Ardennite, tiragalloite and medaite: structural control of (As5+,V5+,Si4+)O4 tetrahedra in silicates

摘要

Several silicate-minerals, such as ardennite - MgAl₅[Si₅(As5+,V5+)O₂₂](OH)₆, Z = 2, tiragalloite - [Si₃As5+O₁₂(OH)], Z = 4 and medaite - [Si₅(V5+,As5+)O₁₈(OH)], Z = 4 possess (V5+,As5+,P5+)O₄ tetrahedra. Using electron-microprobe analysis (EMPA) and single-crystal X-ray diffraction methods, the crystal chemistry of ardennite from Salam-Château, 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 R₁ values of 2.10-5.67%. According to chemical analysis, the (As+V+P) content increases with decreasing Si content. Thus, Si replaces pentavalent cations in tetrahedral coordination. The (As5+,V5+,P5+,Si4+)O₄ tetrahedra are categorized by their connections to SiO₄ tetrahedra. The (As5+,V5+,P5+,Si4+)O₄ tetrahedron of ardennite is isolated, and those of tiragalloite and medaite terminate a tetrahedral chain. The of the isolated (As5+,V5+,P5+,Si4+)O₄ tetrahedron shows a positive correlation with the mean ionic radius. For (As5+,V5+,P5+,Si4+)O₄ 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)O₄ and the adjacent SiO₄ tetrahedron. The bridging O atom is over-bonded to satisfy the charge requirement of (As+V+Si).