2023ApJ...951...24K

The presence of amorphous silicate particles in interstellar and circumstellar space has been suggested based on the observation of 9.7 and 18 μm emission bands. We have successfully synthesized amorphous silicate samples of an enstatite and forsterite composition by the mechanical milling of mixed powder consisting of SiO₂-MgO and SiO₂-Mg(OH)₂ reagent-grade particles under different rotation frequencies and milling times. These two types of starting materials are prepared to study the effect of the OH bond on synthesis and crystallization. The amorphous samples were characterized by X-ray diffraction and infrared spectroscopy. Amorphous samples with enstatite composition are synthesized from both SiO₂-MgO and SiO₂-Mg(OH)₂ at 300 rpm and for 300 hr. Amorphous samples with forsterite composition are synthesized from both SiO₂-MgO and SiO₂-Mg(OH)₂. The samples from SiO₂-Mg(OH)₂ require 400 rpm and a long milling time of 1600 hr. After crystallization, amorphous samples with an enstatite composition synthesized from SiO₂-Mg(OH)₂ mainly transform into forsterite with small amounts of amorphous silica SiO₂ and enstatite depending on the rotation frequencies and milling time, while those from SiO₂-MgO become enstatite. The amorphous samples with a forsterite composition are crystallized to forsterite from both starting materials. The presence of H₂O or OH bonds significantly affects the final products after the crystallization of amorphous silicates of enstatite composition.