Site occupancies in silicate structures

The site preferences shown by cations in the spinel structure demonstrate that transition metal ions prefer coordination sites that bestow on them greatest electronic stability. In addition, certain cations deform their surrounding in order to attain enhanced stability by the Jahn-Teller effect. These two features suggest that similar factors may operate and cause enrichments of cations in specific sites in silicate structures, leading to cation ordering or intersite (intracrystalline ) partitioning within individual minerals which, in turn, may influence distribution coefficients of cations between coexisting phases. 

In aluminosilicates, too, including epidote and the Al2SiOs polymorphs andalusite, kyanite and sillimanite, Al3+ ions occupy several coordination sites. Again, trivalent cations such as Cr3+, V3+, Mn3+ and Fe3+ may show cation ordering and be relatively enriched in one specific Al3+ site in a crystal structure. 

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