Surface crystallization and texture in cordierite glasses

Systematic work was carried out at 860°C (Tg∼800°C) on the crystal growth of a high-quartz solid solution phase (μ-cordierite), which crystallizes on solid impurities on the surface and in the volume of the same cordierite glass sample. Crystal growth velocities were determined for three different growth fronts: (I) crystals that nucleated on solid inclusions existing in the glass interior and grew radially; (II) isolated crystals that nucleated on the external sample surfaces and grew with the fast axes parallel to the surface, and (III) crystals that nucleated on the external surfaces and grew towards the specimen center, spontaneously forming a textured surface layer. The crystal growth velocities of the three fronts were equal. We conclude, therefore, that the mechanisms of molecular transport at the crystal–liquid interface (or interfacial rearrangement) for crystal growth on the surface and in the interior of cordierite glass are similar. X-ray diffraction investigations have shown that, in case (III), a fiber texture is spontaneously formed with preferential alignment of the c0-axis of the grains normal to the growth front. The texture dispersion angle decreases towards the interior of the glass, in accordance with evolutionary microstructural models. The apparent induction times (≈2 h) for growth of internally nucleated crystals, case (I), and (≈3 h) for the isolated surface crystals, case (II), indicate that different nucleation sites have caused heterogeneous nucleation on the surface and in the glass interior. The long induction time for growth of the textured surface layer (≈11 h) is due to a succession of two different growth modes that form the texture in this glass.