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 Pb_1.02 H_1.02(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. [ABSTRACT FROM AUTHOR]