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Cayalsite-(Y), a new rare-earth calcium aluminium fluorosilicate with OD character

Thomas Malcherek, Jochen Schlueter, Mark Cooper, Neil Ball and Tomas Husdal
Cayalsite-(Y), a new rare-earth calcium aluminium fluorosilicate with OD character
European Journal of Mineralogy (July 2015) 27 (5): 683-694


The crystal structure of cayalsite-(Y), with idealized composition CaY (sub 6) Al (sub 2) Si (sub 4) O (sub i8) F (sub 6) , has been determined on the basis of single-crystal X-ray diffraction data. The mineral occurs in several polytypic modifications arising from the combination of three non-equivalent structural layers. Two maximum degree of order (MDO) polytypes have been identified in cayalsite-(Y). The orthorhombic MDO polytype, cayalsite-(Y)-1O, has lattice parameters a = 15.993(1), b = 5.5306(3) and c = 9.6590(7) Aa. It is described in space group symmetry Pban and refines to R (sub obs) = 2.2%. The monoclinic MDO polytype, cayalsite-(Y)-1M, a = 11.0602(7), b = 5.5280(2), c = 16.0195(9)Aa, beta = 118.925(3) degrees is described in space group symmetry P2/c and refines to R (sub obs) = 3.5% in a two phase mixture with cayalsite-(Y)-1O. Predominantly heavy lanthanide elements substitute for Y and Ca in cayalsite-(Y), with some Ca also substituting for the rare-earth elements (REE). A typical empirical composition, normalized to 24 anions, is Ca (sub 1:03) (Y (sub 4.73) Nd0.02Gd (sub 0.34) Dy (sub 0.43) Er (sub 0.31) Tb (sub 0.22) ) (sub Sigma 6.05) Al (sub 1.87) Si (sub 4.03) (Fe (sub 6.08) O (sub 17.92) ) (sub Sigma 24) . Cayalsite-(Y) forms colourless to faintly pink, prismatic crystals in cavities of yttrian fluorite in two granitic pegmatites located in Tysfjord, Nordland, Norway. Cayalsite-(Y)-1O crystals are optically biaxial (+), with indices of refraction alpha = 1.730(5), beta = 1.740(5), gamma = 1.760(5) and 2V (sub meas) = 56.5(5). The crystal structure of cayalsite-(Y) is composed of linear chains of edge sharing [AlO (sub 6) ] octahedra with isolated [SiO (sub 4) ] tetrahedra. The REE and Ca-cations occur in eight-fold coordination by O and F anions. Apart from substitutional disorder of REE and Ca atoms, the cayalsite structure is characterized by substitutional and positional disorder affecting the local position of [SiO4] tetrahedra in one of its layers. Avoidance of close Si-F contacts causes the partial splitting of O and F positions. The possible interplay of layer stacking and cation ordering is discussed.

ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 27
Serial Issue: 5
Title: Cayalsite-(Y), a new rare-earth calcium aluminium fluorosilicate with OD character
Affiliation: Universitaet Hamburg, Mineralogisch-Petrographisches Institut, Hamburg, Germany
Pages: 683-694
Published: 20150707
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 13
Accession Number: 2015-082113
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
N64°55'00" - N69°17'60", E11°45'00" - E18°15'00"
Secondary Affiliation: University of Manitoba, CAN, Canada
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201535
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