REFINEMENT OF THE CRYSTAL-STRUCTURE OF CRONSTEDTITE-3T

The crystal structure of cronstedtite-3T from Kutna Hora (Bohemia, Cze choslovakia), space group P3(1), was refined to R(w)(all) = 3.1% for 1 336 independent diffractions. There are two and three independent tetr ahedral and octahedral positions, respectively, in this structure. The tetrahedra are occupied by 0.75 Si and 0.25 Fe while the octahedra ar e uniformly occupied by Fe. The refinement process was hindered by two problems: a ''strong'' superposition structure (all atoms of the octh aderal sheets, i.e., divided-by 70% of the total diffraction power con tribute almost solely to the family diffractions with mod(h - k), 3) = 0), and a slight disorder of the investigated crystal. The first prob lem was resolved by a preliminary block-diagonal refinement procedure where the atoms coinciding in the superposition structure were separat ed into individual blocks. The second problem was resolved by includin g two scale factors into the final full-matrix refinement: one for fam ily diffractions, the other for the remaining ones which are character istic for this polytype.