IRIDIUM DISTRIBUTION IN HYDROTHERMALLY SYNTHESIZED FE, CU, ZN, AND PB SULFIDES

Abstract

We have synthesized hydrothermally sulfides of Fe, Cu, Pb, and Zn in the presence of Ir; 192Ir was employed as the radioactive tracer, and its distribution in run products was determined by autoradiography. None of the synthesized minerals were found to contain Ir in their structure. Instead, there is a characteristic enrichment of Ir along the external surface of individual large crystals or polycrystalline aggregates. Internal parts of sulfide crystals and aggregates lack Ir. We conclude that iridium is concentrated on the surface of growing crystals and polycrystalline aggregates at the crystal-solution interface, and is a result of adsorption equilibrium rather than equilibrium between crystal and solution. As a consequence, coefficients of Ir distribution based on bulk analyses, rather than local analyses of coexisting phases, are not reliable.