The structural relation between the new synthetic silicate K2ScFSi4O10 and narsarsukite, Na2(Ti,Fe3+)(O,F)Si4O10

Abstract

During investigations of the system Sc 2 O 3 -Al 2 O 3 -TiO 2- SiO 2 , a new, unusual silicate, K 2 ScFSi 4 O 10 , was synthesised as colourless pseudo-cuboctahedral crystals from a KF-MoO 3 flux. The crystal structure was determined from single-crystal X-ray diffraction data (Mo- Ka radiation, CCD area detector, 2𝛉 max = 65°, R int = 1.07 %) and refined in space group I4/m (no. 87) to R 1( F ) = 1.46% and w R2 all = 4.15% using 976 ‘observed’ reflections with F 0 > 40( F 0 ). Unit-cell parameters are: a = 11.207(2), c = 8.166(2) A, V = 1025.6(4) A 3 , Z = 4. The crystal studied is merohedrally twinned with twin plane {110}, and shows pseudo-symmetry I4/mmm. , The structure contains infinite chains of corner-sharing, nearly regular ScO 4 F 2 octahedra [ d av (Sc-O) = 2.071 A, d av (Sc-F) = 2.042 A] connected via shared F atoms along [001]. These octahedral chains share their O atoms with tubular [001] silicate chains built from four-membered silicate rings. The latter are built from one crystallographically unique, distorted SiO 4 tetrahedron with d av (Si-O) = 1.617 A. The resulting three-dimensional framework hosts voids occupied by a unique, eight-coordinated K atom. K 2 ScFSi 4 O 10 is isotypic with narsarsukite Na 2 (Ti, Fe 3+ )(O, F)Si 4 O 10 , but, although their topologies are basically identical, there exist differences which cause the merohedral twinning and pseudo-holohedry (pseudo- I4/mmm ) of K 2 ScFSi 4 O 10 : the presence of the larger K atom causes a considerable rotation of the octahedral units around [001] and a distinct rearrangement of the framework voids, both leading to the presence of a pseudo-mirror plane {110}. In narsarsukite, no such twinning is possible, as demonstrated by a careful reinvestigation of the crystal structure of a narsarsukite crystal from Mont Saint Hilaire, Canada, which confirmed a previous structure determination. The crystal-chemical relations between K 2 ScFSi 4 O 10 and KTiOPO 4 -type A ScFAsO 4 ( A = Rb, Cs) and other compounds and minerals containing trans or cis MO 4 F 2 ( M = Cr, Al, Fe) octahedra are pointed out. A structure determination of K 2 ScFSi 4 O 10 at 120 K revealed no significant changes of the atomic arrangement. The shrinkage was distinctly stronger along [001] than that along [100].