Arsenic speciation in danburite (CaB2Si2O8): a synchrotron XAS and single-crystal EPR study

Abstract

Gem-quality danburite containing 269 ppm As, from Charcas, San Luis Potosi, Mexico, has been investigated by synchrotron X-ray absorption spectroscopy (XAS) and single-crystal electron paramagnetic resonance (EPR) spectroscopy. Arsenic K -edge X-ray absorption near-edge-structure (XANES) spectra show that the dominant oxidation state is +3, and modeling of the extended X-ray absorption fine structure (EXAFS) spectra suggests that As 3+ mainly occupies the Si site. Single-crystal continuous-wave EPR spectra, measured before and after gamma-ray irradiation, reveal three arsenic-associated electron centers (I, II and III). Centers I and II have similar principal electron Zeeman g values and principal 75 As hyperfine constants [I: A 1 /h = 732.8(2) MHz, A 2 /h = −274.3(3) MHz and A 3 /h = −299.8(3) MHz; II: A 1 /h = 743.8(2) MHz, A 2 /h = −306.3(2) MHz and A 3 /h = −325.8(2) MHz]. These parameters suggest that Centers I and II are varieties of the [AsO 2 ] 2− radical, formed from electron trapping on substitutional As 3+ ions at the Si site. This model for the [AsO 2 ] 2− radical is further supported by 11 B (and 10 B) superhyperfine parameters determined from pulsed electron spin echo envelope modulation (ESEEM) spectroscopy. Center III is the [AsO 3 ] 2− radical on the basis of its characteristic 75 As hyperfine constants [A 1 /h = 2406.0(1) MHz, A 2 /h = 1903.4(1) MHz and A 3 /h = 1892.1(1) MHz]. The [AsO 3 ] 2− radical with its A 1 axis along the Si – O4 bond direction originated from electron trapping on a [AsO 4 ] 3− group after removal of the O4 atom during gamma-ray irradiation. Therefore, arsenic in the borosilicate danburite is present in both the +3 and +5 oxidation states and preferentially occupies the Si site. This study also highlights the advantages (and need) of multiple spectroscopic techniques for determining speciation of trace elements such as arsenic in minerals.