PNNL

Abstract

Ni and Co are critical elements to the world economy and modern technologies. Mafic and ultramafic deposits represent low-grade, yet abundant alternatives to traditional Ni and Co ores. In this work, density functional theory (DFT) with the Hubbard U correction (DFT+U) was used to simulate the incorporation of Ni and Co in forsterite (Mg2SiO4), the Mg endmember of olivine, a common mineral in mafic and ultramafic rocks. Hubbard U terms for Ni and Co were parameterized using a series of oxide, hydroxide, carbonate, silicate, and sulfide minerals relevant to extraction and recovery of Ni and Co from mafic and ultramafic deposits. Electronic, energetic, magnetic, and structural properties were considered in the parameterization. For each of Ni and Co, an effective Hubbard correction (Ueff) value that optimized agreement with either experimental data or a hybrid exchange-correlation functional, for all minerals considered, is reported. DFT+U ab initio molecular dynamics (AIMD) simulations of Ni and Co incorporated in the M1 and M2 octahedral sites of forsterite were then performed. Ni and Co substitution in the M1 site was more energetically favorable than substitution in the M2 site, in agreement with published partition coefficients. AIMD trajectories were used to compute extended X-ray absorption fine structure (EXAFS) spectra of Ni in the M1 and M2 sites for direct fitting to a published experimental spectrum of Ni in a natural San Carlos olivine sample. The results of the fit indicated that ordering of Ni in the M1 site was not as strong at the low Ni concentrations relevant to mafic and ultramafic silicate minerals as that at the higher concentrations of the Ni-Mg olivine solid solutions studied to date.