A proposed new type of arsenian pyrite: Composition, nanostructure and geological significance

Abstract

This report describes a new form of arsenian pyrite, called As3+-pyrite, in which As substitutes for Fe [(Fe,As)S2], in contrast to the more common form of arsenian pyrite, As1−-pyrite, in which As1− substitutes for S [Fe(As,S)2]. As3+-pyrite has been observed as colloformic overgrowths on As-free pyrite in a hydrothermal gold deposit at Yanacocha, Peru. XPS analyses of the As3+-pyrite confirm that As is present largely as As3+. EMPA analyses show that As3+-pyrite incorporates up to 3.05 at % of As and 0.53at. %, 0.1at. %, 0.27at. %, 0.22at. %, 0.08at. % and 0.04at. % of Pb, Au, Cu, Zn, Ni, and Co, respectively. Incorporation of As3+ in the pyrite could be written like: As3++yAu++1-y(□)⇔2Fe2+; where Au+ and vacancy (□) help to maintain the excess charge. HRTEM observations reveal a sharp boundary between As-free pyrite and the first overgrowth of As3+-pyrite (20–40nm thick) and co-linear lattice fringes indicating epitaxial growth of As3+-pyrite on As-free pyrite. Overgrowths of As3+-pyrite onto As-free pyrite can be divided into three groups on the basis of crystal size, 8–20nm, 100–300nm and 400–900nm, and the smaller the crystal size the higher the concentration of toxic arsenic and trace metals. The Yanacocha deposit, in which As3+-pyrite was found, formed under relatively oxidizing conditions in which the dominant form of dissolved As in the stability field of pyrite is As3+; in contrast, reducing conditions are typical of most environments that host As1−-pyrite. As3+-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 As3+ and toxic heavy metals in oxidizing natural systems.