Rietveld refinement of okayamalite, Ca2SiB2O7:: Structural evidence for the B/Si ordered distribution

The structure of okayamalite from Arendal, Norway, was refined using the Rietveld method, with CoK alpha powder X-ray diffraction data (R-F = 3.69%). Okayamalite exhibits a melilite-type structure, space group <P(4)over bar 2(1)m>, with cell edges a = 7.1248(2), c = 4. 8177(2) Angstrom. Si and B are ordered on the T1 and T2 sites respectively, in agreement with the refined tetrahedral distances ([T1-O] = 1.657 Angstrom and [T2-O] = 1.498 Angstrom). In comparison with the other melilite-type compounds, the cation population in okayamalite leads to the minimum structural misfit between tetrahedral and square-antiprism layers.