Single-crystal X-ray- and neutron-diffraction refinements (R = 3.9% and 6.6%, respectively) were performed on a sample of 4/n vesuvianite [a 15.532(1), c 11.821(1) Angstrom, V 2851.7(5) Angstrom(3)] from a rodingite in Val d'Ala, Piemonte, Italy. The results demonstrate an ordered distribution of Ca over Ca(4)a,b and of Fe over M(3)a,b sites, whereas no preference is shown by Al, Fe and Mg atoms at M(2)n, b sites. Moreover, the neutron refinement located five hydrogen positions [H(10)a, b, c and H(11)a, b], statistically occupied. H(ll)a,b atoms are bonded to O(11)a and b, whereas H(10)a,b,c atoms are located along the 4-fold axis, and form hydroxyl groups with O(10)a and b. The presence of hydrogen gives rise to several possible local configurations, leading to complex schemes of order.
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