Abstract
Nucleation, stability and change of hexagonal CaAl2Si2O8 crystals in the solution of anorthite70forsterite10silica20 wt% were investigated by high temperature in-situ observations, ex-situ runs in a furnace, and examinations of the quenched products.
Hexagonal CaAl2Si2O8 was observed to crystallize dominantly in the supercooled liquid at temperatures from 1100 to 1000°C. The incubation time becomes shorter as the temperature decreases. At 1000°C, it nucleates within a few hours. At these temperatures anorthite rarely nucleates in spite of large ΔT (300–400°C), but does at 1250–1150°C (ΔT=150–250°C). Its nucleation, however, needs time more than several hours. The nucleation sequence is explainable with supercooling degree and interfacial free energy difference for each polymorph. It was also demonstrated that the hexagonal polymorph has a metastable liquidus at 1240°C, 160°C lower than that of anorthite. This difference is consistent with that in the An100 composition. The crystals of hexagonal CaAl2Si2O8 grow steadily below this metastable liquidus. This can exist permanently if anorthite is not present in the system. However, hexagonal CaAl2Si2O8 changes easily to anorthite without dissolution once the latter starts crystallization and gets in contact with the former. The contact of the two phases triggers the change from hexagonal to triclinic CaAl2Si2O8.
