The crystal structure of peprossiite-(Ce), an anhydrous REE and Al mica-like borate with square-pyramidal coordination for Al (Articolo in rivista)

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Label
  • The crystal structure of peprossiite-(Ce), an anhydrous REE and Al mica-like borate with square-pyramidal coordination for Al (Articolo in rivista) (literal)
Anno
  • 2000-01-01T00:00:00+01:00 (literal)
Alternative label
  • Callegari A. 1,2; Caucia F. 1,2; Mazzi F. 2; Oberti R. 1; Ottolini L. 1; Ungaretti L. 1,2; (2000)
    The crystal structure of peprossiite-(Ce), an anhydrous REE and Al mica-like borate with square-pyramidal coordination for Al
    in The American mineralogist (Online)
    (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#autori
  • Callegari A. 1,2; Caucia F. 1,2; Mazzi F. 2; Oberti R. 1; Ottolini L. 1; Ungaretti L. 1,2; (literal)
Pagina inizio
  • 586 (literal)
Pagina fine
  • 592 (literal)
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  • 85 (literal)
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  • 3-4 (literal)
Note
  • ISI Web of Science (WOS) (literal)
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  • 1. CNR, CS Cristallochim & Cristallog, I-27100 Pavia, Italy; 2. Univ Pavia, Dipartimento Sci Terra, I-27100 Pavia, Italy; (literal)
Titolo
  • The crystal structure of peprossiite-(Ce), an anhydrous REE and Al mica-like borate with square-pyramidal coordination for Al (literal)
Abstract
  • Single-crystal structure refinements are presented of the holotype of crystal peprossiite-(Ce) (Monte Cavalluccio) and of a new sample from Cura di Vetralla (Viterbo, Italy) with slightly different composition, together with new EMP-SIMS chemical analyses. These results allow us to propose a new unit formula: [REE1-x-y(Th,U)(x)Ca-y](Al3O)(2/3)[(B4-zSi2)]O-10 with x - y + z = 1/3 (Z = 1) for the peprossiite group. Lattice constants for the holotype crystal are: a = 4.612(1), c = 9.374(3) Angstrom, V = 172.6 Angstrom(3), Z = 1. space group P (6) over bar 2m. The crystal structure was solved by Patterson methods and refined to R-obs = 1.8% (R-all = 2.2%) for 706 unique reflections in the 2 theta-range 6-136 degrees. Lattice constants for the thorian peprossiite-(Ce) from Cura di Vetralla are: a = 4.596(3), c = 9.309(16) Angstrom, V = 170.3 Angstrom(3), and the structure was refined to R-obs = 2.9% and R-all = 3.0% for 271 unique reflections in the 2 theta-range 4-80 degrees The topology of the tetrahedral layer and the site of the inter-layer cation (REE) in peprossiite resembles that of dioctahedral micas. The main difference lies in the presence of layers of pyramids instead of layers of octahedra typical of mica. In peprossiite. Al is coordinated by five O atoms in a nearly square-pyramidal arrangement, the base of which is Termed by pairs of epical O atoms from two layers of tetrahedra related by a mirror plane. Three of these pyramids share their apical O forming Al3O groups with occupancy of 2/3 according to the structure refinement. A model is proposed that explains the apparent disorder in the pyramidal layer of peprossiite by the stacking within a triple cell (with a' = a root 3 and a <^> a' = 30 degrees) of three ordered layers randomly translated by +/- a. (literal)
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