STRUCTURE DETERMINATION OF VISTEPITE SNMN4B2SI4O16(OH)(2) - ISOTYPISMWITH BUSTAMITE, REVISED CRYSTALLOGRAPHIC DATA AND COMPOSITION

Vistepite, ideal composition SnMn4B2Si4O16(OH)(2), triclinic, P (1) ov er bar, a 6.973(2), b 7.365(2), c 7.665(1) Angstrom, alpha 89.94(2), b eta 62.94(2), gamma 76.88(2)degrees, V 339.05(10) Angstrom(3), Z = 1, occurs in the rhodonite ore deposit on the northern flank of the Inyl' chek mountain chain, Tien-Shan, in southeastern Kirghizia. It forms ti ny pale orange-yellow acicular crystals. A new electron-microprobe ana lysis yielded (in wt.%): Sn 13.70, Mn 27.91, Ca 0.71, Fe 0.28, Si 15.2 1, B 2.50, Al 0.27, O 36.64. The crystals are twinned, with [100] as t win axis and (010) as composition plane. The monoclinic unit-cell orig inally described by Pautov et al. (1992) corresponds to the F-centered cell of the twin lattice. In the structure analysis of cotype materia l, we used special procedures for the refinement of the structure invo lving twins. The refinement converged to R = 4.15% for 1244 observed r eflections of both twin individuals (R = 3.34% for 968 observed reflec tions of one individual). One Sn and two independent Mn atoms are octa hedrally coordinated. One B and two Si atoms occupy tetrahedrally coor dinated positions. Corner-sharing tetrahedra form a dreier chain paral lel to [100]. Edge-sharing octahedra form partially unoccupied infinit e band, three octahedra wide, elongate along [100], with the plane of octahedra parallel to (011). Mn atoms occupy peripheral octahedra, whe reas Sn atoms and vacancies alternate in central octahedra. This arran gement can be derived from the structure of bustamite if the Si(3) ato m is replaced by B(1), and if the M1, M2, M3 octahedral positions are occupied by Mn(2), Mn(1), Sn(1) atoms, respectively, the M4 remaining vacant. The Sn(1) octahedron is smaller, and the vacant octahedron lar ger, than the corresponding octahedra of bustamite. Therefore, the adj acent Mn octahedra are deformed. A bond-valence calculation reveals th at one oxygen atom, O(9), represents an OH group to maintain charge ba lance. The hydrogen atom has been found on the difference-Fourier map. The refinement revealed that the octahedrally coordinated position of Sn(1) is either partially vacant or partially substituted by Mn. Sinc e several various triclinic unit-cells of bustamite have appeared in t he literature, transformation matrices are provided. On the basis of r esults of the revision of cotype material for vistepite, we suggest a redefinition of the mineral species. The five most intense powder-diff raction maxima [d in Angstrom(I)(hkl)] are: 3.392(100)(112,201), 3.210 (82)(01 (2) over bar), 2.230(69)(01 (3) over bar), 2.826(59)(2 (1) ove r bar 1) and 1.6952(56)(224,004).