Microscopic substitution mechanism of Ni²⁺ in olivine lattice

Olivine is the most abundant silicate mineral in the upper mantle and a key host phase for nickel. It plays a key role in the Ni distribution and cycling in the deep Earth’s interior. In this study, a series of Mg-Ni olivine solid solutions were synthesized under high-temperature and high-pressure conditions using the piston-cylinder apparatus. The crystal structure refinement and Raman spectroscopic measurement of these synthetic samples were conducted. The results show that their unit-cell volumes and average M—O (M = Mg, Ni) bond lengths are generally decreased with the increase of Ni contents. When the Ni²⁺ molar content of olivine is less than 30%, Ni²⁺ is preferentially concentrated in the M1 octahedral site of olivine. However, with the further increase of the Ni²⁺ concentrations, the distribution of Ni²⁺ in the M₁ and M₂ sites of olivine is progressively disordered. This ordered-to-disordered substitution mechanism of Ni²⁺ at the M₁ and M₂ sites of olivine is also observed in the Raman spectra of olivines. With the increase of Ni²⁺ content in olivine, the vibrational frequency of lattice of the olivine systematically shifts from the slow increase at first to the decrease later. Hence, for the natural olivine sample with low Ni content, the ordered substitution of Ni²⁺ in its lattice determines the occurrence and partitioning behavior of Ni²⁺ in olivine, from a microscopic perspective.