Electrostatic characterization of oxygen sites in minerals

Abstract

The electrostatic potentials of approximately 500 anion sites in 165 rock-forming mineral end-members have been computed and presented along with coordination numbers, coordinating cations, and mean cation-anion distances. The electrostatic potentials computed for the individual oxygen sites range from a low of 19.56 v for the O2 site of (β-Mg 2SiO 4 (wadsleyite) to a high of 36.4 v for soda niter, with an unweighted average of 28.58 v and standard deviation of 2.51 v. The site potentials tend to vary inversely with coordination number, so that in the silicates the bridging oxygen sites have the highest potentials, oxygens bonded to one Si have lower potentials, and oxygens not bonded to silicon have the lowest. The site potentials of hydroxyl oxygens (with well-determined H-positions) average 28.3 v with a standard deviation of 2.0 v. The potentials of fluorine sites and hydroxyl sites modeled without the H, as a single charge of −1, range from 10.3 v to 16.8 v, with an average value of 10.87 and a standard deviation of 1.85 v. The electrostatic site potentials may thus be used along with coordinating cation types to identify possible sites for hydroxyl substitution in minerals. The average oxygen site potentials for various minerals weighted on the number of sites per unit cell range from a low of 20.9 v for lime (CaO) to a high of 36.4 v for soda niter and are strongly correlated with observed oxygen isotope fractionation trends in minerals. This correlation is best for minerals containing light cations ( Z < 20), so that minerals with larger oxygen site potentials tend to concentrate the heavier oxygen isotope. This correlation is sufficiently good to permit qualitative estimation of oxygen isotope fractionation factors of minerals such as high-pressure silicates that have not been measured experimentally. With further work on experimental measurement of oxygen isotope fractionation between mineral pairs, quantitative prediction of isotope fractionation factors may become possible based on electrostatic site potentials of anion sites.