In MgTiO3 ilmenite (geikielite), layers of MgO6 and TiO6 octahedra alternate in an ordered fashion along the hexagonal c direction [17–19]. On the other hand, Mg2TiO4 qandilite (Fd3m spatial group) is an almost fully inverse spinel with Mg2+ ions being equally partitioned between both octahedral and tetrahedral sites
[17,18,20]. In this spinel, Ni2+ ions are expected to replace preferentially Mg2+ occupying octahedral sites ([Mg]tet[Mg1−x NixTi]octO4) due to its high octahedral site preference energy(OSPE) [21].
Therefore, incorporation of Ni2+ as solid solution in both MgTiO3 geikielite and Mg2TiO4 qandilite crystal structures would involve allocating Ni2+ ions in a single octahedral site and with slightly different Ni2+–oxygen distances to those of MgTi2O5 pseudobrookite. Accordingly, the yellowish color hue and intensity could be changed
or modulated as a result of variations in crystal field intensity and energy arising from the modification of Ni2+ octahedral environments.