First-principles study of the structural and isotopic properties of Al- and OH-bearing hematite

A series of hematite structures containing various amounts of aluminum and hydroxyl groups was modeled using first-principles methods based on the density functional theory. Evolution of the lattice parameters was quantified as a function of Al and H concentrations. The a and c lattice parameters decrease with the aluminum content and increase with the water content. This allows explaining experimental data reported for synthetic hematites, in particular the observed deviation from the Vegard’s rule. Investigation of several hydroxyl configurations associated with cationic vacancies suggests that the speciation of water also significantly affects the hematite structure. The 57Fe and 18O reduced partition functions ( β-factor) were determined. Results show a linear dependence of the iron and oxygen β-factors on the aluminum content. An incorporation of 18 mole % Al 2O 3 in hematite would increase the iron β-factor of about 0.6‰ and the oxygen β-factor of about 5.5‰ at 0 °C. This effect is sufficiently large to be measurable and to affect the interpretation of natural isotopic compositions. On the other hand, the effect of water is found to be negligible for the hydroxyl configuration investigated.