropies at the glasstransition temperature of magmatic silicate melts are almost constant if we use an average molecular weight (amw) or bead as a unit; 8.0±1.2 J/K·amw, 1.1 ±0.2cal/K·bead. The latter value coincides well with the value from the literature for organic polymers. The negative deviation from linearity of the glass-transition temperature of intermediate melts may be interpreted as the effect of the mixing entropy. The calculated glasstransition temperature-composition curve using the mixing entropy agreed well with the experimental values.