Poorly crystalline Pd-Hg-Au intermetallic compounds from Corrego Bom Sucesso, southern Serra do Espinhaco, Brazil

Abstract

Potarite, ideally PdHg, is reported in the literature to have compositions varying from PdHg or Pd(Hg,Au) to Pd3Hg2. Such a Pd3Hg2 phase is unknown in the synthetic Pd–Hg binary system. For the first time, Pd–Hg grains recovered from the historical Bom Sucesso alluvium, regarded as the type locality of Pd, are shown to consist of arborescent and lamellar intergrowths of two intermetallic compounds, compositionally close to empirical Pd(Hg,Au), i.e. auriferous potarite, and (Pd,Au)3Hg2. The Pd–Hg–Au grains have a rim of palladiferous Pt. The otherwise sharp Pd–Hg–Au intergrowths become diffuse at the contact with the palladiferous Pt rim. Both the Pd–Hg–Au compounds and the palladiferous Pt rim did not diffract using the electron-backscattered diffraction (EBSD) and powder X-ray microdiffraction techniques, indicating that they are poorly crystalline. Their poor crystallinity and the diffuse zone between the Pd–Hg–Au core and the Pt-rich overgrowth are suggestive of electrochemical metal precipitation from dilute solutions within the alluvium.