"Rod" polytypism in vesuvianite: crystal structure of a low-temperature P4nc vesuvianite with pronounced octahedral cation ordering

Vesuvianites are known to crystallize in three different tetragonal space-g roups, P4/nnc, P4/n, and P4nc. These differ ent varieties are interpreted a s "rod" polytypes. Positional cation disorder on strings parallel to the 4- fold axes, characteristic of vesuvianites crystallized between 400 and 800 degrees C, leads to P4/nnc symmetry. Low temperature (<300 degrees C) vesuv ianites have long-range ordered strings leading to either P4/n (centrosymme tric) or P4nc symmetry (noncentrosymmetric). The crystal structure of a low-temperature vesuvianite (a = 15.569(2), c = 11.841(1) Angstrom) of Ca19Al9.0Mg2.2Fe1.83+Si18O69(OH)(9) composition, fro m a rodingitized dolerite dike of the Bela Ophiolite in Pakistan, was refin ed from single-crystal Xray data in P4nc symmetry to R = 2.92%. The refinem ent showed that ca. 84% of the strings have a noncentrosymmetric arrangemen t where the apices of the five-fold coordinated Fe3+ square pyramids point in the same direction. In addition, the entire structure relaxes to P4nc sy mmetry leading to pronounced cation ordering on the octahedral Y sites. Y1 hosts only Al, whereas Y2a and Y2b (symmetry equivalent in P4/nnc symmetry) show strongly different bond lengths indicating different occupation by Al , Mg, and Fe3+.