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THE CRYSTAL STRUCTURE OF CERVANDONITE-(Ce), AN INTERESTING EXAMPLE OF As³⁺ Si DIADOCHY

Francesco Demartin^1,, Carlo Maria Gramaccioli¹ and Stefan Graeser²

¹ Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Università degli Studi di Milano, via Venezian 21, I–20133 Milano, Italy
² Naturhistorisches Museum, CH–4001 Basel, and Mineralogisch-Petrographisches Institut der Universität, Bernoullistrasse 30, CH–4056 Basel, Switzerland

E-mail address: francesco.demartin{at}unimi.it

Cervandonite-(Ce), a unique arsenosilicate from Alpine fissures at Pizzo Cervandone (Scherbadung) in the Central Alps between the Binn Valley (CH) and Ossola (I), has proven to be a problem in view of the extreme scarcity of single crystals suitable for the determination of its complex structure. The monoclinic supercell (Z = 6) originally described by the discoverers is confirmed; we have established the crystal structure of a trigonal subcell, with space group R3m and a 6.508(1), c 18.520(3) Å, V 679.4(2) ų and Z = 3, to a final R(F) = 0.0380 for 441 observed independent reflections [I > 2 (I)]. The mechanism of the very unusual As³⁺ Si (not As⁵⁺ Si) diadochy is accounted for by considering the presence in two different sites of sorosilicate anions Si₂O₇^6–, which are statistically replaced by arsenite [AsO₃^3–] anions and minor amounts of OH^–; the positions of the oxygen atoms common to these anions are the same. The asymmetric unit also contains one eight-coordinated atomic site occupied by Ce and other REE lying on the three-fold axis; a bipyramidal six-coordinated site (of symmetry .m) occupied by Fe, Ti, or Al also is present. The originally proposed chemical formula (Ce,Nd,La)(Fe³⁺,Fe²⁺,Ti⁴⁺,Al)₃SiAs(Si,As)O₁₃ is accordingly revised to (Ce,Nd,La)(Fe³⁺,Fe²⁺,Ti⁴⁺,Al)₃O₂(Si₂O₇)_1–x+y (AsO₃)_1+x–y(OH)_3x–3y, where x and y are 0.47 and 0.31, respectively, in the sample here examined.

Keywords: cervandonite-(Ce), crystal structure, arsenites, arsenosilicates, As³⁺ Si diadochy, Alpine minerals, Cervandone, Scherbadung, Binn Valley, Switzerland, Ossola, Italy.