Abstract

The interdiffusion coefficients of zinc and iron in natural iron-poor and iron-rich sphalerites were determined experimentally using electron microprobe analysis. The diffusion couples were heated in sealed silica tubes for predetermined lengths of time together with f (sub S 2 ) buffering materials such as iron and/or synthetic pyrrhotite at temperatures between 950 degrees and 500 degrees C. The interdiffusion rates were determined from measured compositional profiles across natural sphalerite crystal pairs.The penetration profiles of the diffusing Zn and Fe were clearly observed not only at the contact planes of crystals but also at both free end surfaces of the diffusion column and the side surfaces normal to the diffusion interface. Vapor transport probably played an important role in the Zn-Fe interdiffusion. The Zn-Fe interdiffusion coefficient increases slightly with increasing Fe content of sphalerite as expected from the enlargement of the unit cell with Fe concentration. The Arrhenius relation for interdiffusion rates, D (cm 2 /sec), in sphalerite of N mole percent FeS (2 < or = N < or = 19) and at 1 bar pressure, is given by: D = 3.7 X 10 (super -4) exp(0.04 X N) exp{-36 + or - 1 (kcal/mole)/RT} cm 2 /sec.The interdiffusion rate is not dependent on crystallographic orientation for directions normal to (110) and parallel to (110). A high-pressure run at 5 kbars and 800 degrees C shows that interdiffusion rates slightly decrease with increasing total pressure (log D = -10.5 + or - 0.1 cm 2 /sec at 1 bar pressure to log D = -10.8 + or - 0.2 cm 2 /sec at 5 kbars). A dependency of interdiffusion rates on f (sub S 2 ) was also observed in the 950 degrees and 900 degrees C experiments. D values are slower with increasing fugacity of sulfur (e.g., for 900 degrees C, log D = -9.8 + or - 0.1 cm 2 /sec at f (sub S 2 ) = 10 (super -9.6) bars vs. log D = 10.1 + or - 0.2 cm 2 /sec at f (sub S 2 ) = 10 (super -8.0) bars).

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