Tetragonally distorted octahedral sites stability

Table 2.6. Stabilization energies of transition metal ions in tetragonally distorted octahedral sites in oxides... 

A converse situation exists whereby the two oxygen ions along the z axis may move closer to the Mn3+ ion (fig. 2.8 >). This results in the stabilization of the dx2 y2 orbital relative to the dz2 orbital, and shorter Mn-O distances along the z axis compared to the x-y plane. In either of the tetragonally distorted environments shown in fig. 2.8 the Mn3+ ion becomes more stable relative to a regular octahedral coordination site. In most minerals, however, the Mn3+ ion occurs in an axially elongated octahedron (see table 6.1). 

The Co2+, Ti3 and V3 ions are expected to prefer either distorted or small octahedral sites. Thus, Co2+ should be slightly enriched in the orthopyroxene M2 and cummingtonite M4 sites, favour the pseudo-tetragonally distorted olivine Ml site, and be randomly distributed over the amphibole Ml, M2 and M3 sites. The V3+ and Ti3+ ions are expected to occupy the orthopyroxene Ml and alkali amphibole M2 sites, and to be enriched in distorted epidote M3 sites. As noted earlier, the occurrence and stability of Ti3+ ions in lunar and mete-oritic clinopyroxenes ( 4.4.1) may be explained by the availability of the distorted octahedal Ml site in the calcic clinopyroxene structure. 

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