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.

Assessment of the geoavailability of trace elements from selected zinc minerals

“…The silicate tetrahedra are linked by (ZnO 3 (OH)) 5-to form a network that has channels running through it (Kolesov, 2006). Water and metals such as cadmium and lead can fill the channels and have been shown to be exchangeable (Mao, 2012). Weathering of hemimorphite may have an initial release of metals absorbed in the channels followed by breakdown of the network along cleavage planes (silicate tetrahedrons) releasing the metal oxides.…”