Sb5+and Sb3+substitution in segnitite: A new sink for As and Sb in the environment and implications for acid mine drainage

A sample of Sb-rich segnitite from the Black Pine mine, Montana, U.S.A., has been studied by microprobe analyses, single-crystal X-ray diffraction, and μ-EXAFS and XANES spectroscopy. Linear combination fitting of the spectroscopic data provided Sb5+:Sb3+= 85(2):15(2), where Sb5+is in octahedral coordination substituting for Fe3+and Sb3+is in tetrahedral coordination substituting for As5+. Based upon this Sb5+:Sb3+ratio, the microprobe analyses yielded the empirical formula Pb1.02H1.02(Fe3+2.36Sb5+0.41Cu2+0.27)Σ3.04(As5+1.78Sb3+0.07S6+0.02)Σ1.88O8(OH)6.00. The crystal structure refinement and bond valence analysis are consistent with these cation site assignments. The formation of Sb-rich segnitite opens new possibilities for Sb sinks within the supergene zone. Segnitite may, in fact, be an ideal host for the sequestering of several toxic elements for pH < 2. At higher pH values, As is more likely to be incorporated into schwertmannite and ferrihydrite.