SPECTROSCOPIC, STRUCTURAL AND MICROCALORIMETRIC STUDY OF STISHOVITE, A NONPATHOGENIC POLYMORPH OF SIO2

Stishovite, the SiO2 polymorph obtained at extremely high pressure wit h octahedrally coordinated Si atoms, has been characterized by means o f techniques usually employed in surface chemistry. Its surface proper ties 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 t owards stable polymorphs, heating progressively destroys the crystalli ne 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 stishov ite structure is gradually lost, and new absorptions appear due to dif ferent nu(Si-O) modes; (ii) the surface OH groups, almost all of which are involved in strong H-bonding interactions in the original materia l, are different from those in most silicas (no free silanols), and be come 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, monito r the presence of a strong surface field: in particular, the type and strength of the surface sites are qualitatively different, and quantit atively stronger, than those usually observed for most other SiO2 poly morphs.