The tarnishing of electrum (40 atom % Ag) at 750°C and a sulfur fugacity of -2.8 to -2.9 1og₁օ atmospheres is heterogenous and irreversible. This is the case even though these conditions are less than 37°C and .4 log₁օ atmospheres above the point at which tarnish is first observed on the electrum. The tarnish forms initially on the grain boundaries and lattice defects of the electrum. With time, the interiors of the electrum grains become completely masked by tarnish except for narrow untarnished borders adjacent to the grain boundaries. Microprobe analysis indicates that the untarnished borders are depleted in silver and thus cannot tarnish at 750°C. The conclusion is drawn that the tarnish forms as an external layer which destroys the original composition of the adjacent electrum by preferentially removing silver. The silver depletion near the grain boundaries is thought to be caused by rapid diffusion of silver along grain boundaries to the electrum-tarnish interface. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate