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

During investigations of the system Sc₂O₃-Al₂O₃-TiO_2-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, 2𝛉_max = 65°, R_int = 1.07 %) and refined in space group I4/m (no. 87) to R1(F) = 1.46% and wR2_all = 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 [d_av (Sc-O) = 2.071 Å, d_av (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 d_av(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].