Physics
Scientific paper
Sep 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.3795r&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 18, pp.3795-3802
Physics
Scientific paper
Chalcopyrite forms from the reaction between aqueous Cu(II) solutions and pyrite by the overall reaction 2 Cu + + FeS 2 = CuFeS 2 + Cu 2+ The result resolves a long standing controversy about the origin of the electron balance in the reaction. The reaction is a surface chemical reaction-controlled process. The rate is proportional to a fractional power of the geometric pyrite surface area and directly proportional to the dissolved copper concentration. The rate expression may be represented as - d ([ Cu ] aq )/ dt = kA 0.55 ([ Cu ] aq ), where - d ([ Cu ] aq )/ dt is the rate in moles [Cu] · dm -3 · s -1 , [Cu] is the total concentration of dissolved copper, and k is the apparent first order rate constant in cm -2 · s -1 which varies between 1.27 × 10 -8 cm -2 · s -1 at 100°C and 180.9 × 10 -8 cm -2 · s -1 at 157°C. At 125 ± 2° C the apparent first order rate constant is 5.86 ± 0.31 × 10 -8 cm -2 · s -1 for all observed pyrite reactant size fractions. Interpolation of the rate data to 25°C suggests an apparent first order rate constant for the reaction of 6.47 × 10 -12 cm -2 · s -1 . The apparent Arrhenius activation energy for the reaction is 94.29 ± 0.07 kJ mol -1 · K -1 . The experimentation suggests a strong rate dependence on the concentration of Cu(I) in solution suggesting the following mechanism: Cu 2+ · FeS 2( surface ) Cu + · FeS 2( surface ) (fast) Cu + · FeS 2( surface ) CuFeS 2( surface ) (slow). Chalcopyrite formation through the reaction between aqueous copper and pyrite is very slow at temperatures below ca. 100°C where the reaction with Fe(II) monosulphides is faster. However, it becomes the faster reaction at temperatures above 100°C and is predicted to be extremely rapid at temperatures in excess of 200°C. The results suggest an alternative, kinetic origin for chalcopyrite parageneses in many natural systems.
Cowper Mark
Rickard David
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