Effects of temperature and pressure on interatomic distances in oxygen-based minerals

Abstract

Variations of metal-oxygen bond distances with temperature and pressure have been compiled from previously published studies of oxides and silicates for eighteen types of cation polyhedra. All metal-oxygen distances vary approximately linearly with both temperature (23° to 1000°C) and pressure (to 100 kbar), and mean coefficients of metal-oxygen bond expansion and compression are proposed. Two empirical relationships between bond expansion, compression, and other bonding variables are: $$\displaylines{ (1)\,\bar \alpha \, = \,32.9\,(0.75\,-\,z/p)\, \times \,10ˆ{-6} \,ˆ\circ {\rm C}ˆ{-{\rm 1}} ,\,{\rm and} \cr {\rm (2)}\,\bar \beta \,{\rm = }\,{\rm 37}{\rm .0}\,(d_0ˆ3 /z)\, \times \,10ˆ{-6} \,{\rm k}\,{\rm bar}ˆ{-{\rm 1}} , \cr} $$ where z is the cation charge, p the cation coordination number, d 0 the mean polyhedral metal-oxygen distance at 1 atm and 23°C, $$\bar \alpha $$ the mean coefficient of linear expansion from 23°C to 1000°C, and $$\bar \beta $$ the mean coefficient of linear compression to 100 kbar. For many cation polyhedra, including IVSi4+, VIAl3+, IVBe2+, VIMg2+, VIFe2+, VINi2+, and VICo2+, the observed ratio $$\bar \alpha /\bar \beta $$ is approximately constant. Consequently, for minerals containing these cations, bond-length changes during thermal expansion will be the inverse of those during compression. Using the two empirical relationships it is possible to predict bond compressions and expansions for other cation polyhedra, and thus provide a basis for estimating structural variations with temperature and pressure for many oxygen-based minerals.