Crystal growth kinetics in cordierite and diopside glasses in wide temperature ranges

Abstract

We measured and collected literature data for the crystal growth rate, u(T), of -cordierite (2MgO.2Al2O3.5SiO2) and diopside (CaO.MgO.2SiO2) in their isochemical glass forming melts. The data cover exceptionally wide temperature ranges, i.e. 800 1350 C for cordierite and 750 1378 C for diopside. The maximum of u(T) is reached at about 1250 C for both systems. A smooth shoulder occurs around 970 C for -cordierite. Based on measured and collected viscosity data, we fitted u(T) using standard crystal growth models. For diopside, u(T) can be well fitted with the 2D surface nucleation as well as with the screw dislocation growth mechanism. The screw dislocation model, however, yields parameters of more significant physical meaning. For cordierite, these two models also describe the experimental growth rates. However, best fittings of u(T) including the observed shoulder, were attained for a combined mechanism, assuming that the melt/crystal interface growing from screw dislocations is additionally roughened by superimposed 2D surface nucleation at large undercoolings, starting at a temperature around the shoulder. The good fittings indicate that viscosity can be used to assess the transport mechanism that determines crystal growth in these systems, from the melting point Tm down to Tg, with no sign of a breakdown of the Stokes-Einstein / Eyring equation