The Effect Of The Silicate System Crystallization Condition To The Melt Structure And The Morphology Of The Crystal\Melt Interface

As an important component of non-metallic material, silicates have been applied to various fields of social activities. In mineralogy, the silicate is the largest and most complex type of the world's minerals. Therefore, to fully study the character and the structure of the silicate is one of the most important missions. In the crystallization of the silicate melt, the changes in the crystal structure are closely related with the melt structure. When crystal growth from melt, there is always one, wide or narrow boundary layer (several microns to tens of microns) between the crystal and melt. In the front of the crystal growth, with the solid / liquid interface moving, the large dimension units which are medium-range order and composed by the anion polyhedron, arise in the boundary layer. The structure of the units is similar to the crystal structure, which is largely different from the composition and structure of the melt in the distance. Therefore, when we study the melt's whole structure and character, we also need to pay attention to the structure and composition of the boundary layer. Because the silicate melt is easy to form glass, it is easy to quench the melt to freeze the state of crystal growth and high-temperature melt. It is suitable for the detail study of crystal growth boundary layer.In this paper, we first studied the ternary system Ab-An-Di. In the Ab-An-Di phase diagram, we selected two components near the cotectic-line. After mixing the ingredients, we have prepared the samples at different undercooling degrees and different growth time, which crystal crystallized in an appropriate content. Through quenched the melt, we analyzed the structure of crystals and melt. The samples' surface morphology were observed by the optical microscope observes. Te laser Raman spectroscopy were used to test and analysis the structure. It is found that the crystal in sample 1 is dendritic diopside, the crystal in sample 4 is spherical plagioclase. Diopside in samples 1 is feathered or dendritic. Some branches grow on both sides of the stem. On some branches there are sub-branches. Plagioclase in sample 4 is like radial fibers or needle-like collection, having no branches. The Raman spectroscopy results show that, in the process of crystallization, the main unit or the structural fragments in the melt were quickly gathered implying the process of vibration characteristics rapid convergence. It is shown that the structure of crystal inherited the melt's structure characteristics. At the same time, there are some new peaks appeared in the crystal which are not in melt. Those reflect that the structure of the crystal not only inherits the melt's structure characteristic, but also occurs some changes.In An-Di binary phase diagram, the composition point of Di 80%, An 20% was selected to study. After mixing the ingredients and sampling, the original samples were melt at a high temperature. A natural diopside crystal seed was put into the melt at the liquidus temperature, and the crystal grew at different undercooling degrees and in appropriate time to obtain diopsite crystal with an appropriate content. The samples were accurate treated. The crystal morphology was studied with an optical microscope. The results showed that the diopside crystals grew relatively larger than those without seed. With the undercooling degree increasing, the shape of the diopside crystal evolved from the self-shaped, semi-self-shape to the skeleton-like, dendritic-like, and the crystal size is becoming smaller and smaller. It is suggested that with the undercooling increasing, the melt viscosity increasing, the rate of collocation [Si-O⁴]^4- tetrahedron along the c-axis tends to speed up, meanwhile, the rate in other directions relatively slow down. With the undercooling increasing, the spontaneous nucleation rate is also accelerated and the mutual influence of adjacent crystals is increscent.The Raman spectra of the samples have shown that the FWHM of the corresponding peaks of the crystalline Raman spectra becomes wider with the increasing of the undercooling degree. By comparing the Raman spectra of crystal with that of glass, it indicates that the structure of crystal is partially similar and successive to the structure of glass. Concretely speaking, there are [Si-O₄]^4- tetrahedron, [Mg(Ca)-O₆]^10- octahedron,[Al-O₄]^5- tetrahedron and [Al-O₆]^9- octahedron both in the crystal and glass.The EPMA was used to investigate the characteristics and the composition changes of the growth interface and diopside crystal. The energy spectra and wave spectra indicates that there is a several micron meter thick transition layer between the crystal and glass, where exists an evidently element variation. In this transition layer, there is a medium-range order structural unit composed of anion polyhedron coupling with each other, which makes the component of the melt in the transition layer different from that of the structural unit. As the increasing of undercooling degree, more Al element enters into diopside crystal, which results in more [Al-O₄]^5- tetrahedron replaced the [Si-O₄]^4- tetrahedron and the content of [Al-O₆]^9- octahedron became larger to ensure the balance of the charge. It is found that Al and Mg cooperated with each other strongly in the crystallization process.