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The crystal structure of vicanite-(Ce), a borosilicate showing an unusual (Si₃B₃O₁₈)^15– polyanion

¹ Dipartimento di Scienze della Terra, Università di Roma "La Sapienza", P.le A. Moro 5, I-00185 Roma, Italy
² Dipartimento di Scienze della Terra, Università di Pavia, Via Ferrata 1, I-27100 Pavia, Italy
³ CNR-Istituto di Geoscienze e Georisorse, Sezione di Pavia, Via Ferrata 1, I-27100 Pavia, Italy

Correspondence: ^* E-mail: adriana.maras{at}uniroma1.it

The crystal structure of holotype vicanite-(Ce) has been solved and refined to R = 1.8% for 1398 observed reflections with the aid of a new crystal from the same locality (Tre Croci, Vetralla, Italy), found more than 10 years after the first. The new unit formula is (Ca,REE,Th)₁₅Fe³⁺(SiO₄)₃ Si₃B₃O₁₈)(BO₃)(As⁵⁺O₄)(As³⁺O₃)_x(NaF₃)_1–xF₇ · 0.2 H₂O with x = 0.4. The structure is trigonal, R3m, Z = 3, a = 10.8112(2), c = 27.3296(12) Å, and layered along [001] with three distinct layers.

Layer A at z ca. 0 (1/3, 2/3) contains an Fe(SiO₄)₆ group and a threefold B₃O₉ borate ring. Each tetrahedron of the ring shares one oxygen atom with one Si tetrahedron, forming an unusual Si₃B₃O₁₈^15– polyanion.

Layer B at z ca. 1/9 (4/9, 7/9) contains an AsO₄ tetrahedron and a BO₃ triangle.

Layer C at z ca. 2/9 (5/9, 8/9) represents the disordered part of the structure, containing two very close (0.85Å) As³⁺O₃^3– and NaF₃^2– polyhedra, the occurrence of which is mutually exclusive and statistically disordered.

A 3-dimensional network of M-(O,F)_n polyhedra (M = Ca, REE, Th; 8 < n < 10) provide connections among neighboring layers.

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