X-RAY AND NEUTRON CRYSTAL-STRUCTURE REFINEMENTS OF A BORON-BEARING VESUVIANITE

The crystal structure of a sample of B-bearing vesuvianite, Ca-19(Al6. 20Fe1.463+Mg3.65Fe1.592+Ti0.16Mn0.07) (Si17.57Al0.43)B2.73O68([OH]2.94 F0.65O6.41), a 15.734(1), c 11.719(1) Angstrom, V 2902.2(5) Angstrom(3 ), P4/nnc, has been refined to an R index of 3.8% based on 1709 observ ed reflections measured at 298 K with MoK alpha X-radiation, and to an R(F-2) index of 9.4% based on 7482 observed reflections measured at 1 5 K by TOF (time-of-flight) neutron diffraction, The results of thr tw o refinements confirm the findings of Great er 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 o ccurs only at the H(1) position in the crystal examined here. The abse nce of H at the H(2) channel position, together with the refined scatt ering 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 s ite-populations of the O(10) and O(12) sites are compatible only with 0.15 B-[3] and 0.85 B-[3] at the T(2) site. Other crystals of B-bearin g vesuvianite also contain B-[3] at T(2) (Groat er al. 1994b). The inc orporation of B at T(2) in different coordination environments provide s a flexible mechanism that can act in conjunction with other substitu tions elsewhere in the structure to satisfy requirements of long-range electroneutrality.