Crystal structure of non-metamict Th-rich hellandite-(Ce) from Latium (Italy) and crystal chemistry of the hellandite-group minerals (Articolo in rivista)

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Label
  • Crystal structure of non-metamict Th-rich hellandite-(Ce) from Latium (Italy) and crystal chemistry of the hellandite-group minerals (Articolo in rivista) (literal)
Anno
  • 1999-01-01T00:00:00+01:00 (literal)
Alternative label
  • Oberti R. 1; Ottolini L. 1; Camara F. 1; della Ventura G. 2; (1999)
    Crystal structure of non-metamict Th-rich hellandite-(Ce) from Latium (Italy) and crystal chemistry of the hellandite-group minerals
    in The American mineralogist (Online)
    (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#autori
  • Oberti R. 1; Ottolini L. 1; Camara F. 1; della Ventura G. 2; (literal)
Pagina inizio
  • 913 (literal)
Pagina fine
  • 921 (literal)
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  • 84 (literal)
Rivista
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  • 5-6 (literal)
Note
  • ISI Web of Science (WOS) (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#affiliazioni
  • 1. CNR, CS Cristallochim & Cristallog, I-27100 Pavia, Italy; 2. Univ Roma Tre, Dipartimento Sci Geol, I-00146 Rome, Italy; (literal)
Titolo
  • Crystal structure of non-metamict Th-rich hellandite-(Ce) from Latium (Italy) and crystal chemistry of the hellandite-group minerals (literal)
Abstract
  • The crystal structure of a Th-U-rich, Y-poor hellandite-(Ce) occurring in a volcanic ejectum from Capranica (Vico volcanic complex, Latium, Italy) was refined to an R index of 1.5% for 2226 observed reflections [I > 3 sigma(I)]. Hellandite from Capranica is monoclinic P2/a with a = 19.068 (8), b = 4.745 (2), c = 10.289 (3) Angstrom, beta = 111.18 (3)degrees. The extra-framework (distorted) tetrahedral cavity, usually occupied by H in hellandite, is partially (35-40%) occupied by Be and/or Li. When this is the case, the [B4Si4O22] tetrahedral chains of hellandite are locally cross-linked to form a sheet, which is built up by the same types of rings as in semenovite but in a different arrangement. The structure refinement and the electron- and ion-microprobe analyses provide constraints on hellandite-group crystal chemistry. The presence of ([4])(Be, Li) at the Be site is coupled with the substitution of F and/or O for OH at the O5 site; the amount of extra-framework cations thus constrains the OH content in the general formula. X-ray data exclude the presence of other OH sites in the structure, in agreement with direct H determination by ion probe (1.02 H apfu + 0.98 F apfu = 2.0 apfu of monovalent anions at O5 vs. 4.5-8 apfu proposed previously). The complete chemical analysis also eliminates the need for a vacancy at the REE sites. The general formula of hellandite-group minerals is thus (Ca,REE)(8)(Th,U,Y,REE)(4)(Ti,Fe3+,Al)(2)(O,F,OHx)(4)(Be,Li)(4-x)Si8B8O44, where electroneutrality is assured by the appropriate mixing of heterovalent substituents at the M sites. (literal)
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