Arsenic and sulfide at

Thermodynamic data indicate that realgar forms in more reducing environments than orpiment (Nordstrom and Archer, 2003), 13. However, realgar and its polymorphs are rarely seen forming below 100 °C (O Day et al., 2004), (Nordstrom and Archer, 2003), 11. Similarly, thermodynamics suggest that As(0) could exist under extremely reducing conditions. In reality, however, it probably only forms in hydrothermal fluids (Nordstrom and Archer, 2003), 14, 25. 

Bostick and Fendorf (2003), 918 performed detailed laboratory studies and concluded that arsenic could form surface precipitates on FeS (including troilite, pyrrhotite, and mackinawite) under anoxic conditions as shown in the following reaction  

In further sorption studies at pH 7, Bostick, Chen and Fendorf (2004) concluded that the FeAsS surface precipitates may convert to AS2S3 over time (also see (Bostick and Fendorf, 2003)). 

Under acidic to slightly alkaline conditions with limited reactive Fe(II), AS2S3 precipitates and dissolved Fe(II) are likely to immediately form through this reaction (Wilkin and Ford, 2006), 169  

As shown in the following reaction, H3ASO30 may sorb onto pyrite surfaces under anaerobic conditions, which leads to the formation of FeAsS, Fe(III) hydroxides, and polysulfides, such as FeS4 (Bostick and Fendorf, 2003)  

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