Variations of bond lengths and volumes of silicate tetrahedra with temperature

AesrRAcr The apparent SiO bond lengths recorded for silicates over a range oftemperatures either typically are invariant or exhibit a contraction with increasing temperature. A rigid-body thermal analysis was completed for the tetrahedra in nine silicates whose structures have been determined over a range of temperatures from 15 to 1250 K and whose tetrahedra seem to behave as rigid units. The coordinates provided by the analysis yield bond lengths and polyhedral volumes corrected for the librational motion of each silicate tetrahedron. The bond lengths and volumes estimated for tetrahedra with four bridging O atoms seem to increase with temperature at a faster rate than those with four nonbridglng O atoms. Those for tetrahedra with two or three nonbridging O atoms tend to increase at an intermediate rate. An analysis of the rigid-body motion of coordinated polyhedra yields a simple but accurate expression for correcting bond lengths for thermal vibrations. It also indicates that the temperature factors ofthe coordinating anions ofrigid polyhedra should be larger than those ofthe coordinated cations, regardless ofthe masses ofthe constituent atoms.