Host: The Mineralogical Society of Japan
The 10mum feature of siliceous dust that has been attributed to the stretching mode of the Si-O band has been detected in different objects, such as circumstellar envelopes of evolved stars, planetary nebula, young stellar objects, protoplanetary disks and comets. It is likely that many of grains have undergone some degree of metamorphism since their formation. In a previous paper, the IR spectrum of the film condensed from SiO vapor, which is yellow in color, showed 10mum absorption. After heat treatment in air, IR peak shifted to 9.2mum in spite of exhibiting the diffraction halo. These phenomena can be explained as the result of phase transitions of microcrystallites among polymorphism of SiO2 structure. In the present study, IR spectra and high resolution transmission electron microscopy(HRTEM) of two grains of SiO with yellow in color and SiO2 with white in color have been used in order to examine the effect of the heating in vacuum. Experimental studies on the metamorphism of SiO and SiO2 grains by heating at 10-6 Pa have been conducted by using a special heating holder attached with a high resolution electron microscope. Si crystallites were predominantly grown at 500 to 700 C in SiO particles. The Si crystallites were disappeared at 800 C and began to evaporate as the SiO phase. In the case of SiO2 grains, the appearance of Si crystallites seen on a part of SiO2 grain surface at 900 C accelerated partial evaporation from a surface of SiO2 particle. These results are analyzed based on electron diffraction patterns and high resolution transmission electron microscopic images.
