Ardennite, tiragalloite and medaite: structural control of (As~(5+),V~(5+),Si~(4+))O_4 tetrahedra in silicates

摘要

Several silicate-minerals, such as ardennite — Mn_4~(2+)MgAl_5[Si_5(As~(5+),V~(5+))O_(22)](OH_)6, Z = 2, tiragalloite - Mn_4~(2+)[Si_3As~(5+)O_(12)(OH)], Z = 4 and medaite - Mng [Si_5(V~(5+),As~(5+))O_(18)(OH)], Z = 4 possess (V~(5+),As~(5+),P~(5+))O_4 tetrahedra. Using electron-microprobe analysis (EMPA) and single-crystal X-ray diffraction methods, the crystal chemistry of ardennite from Salam-Chateau, Belgium and the Vernetto mine, Italy, tiragalloite from the Gambatesa mine, Italy, and medaite from the Molinello mine, Italy and the Fianel mine, Switzerland, were studied. Structure refinements converged to R_1 values of 2.10—5.67%. According to chemical analysis, the Σ(As+V+P) content increases with decreasing Si content. Thus, Si replaces pentavalent cations in tetrahedral coordination. The (As~(5+),V~(5+),P~(5+),Si~(4+))O_4 tetrahedra are categorized by their connections to SiO_4 tetrahedra. The (As~(5+),V~(5+),P~(5+),Si~(4+))O_4 tetrahedron of ardennite is isolated, and those of tiragalloite and medaite terminate a tetrahedral chain. The of the isolated (As~(5+),V~(5+),P~(5+),Si~(4+))O_4 tetrahedron shows a positive correlation with the mean ionic radius. For (As~(5+),V~(5+),P~(5+),Si~(4+))O_4 tetrahedra with one T-O-T link, and mean ionic radius are also correlated. In addition, the longest bridging T— O bond occurs between (As,V,P,Si)O_4 and the adjacent SiO_4 tetrahedron. The bridging O atom is over-bonded to satisfy the charge requirement of Σ(As+V+Si).