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The structural relation between the new synthetic silicate K (sub 2) ScFSi (sub 4) O (sub 10) and narsarsukite, Na (sub 2) (Ti,Fe (super 3+) )(O,F)Si (sub 4) O (sub 10)

Uwe Kolitsch and Ekkehart Tillmanns
The structural relation between the new synthetic silicate K (sub 2) ScFSi (sub 4) O (sub 10) and narsarsukite, Na (sub 2) (Ti,Fe (super 3+) )(O,F)Si (sub 4) O (sub 10)
European Journal of Mineralogy (February 2004) 16 (1): 143-149

Abstract

During investigations of the system Sc (sub 2) O (sub 3) -Al (sub 2) O (sub 3) -TiO (sub 2) -SiO (sub 2) , a new, unusual silicate, K (sub 2) ScFSi (sub 4) O (sub 10) , was synthesised as colourless pseudo-cuboctahedral crystals from a KF-MoO (sub 3) flux. The crystal structure was determined from single-crystal X-ray diffraction data (Mo-Kalpha radiation, CCD area detector, 2theta (sub max) = 65 degrees , R (sub int) = 1.07%) and refined in space group I4/m (no. 87) to R1(F) = 1.46% and wR2 (sub all) = 4.15% using 976 "observed" reflections with F (sub 0) > 4sigma (F (sub 0) ). Unit-cell parameters are: a = 11.207(2), c = 8.166(2) Aa, V = 1025.6(4) Aa (super 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 (sub 4) F (sub 2) octahedra [d (sub av) (Sc-O) = 2.071 Aa, d (sub av) (Sc-F) = 2.042 Aa] 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 (sub 4) tetrahedron with d (sub av) (Si-O) = 1.617 Aa. The resulting three-dimensional framework hosts voids occupied by a unique, eight-coordinated K atom. K (sub 2) ScFSi (sub 4) O (sub 10) is isotypic with narsarsukite Na (sub 2) (Ti,Fe (super 3+) )(O,F)Si (sub 4) O (sub 10) , but, although their topologies are basically identical, there exist differences which cause the merohedral twinning and pseudo-holohedry (pseudo-I4/mmm) of K (sub 2) ScFSi (sub 4) O (sub 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 (sub 2) ScFSi (sub 4) O (sub 10) and KTiOPO (sub 4) -type AScFAsO (sub 4) (A = Rb, Cs) and other compounds and minerals containing trans or cis MO (sub 4) F (sub 2) (M = Cr, Al, Fe) octahedra are pointed out. A Structure determination of K (sub 2) ScFSi (sub 4) O (sub 10) at 120 K revealed no significant changes of the atomic arrangement. The shrinkage was distinctly stronger along [001] than that along [100].


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 16
Serial Issue: 1
Title: The structural relation between the new synthetic silicate K (sub 2) ScFSi (sub 4) O (sub 10) and narsarsukite, Na (sub 2) (Ti,Fe (super 3+) )(O,F)Si (sub 4) O (sub 10)
Affiliation: Universitaet Wien, Institut fuer Mineralogie und Kristallographie, Vienna, Austria
Pages: 143-149
Published: 200402
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 44
Accession Number: 2004-038997
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200411
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