Correlation between electron paramagnetic resonance and thermoluminescence in natural sodalite

Samples of natural sodalite, Na₈Al₆Si₆O₂₄Cl₂, submitted to gamma irradiation and to thermal treatments, have been investigated using the thermoluminescence (TL) and electron paramagnetic resonance (EPR) techniques. Both, natural and heat-treated samples at 500°C in air for 30 min, present an EPR signal around g = 2.01132 attributed to oxygen hole centers. The EPR spectra of irradiated samples show an intense line at g = 2.0008 superimposed by a hyperfine multiplet of 11 lines due to an O⁻ ion in an intermediate position with respect to two adjacent Al nuclei. In the TL measurements, the samples were annealed at 500°C for 30 min and then irradiated with γ doses varying from 0.001 to 20 kGy. All the samples have shown TL peaks at 110, 230, 270, 365, and 445°C. A correlation between the EPR g = 2.01132 line and the 365°C TL peak was observed. A TL model is proposed in which a Na⁺ ion acts as a charge compensator when an Al³⁺ ion replaces a Si⁴⁺ lattice ion. The γ ray destruction of the Al–Na complex provides an electron trapped at the Na and a hole trapped at a non-bridging oxygen ion adjacent to the Al³⁺ ion.     

Electron paramagnetic resonance study of iridium in forsterite

Electron paramagnetic resonance of Ir²⁺ in forsterite is studied at Q-band frequency and room temperature. There are four equivalent spectra superimposed along the three crystallographic axes. The individual spectrum consists of four hyperfine lines of approximately equal intensity separated from each other by 42 G; one axis of the g tensor is near the c axis. Ir²⁺ is certainly substituted for Mg²⁺. Because of the fourfold degeneracy of the EPR spectrum, it may be suggested that iridium occurs at M₁. Taking into account that one eigenvector of the g tensor is nearly parallel to c, it seems also possible that the substitution takes place at the M₂ position. In this case, the lattice relaxation of the crystal structure around Ir²⁺ at M₂ must break the point symmetry m at M₂.     

Electron paramagnetic resonance study of new paramagnetic centers in microcline-perthites from pegmatites

Four new types of paramagnetic centers — NH⁺ ₃, N^2?, Al-O^?, E ₁ — have been detected in microcline perthites from pegmatites in the Ukrainian Shield. Values are tabulated for their g and A tensors and limits of thermal stability determined. The NH⁺ ₃ center substitutes the K⁺ ion. It occurs naturally in potash feldspars but is intensified by gamma or X-ray irradiation. It is regarded as a radiational development of the more general NH⁺ ₄ ? K⁺ isomorphism. It disappears after heating to temperatures higher than 470 K. The N^2? center is an uncommon example of isomorphous substitution of a bridging oxygen, being located on a O _D(m) site between T ₂(o) and T ₁(m) silicon sites. It is stable to 820 K. The hole center, Al-O^?, has been detected on an O _A(l) oxygen shared by T ₁(o) and T ₁(m) tetrahedra. It is stable to 590 K. The E ₁ center in these alkali feldspars is similar to the E ₁ center in quartz, being an electron trapped in an oxygen vacancy in the O _B (o) position. It is stable to 420 K. The NH⁺ ₃, Al-O^? and E ₁ centers can be restored from temperatures above their stability limits by gamma radiation. Concentration of centers varies from sample to sample depending on conditions of formation and subsequent history of the minerals.     

Electron paramagnetic resonance of phytofulgurite

Doklady Earth Sciences -     

Application of electron paramagnetic resonance spectroscopy to examination of carbonized coal blends

Paramagnetic centers in two- and three-component coal blends carbonized at 1000 °C were studied by X-band (9.3 GHz) electron paramagnetic resonance (EPR) technique. The blends were prepared from three different Polish coals with carbon contents [wt.%]: 88.66, 86.21, and 82.67, respectively. The aim of this work was to compare EPR parameters and concentrations of paramagnetic centers in the initial and carbonized coal samples. Furthermore the spin–spin and spin–lattice interactions were characterized.EPR spectra were measured with magnetic modulation 100 kHz and microwave power 0.7 mW. Amplitudes and linewidths of EPR spectra were obtained. g-Factors were calculated from resonance condition. Concentrations of paramagnetic centers in the samples were determined. Influence of microwave power in the range 0.7–70 mW on EPR spectra was analyzed.All the studied samples revealed paramagnetism. Unpaired electrons are localized in the same atoms, because similar g-values in the range 2.0035–2.0038 were obtained for all the original samples. The EPR parameters of coal blends were additive in comparison with the parent coals. EPR spectra strongly changed after carbonization of the coal samples. Narrower EPR lines were measured for the original coal samples than for carbonized ones. We detected lower concentrations of paramagnetic centers in carbonized three-component coal blends than in two-component carbonized blends. EPR lines of the studied carbonized blends were not saturated at the microwave power used, which suggests fast spin–lattice relaxation processes in the samples. EPR examination proved chemical interactions between coal constituents during carbonization of coal blends.     

Optical absorption and electron spin resonance in blue and green natural beryl

A comparative study of blue and green beryl crystals (from the region of Governador Valadares, Minas Gerais, Brazil) using electron paramagnetic resonance (EPR) and optical absorption (OA) spectroscopy is reported. The EPR spectra show that Fe³⁺ in blue beryl occupies a substitutional Al³⁺ site and in green beryl is localized in the structural channels between two O₆ planes. On the other hand the infrared spectra show that the alkali content in the blue beryl is mostly at substitutional and/or interstitial sites and in green beryl is mostly in the structural channels. The OA spectra show two types of Fe²⁺. Thermal treatments above 200° C in green beryl cause the reduction of Fe³⁺ into Fe²⁺ accompanied by a change of color to blue. The blue beryl color does not change on heating. The kinetics of the thermal conversion of Fe³⁺ into Fe²⁺ is composed of two first order processes; the first one has an activation energy ΔE ₁=0.30 eV and the second one has an activation energy ΔE ₂=0.46 eV.     

Thermoluminescence, electron paramagnetic resonance and optical absorption in natural and synthetic rhodonite crystals

Thermoluminescence, electron paramagnetic resonance and optical absorption properties of rhodonite, a natural silicate mineral, have been investigated and compared to those of synthetic crystal, pure and doped. The TL peaks grow linearly for radiation dose up to 4 kGy, and then saturate. In all the synthetic samples, 140 and 340°C TL peaks are observed; the difference occurs in their relative intensities, but only 340°C peak grows strongly for high doses. Al₂O₃ and Al₂O₃ + CaO-doped synthetic samples presented several decades intenser TL compared to that of synthetic samples doped with other impurities. A heating rate of 4°C/s has been used in all the TL readings. The EPR spectrum of natural rhodonite mineral has only one huge signal around g = 2.0 with width extending from 1,000 to 6,000 G. This is due to Mn dipolar interaction, a fact proved by numerical calculation based on Van Vleck dipolar broadening expression. The optical absorption spectrum is rich in absorption bands in near-UV, visible and near-IR intervals. Several bands in the region from 540 to 340 nm are interpreted as being due to Mn³⁺ in distorted octahedral environment. A broad and intense band around 1,040 nm is due to Fe²⁺. It decays under heating up to 900°C. At this temperature it is reduced by 80% of its original intensity. The pink, natural rhodonite, heated in air starts becoming black at approximately 600°C.     

Electron paramagnetic resonance spectroscopy: A suggested approach to trace metal analysis in marine environments

Electron paramagnetic resonance (EPR) spectroscopy analysis of marine samples from different environments appears to differentiate between adsorbed and structural Mn (II) and Fe (III) sites in the sediment. This suggests that EPR may provide a means of distinguishing different environmental influences on sediment. Acid extract solutions from sediment samples exhibit clearly defined EPR spectra due to Mn(II), Ti(III), Fe(III), and VO(IV), which are amenable to qualitative and quantitative analysis at concentrations below one part per million. Spectra of several shellfish vary considerably, both between species, and within a species, depending on sampling localities. Resonances from Mn(II), Mo(V), and Fe(III) can be obtained. Mn(II) is substituted for Ca(II) in the calcite structure of some shells. The low detection limits, small sample size, required and identification of oxidation states by EPR complement other analytical techniques and may prove useful in marine systems.     

An electron spin resonance spectrometric investigation of natural,extracted and thermally altered kerogenous materials

Core cuttings from three wells representing a 1000–7000-ft range of depths were examined by ESR spectroscopy. The spectral parameters of some of the series of samples follow trends, as suggested by the ESR kerogen method for paleotemperature estimation, while the spectral parameters of other series of samples do not. Samples extracted with mineral acids exhibit spectra parameters which follow the proposed trends somewhat better, but show large local variations which appear to be due to factors other than paleotemperature. Thermally treated samples show a general increase in free radical concentration, although there are several anomalies. While the ESR-kerogen method of paleotemperature estimation appears to have potential due to the large variations in some of the observed parameters, factors such as mineral acid extraction and sample lithology, among others, need further investigation to refine the technique.     

Temperature dependence on the electron paramagnetic resonance spectra of natural jasper from Taroko Gorge (Taiwan)

Structural properties of natural jasper from Taroko Gorge (Taiwan) have been investigated by means of powder X-ray diffraction, electron paramagnetic resonance (EPR) and Fourier transform infrared spectroscopic techniques. The EPR spectrum at room temperature exhibits a sharp resonance signal at g = 2.007 and two more resonance signals centered at g ≈ 4.3 and 14.0. The resonance signal at g = 2.007 has been attributed to the E′ center and is related to a natural radiation-induced paramagnetic defect. Two more resonance signals centered at g ≈ 4.3 and 14.0 are characteristic of Fe³⁺ ions. The EPR spectra recorded at room temperature of jasper samples, heat-treated at temperatures ranging from 473 to 1,473 K exhibit marked temperature dependence. The resonance signal corresponding to E′ center disappears at elevated temperatures. A broad, intense resonance signal centered at g ≈ 2.0 appears at elevated temperatures. This resonance signal is a characteristic of Fe³⁺ ions, which are present as hematite in the jasper sample. The intensity of the resonance signal becomes dominant at elevated temperatures at ≥873 K, masking g ≈ 4.3 and g ≈ 14.0 resonance signals. The EPR spectra of jasper heat-treated at 673 K have been recorded at temperatures between 123 and 296 K. The population of spin levels (N) has been calculated for the broad g ≈ 2.0 resonance signal. It is found that N decreases with decreasing temperature. The linewidth (ΔH) of g ≈ 2.0 resonance signal of the heat-treated jasper is found to increase with decreasing temperature. This has been attributed to spin–spin interaction of the Fe³⁺ ions present in the form of hematite in the studied jasper sample.     

Electron spin resonance spectra of marine and fresh-water manganese nodules

Eighty ferromanganese nodules from a wide variety of marine and fresh-water environments have been analyzed by electron spin resonance spectroscopy. The purpose has been to gain information on the forms in which the major constituents of manganese nodules are present. Contributions to ESR spectra of the nodules come mainly from Mn²⁺ and Fe³⁺. Deep-sea samples generally showed only broad resonance lines, and those with larger peaks close to g = 2.0 are believed to contain more Mn²⁺ than others. Some Antarctic and fresh-water nodules lack a strong Mn²⁺ resonance and have a peak around g = 4.0 which is most likely tetrahedral Fe³⁺. A number of smaller peaks in several samples could not be readily interpreted in terms of contributions from individual ionic species because of fundamental problems in preparing standards having the ion of interest in the same micro-environment as it experiences in the nodules.     

Dating of marine aragonite by electron spin resonance

Marine aragonite, in the form of corals and/or shells, provides useful markers of geological and archaeological events. It is, therefore, important to have simple and accurate methods of dating these materials. Electron spin resonance (ESR) has previously been shown to be a reliable method for establishing the age of aragonitic coral samples in the time period approximately 100 ka B.P. The primary purpose of the present work is to discuss the problems encountered in extending this method to considerably older samples, up to 600 ka BP in age. In this time period there are questions about the stability of the ESR signal. The samples investigated are aragonitic corals from reef terraces of Barbados, West Indies, all of which have previously been dated by the methods, and by U-series disequilibrium, for samples below the limit of this method. There is generally good agreement for samples up to about 300 ka in age; older samples, even unrecrystallized, appear younger when dated by ESR than by . The source of this discrepancy is not clear. The explanation of thermal fading is not adequate. However, it appears likely that in most cases ESR will be able to be used to date materials up to this age. Further investigation is needed to determine tests that will distinguish datable samples from non-datable ones.     

Electron spin resonance of Mn2+ in sodalite

Electron spin resonance of allowed (Δm=0) and forbidden (Δm=±1) hyperfine transitions of Mn²⁺ in sodalite, Na₈(Al₆Si₆O₂₄)Cl₂, is reported. No fine structure other than the central M=∣+1/2>?∣?1/2> transition is observed. From intensity ratios of forbidden to allowed transitions and doubling of allowed lines in powder spectra the crystal field parameter |D| was estimated as equal to (8±5) 10^?3 cm^?1. The g-value for the spectrum was obtained as equal to 2.0033±0.0005. The hyperfine structure constant |A| was 83±1 gauss, equal to (77±1) 10^?4 cm^?1.     

Electron paramagnetic resonance characteristics of the humic acids from a podzol

A study has been made of solid and solution electron paramagnetic resonance (EPR) spectra of humic acids from different horizons in a podzolic soil. Hyperfine splitting was observed in the solution spectra of humic acids from all horizons and depended on the strength of alkali and the period of dissolution. The upper organic horizons L, F and O₁ contained humic acids with some spectral characteristics in common with lignin. Humic acid from the lower horizons showed different spectra. At least 5 different radical signals were present.     

The bonding of vanadium in complexes with humic acid: an electron paramagnetic resonance study

Complexes formed by the addition of vanadyl salts to a peat humic acid have been studied by EPR spectroscopy. The spectra show that the vanadium is in an environment with approximately axial symmetry. The g-values and ⁵¹V hyperfine coupling constants indicate that the vanadium remains in the vanadyl state and is complexed possibly either by oxygen donor groups or by mixed oxygennitrogen donor groups in the humic acid. Identical spectra were obtained when the vanadium was added to the humic acid as the metavanadate ion, thus showing that reduction of vanadium from oxidation state (V) to (IV) occurs.