Abstract
Formation and dissolution of secondary arsenic minerals often play significant roles in controlling arsenic mobility in contaminated environments, especially in sulfide mines. Weathering of the orpiment and realgar-bearing tailings from the Shimen realgar deposit, the largest realgar deposit in Asia, was studied. An integrated mineralogical analysis by using X-ray powder diffraction, Raman spectrum, scanning electron microscope, and transmission electron microscope reveals four kinds of As-bearing secondary minerals, including arsenic oxides, arsenates, As-gypsum, and As-Fe minerals. The precipitation of arsenates is due to interaction of As-bearing runoff waters and the underlying carbonate rocks, or the transformation of gypsum into arsenates or As-bearing gypsum through \( {\mathrm{SO}}_4^{2-}/{\mathrm{HAsO}}_4^{2-} \) substitution. Ca-arsenates are mainly weilite and pharmacolite with Ca/As atomic ratio of 1. Scanning transmission X-ray microscope and X-ray absorption fine structure reveal that the valence of arsenic is mainly +3 and +5.
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