Glass Forming Ability of Sub-Alkaline Silicate Melts

Abstract:

The glass forming ability (GFA) and critical cooling rate (Rc) of six natural sub-alkaline melts from basalt to rhyolite (i.e., B₁₀₀, B₈₀R₂₀, B₆₀R₄₀, B₄₀R_60, B₂₀R₈₀, and R₁₀₀) have been quantified through cooling-induced solidification experiments of 9000, 1800, 180, 60, 7 and 1 °C/h conducted at ambient pressure and air buffering conditions, in a temperature range between 1300 °C (superliquidus region) and 800 °C (glass transition region), The phase proportion in each run-product was determined by image analysis on about 500 BS-SEM microphotographs. The phase assemblage consists of glass, clinopyroxene, spinel, and plagioclase with the occurrence of sporadic olivine, orthopyroxene and melilite. Both the glass and crystalline fractions are well correlated with the composition of residual melt. Generally, the amount of crystals decreases with increasing cooling rate. However, some exceptions occurs showing no correlations or even opposite trends. For the example of, Al₂O₃ and CaO in clinopyroxenes from B₁₀₀, B₈₀R₂₀, B₆₀R₄₀ and B₄₀R₆₀, their concentrations scale as a function of both cooling rate and the degree of clinopyroxene crystallization.

The value of Rc changes of 5 order of magnitude from <1 to ~9000 °C/h when the melt composition changes from R₁₀₀ to B₁₀₀, respectively. The most important Rc variations are measured between B₈₀R₂₀ and B₆₀R₄₀, levelling off towards B₁₀₀. This trend scales with NBO/T (non bridging oxygen per tetrahedron) and can be modelled by the following master sigmoid equation: Rc = a / 1+e^{-(NBO/T-xo/b)}, where a, b and xo are fitting parameters equal to 9214, 0.040 and 0.297, respectively.

Our data can be used to retrieve the solidification conditions of aphyric, degassed and oxidised lavas. Indeed, the relationship between crystal content and cooling kinetics suggests that the solidification path is more complex than previously assumed and strongly non-linear. This finding has also implications to design glass-ceramics based on natural, accessible and low-cost starting materials.