During investigations of the system Sc2O3-Al2O3-TiO2-SiO2, a new, unusual silicate, K2ScFSi4O10, was synthesised as colourless pseudo-cuboctahedral crystals from a KF-MoO3 flux. The crystal structure was determined from single-crystal X-ray diffraction data (Mo-Ka radiation, CCD area detector, 2𝛉max = 65°, Rint = 1.07 %) and refined in space group I4/m (no. 87) to R1(F) = 1.46% and wR2all = 4.15% using 976 ‘observed’ reflections with F0 > 40(F0). Unit-cell parameters are: a = 11.207(2), c = 8.166(2) Å, V= 1025.6(4) Å3, 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 ScO4F2 octahedra [dav (Sc-O) = 2.071 Å, dav (Sc-F) = 2.042 Å] 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 SiO4 tetrahedron with dav(Si-O) = 1.617 Å. The resulting three-dimensional framework hosts voids occupied by a unique, eight-coordinated K atom.

K2ScFSi4O10 is isotypic with narsarsukite Na2(Ti, Fe3+)(O, F)Si4O10, but, although their topologies are basically identical, there exist differences which cause the merohedral twinning and pseudo-holohedry (pseudo-I4/mmm) of K2ScFSi4O10: 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 K2ScFSi4O10 and KTiOPO4-type AScFAsO4 (A = Rb, Cs) and other compounds and minerals containing trans or cis MO4F2 (M = Cr, Al, Fe) octahedra are pointed out.

A structure determination of K2ScFSi4O10 at 120 K revealed no significant changes of the atomic arrangement. The shrinkage was distinctly stronger along [001] than that along [100].

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