The structural relation between the new synthetic silicate K2ScFSi4O10 and narsarsukite, Na2(Ti,Fe3+)(O,F)Si4O10
- Author(s)
- Uwe Kolitsch, Ekkehart Tillmanns
- Abstract
During investigations of the system pseudo-euboctahedral crystals from a KF-MoO3 flux. The crystal structure was determined from single-cry stal X-ray diffraction data (Mo-Ka radiation, CCD area detector, 2?max - 65°, Riint, = 1.07 %) and refined in space group 14/m (no. 87) to R1(F) = 1.46 % and wR2all = 4.15 % using 976 'observed' reflections with Fo > 4d(Fo). 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 pseudosymmetry 14/mmm. The structure contains infinite chains of corner-sharing, nearly regular ScO4F2 octahedra [dav(Sc-O) = 2.071 A°, dav(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 SiO4 tetrahedron with dav(Si-O) = 1.617 A°. 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 K2ScFSi4 O10: 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]. Œ 2004 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart.
- Organisation(s)
- Department of Mineralogy and Crystallography
- Journal
- European Journal of Mineralogy: an international journal of mineralogy, geochemistry and related sciences
- Volume
- 16
- Pages
- 143-149
- No. of pages
- 7
- ISSN
- 0935-1221
- DOI
- https://doi.org/10.1127/0935-1221/2004/0016-0143
- Publication date
- 2004
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 1051 Geology, Mineralogy
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/780a7ef1-dcea-41d5-91f5-fc3814c590fd