Surface nucleation and growth in Anorthite glass

Qualitative and quantitative studies of surface nucleation and growth were carried out with a glass having a composition close to Anorthite (CaO.Al2O3.2SiO2). Quasi-isometric hexagonal and elongated hexagonal crystals assumed to belong to the same phase, and a few rectangular crystals were observed after continuous or cumulative (intermittent) thermal treatments. The EDS analysis revealed that the chemical compositions of the hexagonal morphologies were similar to the mother glass composition. The number density on the surface, Ns (crystals/m2), and the relative concentration of the different morphologies depended on the type of polishing powder used to prepare the glass surfaces. Low magnification optical micrographs showed preferential nucleation of crystals along surface scratches. These qualitative experiments indicate that heterogeneous nucleation predominates. Some quantitative analyses were carried out for specimens polished with cerium oxide before heat treatment. The crystal nucleation curves at 960°C (Tg∼850°C) revealed an apparent transient period, τA, of a few hours. That period was mainly due to the time needed for growth of nuclei from the critical size (d∗∼1–2 nm) to detectable dimensions (ε∼1–2 μm). For continuous treatments, there was a tendency to saturate Ns. Growth kinetics were determined for isolated crystals growing on the external surfaces and for the crystalline layer growing towards the specimen center. These growth velocities were equal, within the error limits. This behavior indicates that the mechanisms of interfacial rearrangements controlling crystal growth on the surface and inside the glass are the same and corroborates previous studies for Cordierite glasses.