Jr. Mycroft et al., SPONTANEOUS DEPOSITION OF GOLD ON PYRITE FROM SOLUTIONS CONTAINING AU(III) AND AU(I) CHLORIDES .1. A SURFACE STUDY, Geochimica et cosmochimica acta, 59(16), 1995, pp. 3351-3365
The adsorption and deposition of Au on the surface of naturally occurr
ing pyrite under ambient conditions has been studied by scanning elect
ron microscopy (SEM), X-ray photoelectron (XPS) and Rutherford backsca
ttering spectrometry (RES). Gold is reduced on the pyrite surface from
either Au(I) or Au(III) solutions and tends to nucleate as small sphe
res at microfractures on the mineral surface; each gold cluster is sur
rounded by a reaction halo of unknown composition. The rate of Au depo
sition is approximately inverse first order in Cl- and is independent
of AuCl4- and pH. Pyrite is found to be the reducing agent for Au(III)
; it reacts with surface ferrous iron to produce Au(I) and ferric iron
. The Au(I) in turn reacts with pyrite again to form metallic gold. Th
e need for careful interpretation of the XPS binding energy data is sh
own; the Au 4f(7/2) binding energy is a function of not only oxidation
state but also Au particle size.