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

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) Å3 and Z = 3, to a final R(F) = 0.0380 for 441 observed independent reflections [I > 2s(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 Si2O76–, which are statistically replaced by arsenite [AsO33–] 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.