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A single-crystal EPR study of radiation-induced defects in selected silicates

Date

2012-11-08

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Doctoral

Abstract

This thesis presents a series of single-crystal electron paramagnetic resonance (EPR) studies on radiation-induced defects in selected silicate minerals, including apophyllites, prehnite, and hemimorphite, not only providing new insights to mechanisms of radiation-induced damage in minerals but also having direct relevance to remediation of heavy metalloid contamination and nuclear waste disposal. The NH2 free radical, which is one of the most studied triatomic molecules and is widely used as spin labels in biophysical and biomedical research, in fluorapophyllite has been observed and characterized by single-crystal EPR spectra. Fourier-transform infrared (FTIR) spectrum supports electron-microprobe analyses that fluorapophyllite on the cavity walls in a phonolite (North Bohemia, Czech Republic) contains ammonium NH4+. The spin-Hamiltonian parameters of the NH2 free radical show that this molecule is oriented parallel to (and rapidly rotated about) the crystallographic c axis. The NH2 free radical in fluorapophyllite, most likely formed from radiolysis of the NH4+ ion, remains stable after annealing at 300°C, but is bleached at 340°C. This is the first report of the NH2 free radical in a mineral lattice. An O- center and its biradicals in hydroxylapophyllite have been investigated by use of single-crystal and powder EPR spectroscopy at 290 and 90 K and three-pulse electron spin echo envelope modulation (ESEEM) spectroscopy at 25 K. The spin-Hamiltonian parameters show that the O- center (σ-type) represents an unpaired electron in the 2pz orbital of the hydroxyl oxygen atom. A series of weak satellite peaks accompanying the main lines have been attributed to four geometrically distinct pairs of neighboring O- centers (i.e., biradicals). These biradicals have a point-dipole character and further support the O- model and its location. The O- center in hydroxylapophyllite is most likely produced by natural radiation and can be enhanced by gamma-ray irradiation. It is bleached at 300°C but can be restored readily by gamma-ray irradiation. An Al-O- center in gamma-ray-irradiated prehnite has been investigated by single-crystal EPR spectroscopy at 298 and 160 K. The spin-Hamiltonian parameters g and A(27Al) at 298 K show that the hole traps on an apical hydroxyl oxygen of the octahedral O4Al(OH)2 group, after removal of the proton. Pulsed electron nuclear double resonance (ENDOR) spectra measured at 25 K further confirm the structural model of the Al-O- center. Isothermal and isochronal annealing experiments show that the Al-O- center exhibits second-order decay kinetics. The Al-O- center is bleached after annealing at 375°C but can be restored by gamma-ray irradiation. These results from the Al-O- center in prehnite provide support for and new insights into Clozel et al. (1995)’s VIAl.O..VIAl model for the B center in kaolinite. Two arsenic-centered oxyradicals ([AsO4]4- and [AsO4]2-) in gamma-ray irradiated hemimorphite (Mapimi, Durago, Mexico) have been observed and characterized by single-crystal EPR at ~295 K. The spin-Hamiltonian parameters suggest that the [AsO4]4- radical is produced from electron trapping by a locally uncompensated [AsO4]3- group substituting for the [SiO4]4- group. The spin-Hamiltonian parameters of the [AsO4]2- radical, including its 29Si and 1H superhyperfine coupling constants, suggest hole trapping on the bridging oxygen linked to a Si4+ ion. Hydrothermal experiments at 200 °C and ~9.5 MPa show that hemimorphite is able to accommodate up to 2.5 wt% As2O5. These results demonstrate that hemimorphite is capable of sequestering arsenate in its crystal lattice, so it is a natural sink for attenuating As in supergene non-sulfide Zn deposits and Zn mine tailings. Hemimorphite commonly contains elevated contents of heavy metalloids such as As, Cu, Cd, and Pb. Cation-exchange experiments of hemimorphite with 0.1M CaCl2 solution at 110°C show that As and Cu are retained, whereas Cd and Pb are readily exchanged. This exchange behavior of Cd and Pb suggests that they may reside in the channel. Single-crystal EPR results at 295 and 120 K show that Cu2+ resides at the tetrahedral Zn site, not in the channel as previously suggested by the powder EPR study of Gallegos et al. (2009). These results suggest that hemimorphite is potentially useful for the remediation of heavy metalloid contamination. Single-crystal EPR spectra of gamma-ray-irradiated hemimorphite after storage at room temperature for three months reveal a hydroperoxy radical HO2 with complex proton hyperfine and superhyperfine structures. The single-crystal EPR spectra of this HO2 radical, measured from 4 K to 275 K, confirm two reversible phase transitions at ~98 K and ~21 K. Spin-Hamiltonian parameters show that the HO2 radical at 110 K forms from the H2O molecule in the channel and interacts with two equivalent protons of the nearest hydroxyl groups. The HO2 radical changes in site symmetry from monoclinic to triclinic across the ~98 K phase transitions and confirms dynamic ordering and rotation of its precursor water molecule in the channel at <98 K. The EPR spectra of the HO2 radical at <21 K and results from density functional theory (DFT) calculations suggest that hemimorphite adopts the monoclinic space group Im with completely ordered O.H systems at low temperature.

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Keywords

Silicates, EPR, radiation, defects

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Degree

Doctor of Philosophy (Ph.D.)

Department

Geological Sciences

Program

Geology

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