The microstructure of silicate varying with crystal and melt properties under the same cooling condition

Diopside dendrites and plagioclase spherulites were formed from Ab-An-Di silicate melts under exactly the same cooling conditions. The effects of crystal and melt structure on the microstructure of silicate were studied by optical microscopy and Raman spectroscopy. The results show that the anisotropy is truly an important factor in the growth pattern evolution. The phenomena can be described by the morphology diagram. In spite of the dendritic shape, diopside dendrite behaves like a single crystal, while the plagioclase spherulite is polycrystalline. The silicates crystallized from a fast-cooling silicate melt can reflect the structural characteristics of the melts, as is apparent from the Raman spectra. The growth units for silicate crystals are mainly the segments of Si-O polymers with different polymerization degree other than simple ions. The structural units in the silicate melt interact strongly with the crystal at the solid and melt interface; this acts as a buffer layer for structural transform from disordered or less ordered short-range structure to long-range ordered structure. The results imply that the phase-field approach to modelling the growth patterns has a realistic basis at the atomistic scale. (C) 2004 Elsevier Ltd. All rights reserved.