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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.

Single-crystal electron paramagnetic resonance (EPR) spectra of gamma-ray-irradiated prehnite (Jeffrey mine, Quebec, Canada) measured at 298 and 160 K reveal an aluminum-associated oxygen hole center (Al—O − ). Spin Hamiltonian parameters g and A ( 27 Al) fitted from the 298 K spectra suggest that this Al—O − center represents hole trapping on an apical hydroxyl oxygen atom (after removal of the proton) coordinated to an octahedral Al 3+ ion ( i.e. , an [ • OAlO 4 (OH)] center from the [(OH)AlO 4 (OH)] precursor, where • denotes the unpaired spin). Pulsed electron nuclear double resonance (ENDOR) spectra measured at 25 K allow the identification and quantitative analysis of two sets of 27 Al hyperfine structures and five proton hyperfine structures, which are all consistent with the proposed structural model. Isothermal and isochronal annealing experiments show that this center is bleached out completely at 375 °C, but can be readily restored by gamma-ray irradiation, and exhibits second-order decay kinetics. These results from the Al—O − center in prehnite provide support for and new insights into Clozel et al . (1995)’s VI Al—O − — VI Al model for B-centers in kaolinite.

A hole-like centre present in natural hydroxylapophyllite without any artificial irradiation has been investigated by single-crystal and powder electron paramagnetic resonance (EPR) spectroscopy at 290 K and 90 K, and by three-pulse electron spin echo envelope modulation (ESEEM) spectroscopy at 25 K. Calculated matrices g , A ( 29 Si), A ( 1 H), A ( 39 K) and P ( 39 K) suggest it to be an O − centre at the hydroxyl oxygen site and the unpaired electron in the 2 P z orbital. A series of weak satellite peaks accompanying the main absorption line in single-crystal EPR spectra have been shown to arise from four geometrically distinct pairs of neighbouring O − centres ( i.e. , biradicals). The spin Hamiltonian parameters of these O − −O − pairs provide further support for the O − model and its location at the hydroxyl oxygen site. The O − centre in hydroxylapophyllite most likely formed from natural radiation and can be enhanced by gamma-ray irradiation. It is annealed out at 300°C but can be restored readily by gamma-ray irradiation. The presence of these O − −O − pairs in the hydroxylapophyllite is probably attributable to a high abundance of the O − centre in this sample.