Hemimorphite as a natural sink for arsenic in zinc deposits and related mine tailings: Evidence from single-crystal EPR spectroscopy and hydrothermal synthesis Mao Mao, Jinru Lin, Yuanming Pan * Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5E2 Received 2 September 2009; accepted in revised form 11 February 2010; available online 19 February 2010 Abstract 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: [AsO 4 ] 4 and [AsO 4 ] 2 . Inductively coupled plas- ma 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 ( 75 As) and P( 75 As), show that the [AsO 4 ] 4 radical formed from electron trapping by a locally uncompensated [AsO 4 ] 3 ion substituting for [SiO 4 ] 4 . Matri- ces g, A( 75 As) and P( 75 As) of the [AsO 4 ] 2 radical show it to have the unpaired spin on the bridging oxygen of an [AsO 4 ] 3 ion at a Si site and linked to a monovalent impurity ion. This structural model for the [AsO 4 ] 2 radical is further supported by observed 29 Si and 1 H superhyperfine structures arising from interactions with a single Si atom (A/g e b e = 1 mT at B//c) and two equivalent H atoms (A/g e b e = 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% As 2 O 5 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 zeo- lites in the immobilization and removal of arsenic in surface environments. Ó 2010 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Arsenic has long been the subject of intense research, be- cause it is one of the most common and highly toxic con- taminants in surface environments, including those associated with mining activities and metallurgical pro- cesses (Nriagu, 1994; Foster et al., 1998; Courtin-Nomade et al., 2009; Walker et al., 2009). Numerous studies have demonstrated that the mobility and bioavailability of As are usually closely related to its host phases (i.e., minerals in many cases; Foster et al., 1998; Courtin-Nomade et al., 2009; Walker et al., 2009). Therefore, significant efforts have been devoted to the identification and development of minerals (or their synthetic equivalents) that are capable of fixing or minimizing the mobility and bioavailability of As in the environment (e.g., Bothe and Brown, 1999; Di Benedetto et al., 2006; Blanchard et al., 2007; Ferna ´ndez- Martı ´nez et al., 2008; Lee et al., 2009; Drahota and Filipi, 2009). As part of our ongoing study on radiation-induced de- fects in silicates (Mao and Pan, 2009; Pan et al., 2009; Mao et al., 2010), we have investigated gamma-ray irradi- ated hemimorphite by electron paramagnetic resonance (EPR) spectroscopy. EPR spectroscopy is a powerful and well-established technique for structural characterization 0016-7037/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2010.02.011 * Corresponding author. Tel.: +1 306 966 5699; fax: +1 306 966 8593. E-mail address: yuanming.pan@usask.ca (Y. Pan). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 74 (2010) 2943–2956