NATURE OF HEAVY METALS IN HEMIMORPHITE: A CATION-EXCHANGE AND SINGLE-CRYSTAL EPR STUDY

Abstract

Hemimorphite, a zeolite-like mineral commonly present in supergene non-sulfide Zn deposits and Zn mine tailings, is known to contain elevated levels of As, Cd, Cu and Pb. Cation-exchange experiments of a natural hemimorphite (Mapimi, Durango, Mexico) with 0.1 M CaCl 2 solution at 110 oC show that As and Cu are retained, whereas Cd and Pb are readily exchanged. The retention of As is consistent with previous single-crystal electron paramagnetic resonance (EPR) results that showed its occurrence as the substitutional As 5+ ion at the Si site. Single-crystal EPR spectra of hemimorphite from the M’Fouati Pb-Zn mine (Reneville, Congo), measured at 120 K and 295 K, show a Cu 2+ center. The best-fit spin Hamiltonian parameters g , A ( 63 Cu), P ( 63 Cu) and A ( 1 H) of this center at 120 K demonstrate that Cu 2+ resides at the tetrahedral Zn site, not in the channels as suggested by a previous powder EPR study. Cadmium and Pb in hemimorphite are probably present in the channels; this would account for their contrasting behavior from As and Cu during the cation-exchange experiments. However, single-crystal EPR spectra of ten samples of hemimorphite from various Pb-Zn deposits, including those taken after gamma-ray irradiation for doses up to 55 kGy, did not detect any paramagnetic Cd or Pb centers.