This report describes a new form of arsenian pyrite, called As^3+ -pyrite, in which As substitutes for Fe [(Fe,As)S2], in contrast to the more common form of arsenian pyrite, As^1&amp;minus; -pyrite, in which As^1&amp;minus; substitutes for S [Fe(As,S)2]. As^3+ -pyrite has been observed as colloformic overgrowths on As-free pyrite in a hydrothermal gold deposit at Yanacocha, Peru. XPS analyses of the As^3+ -pyrite confirm that As is present largely as As^3+. EMPA analyses show that As^3+ -pyrite incorporates up to 3.05 at % of As and 0.53 at. %, 0.1 at. %, 0.27 at. %, 0.22 at. %, 0.08 at. % and 0.04 at. % of Pb, Au, Cu, Zn, Ni, and Co, respectively. Incorporation of As^3+ in the pyrite could be written like: As^3+ + yAu^+ + 1 - y(â–¡) <-> 2Fe^2+ ; where Au+ and vacancy (â–¡) help to maintain the excess charge. HRTEM observations reveal a sharp boundary between As-free pyrite and the first overgrowth of As^3+ -pyrite (20&amp;ndash;40 nm thick) and co-linear lattice fringes indicating epitaxial growth of As^3+ -pyrite on As-free pyrite. Overgrowths of As^3+ -pyrite onto As-free pyrite can be divided into three groups on the basis of crystal size, 8&amp;ndash;20 nm, 100&amp;ndash;300 nm and 400&amp;ndash;900 nm, and the smaller the crystal size the higher the concentration of toxic arsenic and trace metals. The Yanacocha deposit, in which As^3+ -pyrite was found, formed under relatively oxidizing conditions in which the dominant form of dissolved As in the stability field of pyrite is As^3+; in contrast, reducing conditions are typical of most environments that host As^1&amp;minus; -pyrite. As^3+ -pyrite will likely be found in other oxidizing hydrothermal and diagenetic environments, including high-sulfidation epithermal deposits and shallow groundwater systems, where probably kinetically controlled formation of nanoscale crystals such as observed here would be a major control on incorporation and release of As^3+ and toxic heavy metals in oxidizing natural systems.
