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Crystal structure of isolueshite and its synthetic compositional analogue

Sergey V. Krivovichev, Anton R. Chakhmouradian, Roger H. Mitchell, Stanislav K. Filatov and Nikita V. Chukanov
Crystal structure of isolueshite and its synthetic compositional analogue
European Journal of Mineralogy (June 2000) 12 (3): 597-607


The crystal structure of isolueshite, a recently described member of the perovskite group from the Khibina complex (Russia), was refined by single-crystal methods in the space group Pm3m [a = 3.909(1) A) to R = 0.031 and wR = 0.061. The previously proposed cubic symmetry of isolueshite was confirmed. In contrast to the known perovskite-group minerals crystallizing with the cubic symmetry, isolueshite shows a disordered arrangement of oxygen atoms in a Pm3m-type cell. In isolueshite, the oxygen atoms are disordered from their "ideal" site at 3c (1/2, 0, 1/2) over four 12h sites (x, 0, 1/2) with a refined x of 0.579(5). The refinement was based on the structural formula (Na (sub 0.75) La (sub 0.19) Ca (sub 0.06) ) (sub Sigma 1.00) (Nb (sub 0.50) Ti (sub 0.50) ) (sub Sigma 1.00) O (sub 3.00) closely corresponding to the empirical formula of the mineral. The presence of hydroxyl groups in the structure of isolueshite was confirmed by infrared spectroscopy (absorption lines at 3450 cm (super -1) and 1080 cm (super -1) ). Structural formulae of the mineral accounting for the presence of (OH) (super 1-) groups are given. A synthetic compound of the composition (Na (sub 0.75) La (sub 0.25) ) (Nb (sub 0.50) Ti (sub 0.50) )O (sub 3) was prepared using the ceramic technique (final heating at 1200 degrees C), and shown not to crystallize with cubic symmetry. The crystal structure of this compound is best refined (Rietveld method) in orthorhombic symmetry [Cmcm, a = 7.7841(9), b = 7.8045(2), c = 7.7831(9) A). This structure is derived from the ideal perovskite lattice (Pm3m) by rotation of the (Nb,Ti)O (sub 6) octahedra about two tetrad axes of the cubic subcell (tilt system a (super 0) b (super +) c (super -) ). Our data and, in particular, the presence of oxygen disorder (incongruent octahedral tilting) in the structure of isolueshite, suggest that stabilization of the cubic symmetry of this mineral may be controlled by thermodynamic or kinetic factors (crystallization temperature, pressure or cooling rate), rather than by the isomorphic substitutions involving La and Ti. Hence, isolueshite should probably be considered a quenched or "frozen" polymorph of NaNbO (sub 3) .

ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 12
Serial Issue: 3
Title: Crystal structure of isolueshite and its synthetic compositional analogue
Affiliation: Saint Petersburg, Department of Crystallography, Saint Petersburg, Russian Federation
Pages: 597-607
Published: 200006
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 39
Accession Number: 2000-049402
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
N37°00'00" - N83°00'00", W170°00'00" - E28°00'00"
Secondary Affiliation: Lakehead University, CAN, Canada
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2021, American Geosciences Institute.
Update Code: 200016
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