Back to Research Back to Alkaline Rocks Crystal structure of garnet-group minerals. The purple tetrahedra represent isolated SiO4 radicals, in which some of the Si may be replaced by Fe3+, Al and H4. The green octahedra denote six-coordinated crystallographic sites that can host a variety of cations. In calcic garnets from alkaline rocks, this site typically hosts Fe3+, Ti, Al, Fe2+, Mg and Zr. The yellow spheres stand for eight-coordinated large cations (predominantly Ca).