Spectroscopic, structural and microcalorimetric study of stishovite, a non-pathogenic polymorph of SiO2
Stishovite, the SiO2 polymorph obtained at extremely high pressure with octahedrally coordinated Si atoms, has been characterized by means of techniques usually employed in surface chemistry. Its surface properties have been investigated in order to explain why stishovite is the only non-pathogenic SiO2 crystalline polymorph. X-Ray diffraction and HRTEM studies indicate that, instead of promoting a phase transition towards stable polymorphs, heating progressively destroys the crystalline structure, and the solid undergoes a nearly complete amorphisation. Photoacoustic and FTIR spectroscopies show that, on heating from room temperature to the treatments at higher temperatures (up to ca. 1073 K): (i) the unusual octahedral coordination of Si atoms in the stishovite structure is gradually lost, and new absorptions appear due to different νSi—O modes; (ii) the surface OH groups, almost all of which are involved in strong H-bonding interactions in the original material, are different from those in most silicas (no free silanols), and become more and more similar to those of ordinary SiO2 polymorphs. Both FTIR spectra and microcalorimetric data, relative to the adsorption of H2O on stishovite thermally activated in the 300–1073 K range, monitor the presence of a strong surface field: in particular, the type and strength of the surface sites are qualitatively different, and quantitatively stronger, than those usually observed for most other SiO2 polymorphs.