Crystal structure of filatovite, K[(Al,Zn)2(As,Si)208], the first arsenate of the feldspar group

The crystal structure of filatovite, K[(Al,Zn)(2)(As,Si)(2)O-8] (monoclinic, I2/c, a = 8.772(1), b = 13.370(2), c = 14.690(2) Angstrom, beta =115.944(6)degrees, V= 1549.1(4) Angstrom(3)) has been solved by direct methods and refined to R-1 = 0.033 on the basis of 1959 unique observed reflections with \F-o\ greater than or equal to 4sigmaF. It is isotypic with the structures of celsian, Ba[Al2Si2O8], and its synthetic analogues. The structure contains four symmetry independent tetrahedral sites. The T(1) and T(4) sites have the same refined occupancy of AS(0.55)Si(0.45) and are characterized by average <T-O> bond lengths of 1.634 Angstrom. The occupancies of the T(2) and T(3) sites are Al0.94Zn0.06 and Al0.96Zn0.04, respectively, with average <T(2)-O> and <T(3)-O> bond lengths of 1.753 and 1.754 Angstrom, respectively. Consistent with Al-Si ordering in celsian, (As,Si)O-4 tetrahedra are surrounded by (Al,Zn)O-4 tetrahedra and vice versa. One symmetry independent K+ cation resides in framework cavities and is coordinated by nine O atoms with the average <K-O> bond length of 3.02 Angstrom. Incorporation of pentavalent T5+ cations into the feldspar-like tetrahedral framework is possible if it is accompanied by incorporation of T2+ and T3+ cations into adjacent tetrahedral sites.