The crystal structure of a sample of B-bearing vesuvianite, Ca 19 (Al (sub 6.20) Fe (super 3+) (sub 1.46) Mg (sub 3.65) Fe (super 2+) (sub 1.59) Ti (sub 0.16) Mn (sub 0.07) )(Si (sub 17.57) Al (sub 0.43) )B (sub 2.73) O 68 ({OH} (sub 2.94) F (sub 0.65) O (sub 6.41) ), a 15.734(1), c 11.719(1) Aa V 2902.2(5) Aa 3 , P4/nnc, has been refined to an R index of 3.8% based on 1709 observed reflections measured at 298 K with MoKalpha X-radiation, and to an R(F 2 ) index of 9.4% based on 7482 observed reflections measured at 15 K by TOF (time-of-flight) neutron diffraction. The results of the two refinements confirm the findings of Groat et al. (1992, 1994a, b) on the mechanisms of incorporation of B into the vesuvianite structure. Furthermore, the neutron refinement allowed location of the H, which occurs only at the H(1) position in the crystal examined here. The absence of H at the H(2) channel position, together with the refined scattering at the T(2) site and local bond-valence considerations at the O(10) anion, showed that T(2) is completely occupied by B. The refined site-populations of the O(10) and O(12) sites are compatible only with 0.15 (super [2]) B and 0.85 (super [3]) B at the T(2) site. Other crystals of B-bearing vesuvianite also contain (super [3]) B at T(2) (Groat et al. 1994b). The incorporation of B at T(2) in different coordination environments provides a flexible mechanism that can act in conjunction with other substitutions elsewhere in the structure to satisfy requirements of long-range electroneutrality.

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