Compositional zoning in Fe oxyhydroxides: An electron microprobe study

Abstract

Compositional zoning in iron oxyhydroxides precipitated from mine drainage-contaminated groundwater was studied by electron microprobe (EMP) analysis. The observed zoning is characterized by variations in Al, Fe, S, and Si concentrations, with mean precipitate concentrations of ca. 56.0 wt.% Fe, 1.1 wt.% Al, 2.0 wt.% Si, and 0.5 wt.% S. The analyses indicate an inverse relationship between Al-Si concentrations and Fe-S concentrations in the Fe oxyhydroxides. The results and groundwater data from the field site suggest that the origin of com-positional zoning in the Fe oxyhydroxides is controlled by both surface complexation reactions involving Al³⁺ and H₄SiO₄, which affect goethite formation kinetics, and the Al/Fe and Si/Fe molar ratios in solution.

Key Words:

• Iron oxyhydroxide
• zoning
• aluminum
• silicate
• groundwater
• electron microprobe