This work examined the oxidative/dissolution rates of pyrite and marcasite in seawater under various solution conditions. A spectrophotometric method for determining seawater pH was developed in the course of this work. The evolution of protons during pyrite and marcasite oxidation was monitored spectrophotometrically followed by spectrophotometric measurements of total evolved iron. Oxidation rate observations are described in the context of the evolution of hydrothermally derived particles at the Juan de Fuca Ridge.; It was determined that the oxidative dissolution of marcasite and pyrite is strongly influenced by temperature and weakly influenced by oxygen concentration. A temperature decrease from 25{dollar}spcirc{dollar}C to 0.5{dollar}spcirc{dollar}C resulted in an eleven-fold decrease in the oxidation rate of marcasite and a nine-fold decrease for pyrite. The sulfur oxidation products for pyrite are more oxidized than those of marcasite. The sulfur oxidation products for both pyrite and marcasite are less oxidized in 0.7 M NaCl than in seawater. Observations indicate that the oxidation rates of marcasite and pyrite are very similar. Under conditions similar to those found in-situ around the Juan de Fuca Ridge, the oxidation rate of marcasite was 3.7 {dollar}times{dollar} 10{dollar}sp{lcub}-7{rcub}{dollar} moles FeS{dollar}sb2{dollar}/hr/m{dollar}sp2{dollar} and the oxidation rate of pyrite was 4.0 {dollar}times{dollar} 10{dollar}sp{lcub}-7{rcub}{dollar} moles FeS{dollar}sb2{dollar}/hr/m{dollar}sp2{dollar}.
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