Home – Home – Jornals – Russian Geology and Geophysics 2002 number 8

Native gold samples (Au-Ag system) in the range of fineness from 200 to 1000 received complex comparative study. It has been established that HNO₃ etched samples with gold fineness of about 700 and less have a two-phase structure. Atomic absorption and microprobe analyses show that the phases are pure Au and Ag. Gold and silver particles are a few hundred nanometers in size. Distribution of particles is uniform, i.e., this is an ultradispersive statistically homogeneous mixture composed of native gold and native silver.
We have observed no obvious two-phase structure in the HNO₃ etched samples with gold fineness in the range of 700 to 850 . The fineness of gold increases in the surface layer, and traces of silver are observed in the solution. At some points, the microprobe analysis of fineness (without etching) reveals a drastic change in fineness, which either increases or decreases relative to the average value (deviations up to 250). We suggest that the samples of this group are similar to those from the previous group, but predominant gold prevents silver particles from contacts with HNO₃. Therefore, the samples of this composition do not dissolve in HNO₃.
There is no evidence of phase heterogeneity in samples with a gold fineness of 850 to 1000 . We suppose that Au and Ag substitute isomorphically for each other in the crystal lattice and form a homogeneous solid solution.
Possible reasons for the phase heterogeneity of native gold are discussed. We suggest that it is the result of micro block crystallization associated with high over saturation of solutions.