Nucleation mechanisms: A crystal-chemical investigation of phases forming in highly supercooled aluminosilicate liquids

Crystal nucleation and growth have been investigated approximately 100 K above the glass transition for 12 liquids in the system CaO-Al2O3-SiO2. Nucleation takes place heterogeneously at the gas-liquid interface, yielding various crystal structures, namely, those of yoshiokaite, gehlenite, larnite, anorthite and wollastonite. Characterization by Raman spectroscopy, transmission electron microscopy and electron microprobe indicate that these phases are highly disordered, poorly crystallized and generally non-stoichiometric. Even for incongruent crystallization, crystals have the same Si/Al ratio as their parent melts but variable CaO contents. These features may be rationalized in terms of the contrasting mobility of network-forming cations, which practically vanishes at the glass transition, and that of calcium which remains significant. The fact that the mobility of network-modifying cations controls the structure and composition of nucleating phases implies that liquid viscosity (controlled by the mobility of network formers, i.e. the frequency of Si-O and Al-O bond breaking) is not an appropriate scaling parameter for crystal nucleation. In turn, this may provide an explanation for the discrepancies, reaching many orders of magnitude, between experimentally determined nucleation rates and those predicted from classical and non-classical nucleation theories. An additional factor contributing to such discrepancies is that thermodynamic data obtained on stoichiometric phases stable at liquidus temperatures are not appropriate for estimating the thermodynamic barrier to nucleation of the metastable crystals that actually form at large degrees of supercooling.