SiO₄ network structure changes and crystallization of diatom shells in diatomaceous earth by heat treatment

Abstract

Diatomaceous earth (DE) samples from the Nevada, United States of America and the Noto region, Ishikawa Prefecture, Japan were subjected to heat treatment of up to 1200 °C, and their SiO₄ network structurer changes and crystallization were characterized in detail by thermogravimetric and X–ray diffraction analyses, and infrared and Raman spectroscopies. Raman spectra for the Nevada–DE revealed that the SiO₄ network structure of the unheated diatom shells in Nevada–DE may be mainly composed of 6–membered rings of SiO₄ tetrahedra. In addition, with heat treatment above 600 °C, this rings structure increased and 4– and 3–membered rings of SiO₄ tetrahedra appeared, whereby the SiO₄ network structure became similar to that of silica glass. The first diffraction peak (FSDP) position of X–ray diffraction patterns showed the size of the medium–range structure of diatom shells in Nevada–DE and Noto–DE may be smaller than that of silica glass but larger than that of silica gel. Since the FSDP position of Nevada–DE and Noto–DE is the same, the medium–range structure of Noto–DE may also compose of the 6–membered rings of SiO₄ tetrahedra. Furthermore, the crystallization temperature (1100 °C) from biogenic amorphous silica such as diatom shells in Nevada–DE and Noto–DE to cristobalite was lower than that (1200 °C) of inorganic amorphous silica such as silica gel. Raman spectra show that the SiO₄ network structure in diatom shells for unheated Nevada–DE is mainly composed the 6–membered rings of SiO₄ tetrahedra such as cristobalite, and the 6–membered rings in SiO₄ network structure is increased at a lower temperature than silica gel. It suggested that the diatom shells in Nevada–DE easily crystallize to cristobalite at a lower temperature than silica gel.