Hemimorphite as a natural sink for arsenic in zinc deposits and related mine tailings: Evidence from single-crystal EPR spectroscopy and hydrothermal synthesis

Hemimorphite is a refractory mineral in surface environments and occurs commonly in supergene non-sulfide Zn deposits and Zn mine tailings. Single-crystal electron paramagnetic resonance (EPR) spectra of gamma-ray-irradiated hemimorphite from Mapimi (Durango, Mexico) reveal two arsenic-associated oxyradicals: [AsO4]4− and [AsO4]2−. Inductively coupled plasma mass spectrometry analyses confirm this sample to contain 270 ppm As and that hemimorphite from other Zn deposits has appreciable amounts of arsenic as well. Spin Hamiltonian parameters, including matrices g, A (75As) and P(75As), show that the [AsO4]4− radical formed from electron trapping by a locally uncompensated [AsO4]3− ion substituting for [SiO4]4−. Matrices g, A(75As) and P(75As) of the [AsO4]2− radical show it to have the unpaired spin on the bridging oxygen of an [AsO4]3− ion at a Si site and linked to a monovalent impurity ion. This structural model for the [AsO4]2− radical is further supported by observed 29Si and 1H superhyperfine structures arising from interactions with a single Si atom (A/geβe = ∼1 mT at B//c) and two equivalent H atoms (A/geβe = ∼0.3 mT at B∧b = 10°), respectively. Hydrothermal experiments at 200 °C and ∼9.5 MPa show that hemimorphite contains up to ∼2.5 wt% As2O5 and suggest that both the arsenate concentration and the pH value in the solution affect the As content in hemimorphite. These results demonstrate that hemimorphite is capable of sequestering arsenate in its crystal lattice, hence is a natural sink for attenuating As in supergene non-sulfide Zn deposits and Zn mine tailings. Moreover, results from hemimorphite potentially have more far-reaching implications for major silicates such as zeolites in the immobilization and removal of arsenic in surface environments.