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University of British Columbia, Department of Geological Sciences, Vancouver, BC, Canada
University of Manitoba, Canada
University of Louisville, United States
Argonne National Laboratory, United States
Canadian Museum of Nature, Canada
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.
This record provided courtesy of AGI/GeoRef.
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