Electron spin resonance of trivalent chromium in forsterite,Mg2SiO4

The electron spin resonance (ESR) spectrum of Cr³⁺ in a synthetic single crystal of forsterite doped with Cr₂O₃ was studied at room temperature in the X-band frequency range. The dependence of the observed spectra on the crystal orientation with respect to the applied magnetic field was investigated. The ESR spectra are described by the spin Hamiltonian \(H = \beta HgS + D(S_Z^{\text{2}} - {\text{1/3}}S{\text{(}}S{\text{ + 1)) + }}E{\text{(}}S_x^{\text{2}} - S_y^{\text{2}} {\text{)}}\) with S=3/2. The spin resonance reveals that the chromium ions are located at both the M1 and M2 positions. Other possible substitutional or interstitial Cr³⁺ positions may be possible, but were not observed. The site occupancy numbers of Cr³⁺ at M1 and M2 are roughly 1.2×10^?4 and 0.8×10^?4, respectively, assuming that chromium is oxidized completely. The preference of the chromium ions for M1 was interpreted qualitatively in terms of crystal field criteria. The rhombic and axial spin Hamiltonian parameters, D and E, and the directions of the magnetic axes obtained for M1 and M2 are consistent with the respective oxygen coordination polyhedra.     

Electron spin resonance of stabilized free radicals in sedimentary organic matter

Electron spin resonance (ESR) is evaluated as a method to study the thermal degradation of sedimentary organic matter which consists mainly of kerogen. Whole rock and separated kerogen samples were pyrolysed stepwise (ambient to 700°C at 50°C increments), extracted and analysed for elemental composition and ESR spectra at each step. Whole rock samples give rise to complex spectra which include those of paramagnetic metals and are therefore unsuitable in most cases for this purpose.The ESR parameters g value, ΔH and N_g differ for different types of immature organic matter. An increase in N_g,shift of g value to 2.0026–2.0028 and reduction in h are the main trends during pyrolysis and in natural heating of sedimentary organic matter.The peak generations of liquid and gaseous hydrocarbons coincide with maxima of free radical density. ESR spectroscopy in combination with complementary geochemical characterization of the sedimentary organic matter can serve to indicate maturity with respect to peak oil-gas generation.     

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.     

Electron spin resonance dating of burned flint from Kebara Cave,Israel

Electron spin resonance (ESR) measurements of heated flint from Kebara cave show the presence of two radiation sensitive signals: E' and Al. Both are strongly interfered with by organic signals produced during heating of the flint. The signal subtraction method of Porat and Schwarcz (1991) was used to determine equivalent doses for flint samples from levels VII-XII, for which thermoluminescence (TL) ages on the same samples are known. Ages were determined using dose rates based on analyses of the flint and TLD measurements of external dose rates. The average E' age of 48.5 ± 5.1 ka is much less than the average TL age (62.2 ± 4.3 ka), whereas the average age of 64.6 ± 12 ka given by the Al signal is in good agreement with the TL age, but with much larger dispersion. © 1994 John Wiley & Sons, Inc.     

Electron paramagnetic resonance and electron spin echo spectroscopy study of natural bornite

Summary A minero-chemical and spectroscopic characterisation has been performed on a natural bornite sample from the Natural History Museum of the University of Florence, through the use of EPMA, XRD, continuous-wave (cw) and pulsed EPR spectroscopy. The use of different EPR techniques allowed to fully investigate the distribution and valence states of Cu and Fe and to study the early oxidation products of bornite. The microanalytical and XRD characterisation proved bornite to be homogeneous and stoichiometric. Cw-EPR measurements confirmed the presence of Fe(III) as fundamental valence state in bornite; moreover, no evidence of Cu(II) as a bulk transient species, due to a charge transfer process, has been revealed. The broadening of the Fe(III) EPR spectrum is ascribed to a partial transfer of spin density from Fe(III) to the surrounding cationic sublattice. In contrast, the pulsed EPR experiments, registered at 4.2 K, revealed the presence of both Fe(III) and Cu(II). These species were supposed to be present at the early stage of the surface alteration. The data provided by the electron spin-echo decay and modulation suggest both the oxidised Cu(II) and Fe(III) species to belong to premature surface water adducts (evidenced by proton nuclear modulation), which may evolve to sulphate species.     

Electron paramagnetic resonance and polarized optical absorption spectra of Ni2+ in synthetic forsterite

The investigated Ni doped forsterite was grown with the floating zone technique. The EPR spectra were taken at room temperature using both 9.5 and 35 GHz. All specimens show EPR signals resulting from Mn²⁺ at M2 and Fe³⁺ at M1, M2, and Si positions. Ni²⁺ EPR signals are observed at 35 GHz but not at 9.5 GHz. The Ni²⁺ spectra are described by the spin Hamiltonian

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 spin resonance of auriferous and barren quartz at beaconsfield, Northern Tasmania

The intensity of ESR spectra of powdered quartz from auriferous veins at Beaconfield is greater that the intensity of the spectra from barren vein quartz at the same locality. Lattice impurities are presumed to be the cause of the electron paramagnetism of the mineralised quartz species. The method can be used for the characterisation of different types of quartz and may provide a tool useful in exploration geochemistry.     

怀来盆地珠窝堡遗址ESR年代学初步研究

作为"大泥河湾"考古的重要组成部分,怀来盆地是研究和探讨中国北方乃至东北亚地区更新世期间古人类生存行为、人群迁徙扩散以及文化交流等科学问题的重要地区。珠窝堡遗址的发现,不仅为怀来盆地的史前史研究提供了重要依据,更为泥河湾考古工作增添了新的亮点和坐标。珠窝堡遗址试掘后尚未进行年代学方面的研究,作为怀来盆地最新发现和首次试掘的旧石器遗址,其年代的确定不但有助于厘清古人类在这一地区出现的时代,而且对于建立该地区古人类行为适应、技术发展以及古环境演化的整体年代框架具有重要意义。本研究利用石英Ti-Li心ESR法对珠窝堡遗址样品进行了年龄测定,珠窝堡遗址的ESR年龄为504±76 ka,是怀来盆地内目前发现的年代最早的文化遗存。     

Electron spin resonance dating of the fossil deposits in the Naracoorte Caves,South Australia

The caves near Naracoorte, South Australia, contain one of the richest and most diverse fossil faunal assemblages on the Australian continent. Three sites were selected for electron spin resonance (ESR) dating because clastic, fossiliferous sediments were sandwiched between speleothem layers. This allows independent age control by highly precise thermal ionization mass‐spectrometry (TIMS) U‐series dating. We find that all ESR results agree within the constraints given by the U‐series dates, and allow further refinement of the age of the fauna analysed, indicating that most of the fauna in the large Victoria Cave Fossil Chamber is twice as old as reported previously. Our dating results, spanning from 280 to 500 ka for the Fossil Chamber, Victoria Cave, to about 125 ka for the Grant Hall, Victoria Cave, and 170 to 280 ka for the Fossil Chamber, Cathedral Cave, indicate little change, if any, in the megafaunal assemblage from the early Middle to the early Late Pleistocene. This changed dramatically after the last interglacial, when a large proportion of the megafauna suddenly disappeared. Copyright © 2001 John Wiley & Sons, Ltd.     

Electron spin resonance optical dating of marine,estuarine, and aeolian sediments in Florida,USA

For the first time, electron spin resonance optical dating (ESROD) has been conducted on littorally transported and aeolian siliciclastic sediments in Florida. ESROD utilizes light-sensitive radiation-sensitive defects at silicon sites that have been replaced by aluminum and titanium atoms to give rise to a time-dependant signal. These defects saturate at higher levels of radiation dose, compared to optically stimulated luminescence, and therefore extend the optical dating range back into the millions of years. Our results show that the Trail Ridge Sequence is a multi-depositional unit that began deposition around 2.2 Ma and continued until 6 ka. The Osceola Cape, of the Effingham Sequence, was deposited around 1.5 Ma, and the Chatham Sequence was a multi-depositional terrace with at least three events preserved.     

Temperature-dependent Fe2+–Mn2+ order–disorder behaviour in amphiboles

The partitioning of Fe and Mn between the large M(4) site and the octahedral sites, M(1,2,3) in the amphibole structure has been investigated in two natural manganogrunerites of compositions Ca_0.1Mn_1.9 Mg_1.25Fe²⁺ _3.56Fe³⁺ _0.38Si_7.81O₂₂(OH)₂ and Ca_0.24Mn_1.57 Mg_2.27 Fe²⁺ _2.76Fe³⁺ _0.32Si_7.84O₂₂(OH)₂. The long-range cation distribution in the two samples has been elucidated by in situ neutron powder diffraction revealing that Mn is preferentially ordered onto M(4) ? M(2) >M(1) >M(3) in both samples. Partitioning of Mn from M(4) into the octahedral sites begins at 350 °C, with site exchange energies of ?16.6 kJ mol^?1 and ?14.9 kJ mol^?1, in samples containing 1.90 and 1.57 Mn apfu, respectively. Mössbauer and infrared spectroscopy have been used to study the samples at room temperature, and Mössbauer data agree well with the diffraction results, confirming that high-temperature cation distributions are retained during cooling. The fine structure in the hydroxyl-stretching region of the IR absorption spectra has been used to discuss qualitatively the site occupancies of the coordinating M(1)M(3)M(1) triplet, linked by O(3). On the basis of such modelling, we conclude that a degree of local clustering is present in both samples.     

Electron paramagnetic resonance and ENDOR studies of Cr3+ - Al3+ pairs in forsterite

The electron paramagnetic resonance (EPR) spectrum of Cr³⁺ in synthetic crystals of forsterite consists primarily of lines of Cr³⁺ “isolated” at the M1 and M2 positions in a “perfect” crystal environment without local charge compensation. In addition it shows two nonequivalent superhyperfine-split sextets with different intensities which are due to strong interaction of the Cr³⁺ electron spin S (S=3/2) with an adjacent nuclear spin I(I=5/2). Electron nuclear double resonance (ENDOR) experiments revealed that the sextets result from Cr³⁺ (M1) - Al³⁺ and Cr³⁺ (M2) - Al³⁺ pairs, Al being located at adjacent tetrahedral Si sites. The g, D, A, and A′ tensor components of the Cr³⁺ - Al³⁺ pairs have been determined at room temperature. The values of the pairs are distinct although they are not very different from the corresponding data of “isolated” Cr³⁺. From the intensities of the EPR spectra the relative concentration of the Cr³⁺ - Al³⁺ pairs with respect to “isolated” Cr³⁺ has been estimated.     

Electron spin resonance and related studies of lignite and ball clay from South Devon,England

Electron spin resonance and infrared spectroscopic studies of lignite and ball clay from South Devon, and of extracts obtained from them by solvent fractionation, revealed similarities between corresponding organic components associated with both materials. All fractions exhibited a free radical resonance at g = 2.0037, which occurred with greatest intensity in the humic acids. Additional ESR features due to Fe³⁺, Mn²⁺ and VO²⁺ complexes were observed. Ferric ions give rise to resonances at g = 4.2 which have not been previously reported in the case of natural carbonaceous materials. It is shown that the paramagnetic species associated with the ball clay and lignite extracts do not significantly contribute to the observed ESR spectra of kaolinites, the latter being attributable to substituted Fe³⁺ ions and defect centres within the kaolinite lattice.     

An EPR study of Mn2+ and Fe3+ in dolomites

Electron paramagnetic resonance (EPR) measurements on dolomites from 9 different localities revealed contents of Mn²⁺ on two axial sites in all of them. The center with largerzero-field splitting (ZFS) was always present in much higher concentrations, except for a sample from Oberdorf it amounted to 95 percent or more of the total. This dolomite was the only one with a considerable content of Fe³⁺ on one axial site, almost certainly substituting for Mg²⁺. With X-ray irradiation the concentration of Fe³⁺ increased by about 30 percent showing that at least some of the divalent iron also substitutes for Mg. The ZFSs for Fe³⁺ and Mn²⁺ with larger ZFS increase with decreasing temperature in the same manner. The previous assignment of this Mn²⁺ to Mg sites is thus confirmed. An almost regular increase of the trigonal distortions at the divalent ions in different carbonates with increasing ionic radius is indicated by their crystal structure data. The very small ZFS for Mn²⁺ on Ca sites in dolomite must thus result from a strong local relaxation in the direction of a more regular octahedral arrangement. It is difficult to explain the different distribution ratios of Mn²⁺ on Ca and Mg sites with differences in growth and/or annealing temperatures alone. Thus different supply of Mg²⁺ and Ca²⁺ in the growth solutions may also contribute.     

Quantitative electron spin resonance (ESR) determinations of forms and total amounts of Mn in aqueous environmental samples

An electron spin resonance (ESR) method for determining quantitatively amounts and forms of Mn in 0.2 ml of marine and lacustrine waters with no chemical processing is described and applied to various samples from the Puget Sound region of western Washington state. Results over the 50 ppb15 ppm (0.9–270 μM) total Mn concentration range are reproducible to within ±3–5% and average 8 ± 7 and 13 ± 7% lower than inductively coupled plasma (ICP) determinations on marine and lacustrine pore waters, respectively. Main features in profiles of total Mn versus depth in Pacific Northwest pore waters resemble those in profiles from other coastal regions.The method reveals that 72–77% of total dissolved Mn is present as free Mn⁺⁺ in most seawaters, 14–16% is complexed with chloride, and 10–12% is complexed with sulfate. Less than 5% is associated with carbonate, bicarbonate or organic ligands. These results should significantly reduce uncertainties in equilibrium calculations of state of saturation of seawaters with respect to precipitation of various Mn-solids. The method can be easily extended to various ionic strengths and temperatures.     

Luminescence excitation spectra of Mn2+ in synthetic forsterite

Detailed ligand-field spectra of Mn²⁺ in both microcrystalline and single-crystal synthetic forsterite are obtained using the technique of luminescence excitation spectroscopy. It is shown that Mn²⁺ has an almost exclusive preference for one particular cation site which is most probably the M2 site. Low temperature measurements reveal a no-phonon (purely electronic) transition at 16,260 cm^?1 (615 nm) which is the energy of the lowest split component of the ⁴ T ₁(G) state above the ground state. Phonon replicas of this transition are evident showing that a particular phonon mode (180 cm^?1) is dominantly involved. An analysis of the polarized spectra of Mn²⁺ in single-crystal forsterite shows the choice of C _2v (C ₂, σ _d ) pseudosymmetry for the M2 site yields the best agreement with the polarization dependence of the transitions between the ligand-field states of the Mn²⁺ ion in this site.     

Structural control of polyhedral compression in synthetic braunite, Mn2+Mn3+ 6O8SiO4

The compression of synthetic braunite, Mn²⁺Mn³⁺ ₆O₈SiO₄, was studied by high-pressure single-crystal X-ray diffraction carried out in a diamond-anvil cell. The equation of state at room temperature (third-order Birch-Murnaghan equation of state: V ₀=1661.15(8) ų, K _0,298=180.7±0.9 GPa, K′=6.5±0.3) was determined from unit-cell volume data to 9.18 GPa. Crystal structures were determined at 6 different pressures to 7.69 GPa. Compression of the structure (space group I4₁/acd) was found to be slightly anisotropic (a ₀=9.4262(4) Å, K _a =499±4 GPa, K _a ′=19.7±0.9; c ₀=18.6964(6) Å, K _c =657±6 GPa, K _c ′=15.7±1.4) which can be attributed to the fact that the Mn³⁺-O bonds, which are the most compressible bonds, are aligned closer to the (001) plane than to the c axis. The large bulk modulus is the result of the structural topology in which 2/3 and 1/2 of the edges of the Mn²⁺O₈ and Mn³⁺O₆ polyhedra share edges with other polyhedra. The Mn²⁺O₈ polyhedra were found to compress isotropically, whereas anisotropic compressional behaviour was observed for all three Mn³⁺O₆ octahedra. Although the polyhedral geometry of all three crystallographically independent Mn³⁺ sites shows the same type of uniaxially elongated distortion, the compression of the individual octahedral configurations was found to be strongly dependent upon both the geometry of the polyhedron itself and the types of, and the connectivity to, the neighbouring polyhedra. The differences in the configuration of the different oxygen atoms, and therefore the structural topology, is one of the major factors determining the type and degree of the pressure-induced distortion, while the Jahn-Teller effect plays a subordinate role.     

Electron paramagnetic resonance of PO 2 2− and SO 2 − radicals in barite

Electron paramagnetic resonance (EPR) measurements of natural barite BaSO₄ have revealed the presence of PO ₂ ^2– and SO ₂ ^– ionic radicals. For the latter a hyperfine structure from the ³³S isotope has been detected and measured for the first time. The temperature dependence of the hyperfine interaction (HFI) constant of the PO ₂ ^2– and SO ₂ ^– radicals has been investigated in the 100–400 K range.     

The luminescence decay-time of Mn2+ activated calcite

Luminescence decay time data are presented for 17 samples of synthetic calcite and a natural calcite between room temperature and approximately 15 K. The majority of the samples have Mn concentration in the range 0.0001–0.01 atoms per formula unit (apfu). Their spectra are consistent with a single exponential decay. It was necessary to use a “stretched” exponential to obtain adequate fits to the spectra of two samples having Mn concentrations of 0.036 and 0.091 apfu. The decay time at room temperature ranges from approximately 40–57 ms. At low temperature the corresponding range is 51–120 ms, indicating that thermal quenching of the decay time takes place. The decay time is dependent on the concentration of Mn, reaching a maximum (at both room- and low-temperature) in the range 0.001–0.003 apfu. The composition dependence is greater at low temperature. The results are discussed in terms of the Mott-Seitz and multiphonon mechanisms of thermal quenching. It is concluded that the latter provides a better fit to the data and is more consistent with models of the luminescence process in calcite:Mn.