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1.
Single-crystal W-band electron paramagnetic resonance (EPR) spectra of an electron-irradiated quartz, measured at room temperature, 110 and 77 K, disclose three previously reported hole centers (#1, G and an ozonide radical). The W-band EPR spectra of these three centers clearly resolve six magnetically nonequivalent sites each, whereas previous X- and Q-band EPR studies reported Centers #1 and the ozonide radical to consist of only three symmetry-related components and interpreted them to reside on twofold symmetry axes in the quartz structure. The calculated g matrices of Center #1 and the ozonide radical show that deviations from twofold symmetry axes are <10°, which are probably attributable to distortion related to neighboring charge compensating ions. The W-band EPR spectra of Center G not only result in improved g matrices but also allow quantitative determination of the nuclear hyperfine (A) and quadrupole (P) matrices of its 27Al hyperfine structure that was incompletely resolved before. In particular, the g-maximum and g-minimum principal axes of Center G are approximately along two pairs of O–O edges of the SiO4 tetrahedron, while the unique A principal axis is approximately along a Si–Si direction. These new spin-Hamiltonian parameters suggest that Center G most likely involves trapping of a hole between two oxygen atoms related to a silicon vacancy and stabilized by an Al3+ ion in the neighboring tetrahedron (hence an O2n−–Al3+ defect, where n is either 1 or 3).  相似文献   

2.
The diffusive behavior of argon in quartz was investigated with three analytical depth profiling methods: Rutherford Backscattering Spectroscopy (RBS), 213 nm laser ablation, and 193 nm (Excimer) laser ablation on the same set of experimental samples. The integration of multiple depth profiling methods, each with different spatial resolution and sensitivity, allows for the cross-checking of methods where data ranges coincide. The use of multiple methods also allows for exploration of diffusive phenomena over multiple length-scales. Samples included both natural clear rock crystal quartz and synthetic citrine quartz. Laser analysis of clear quartz was compromised by poor coupling with the laser, whereas the citrine quartz was more easily analyzed (particularly with 193 nm laser). Diffusivity measured by both RBS and 193 nm laser ablation in the outermost 0.3 μm region of citrine quartz are self-consistent and in agreement with previously published RBS data on other quartz samples (including the clear quartz measured by RBS in this study). Apparent solubilities (extrapolated surface concentrations) for citrine quartz are in good agreement between RBS, 213 nm, and 193 nm laser analyses. Deeper penetration of argon measured up to 100 μm depth with the 213 nm laser reveal contributions of a second, faster diffusive pathway, effective in transporting much lower concentrations of argon into the crystal interiors of both clear and citrine quartz. By assuming such deep diffusion is dominated by fast pathways and approximating them as a network of planar features, the net diffusive uptake can be modeled and quantified with the Whipple-LeClaire equation, yielding δDb values of 1.32 × 10−14 to 9.1 × 10−17 cm3/s. While solubility values from the measured profiles confirm suggestions that quartz has a large capacity for argon uptake (making it a potentially important sink for argon in the crust), the slow rate of lattice diffusion may limit its capability to take up argon in shorter lived geologic environments and in experiments. In such shorter-lived systems, bulk argon diffusive uptake will be dominated by the fast pathway and the quartz lattice (including natural isolated defects that may also be storing argon) may never reach its equilibrium capacity.  相似文献   

3.
The shock-metamorphosed quartz exhibits thermal luminescence (TL) with maxima at 365 nm, 470 nm and 610–680 nm. By electron paramagnetic resonance (EPR) analysis E1 type electron centers and hole centers have been found which originate from vacancies including those from the substitution of Al3+ and/or Fe3+, for Si4+. The EPR and TL spectra may be interpreted mainly in terms of vacancy type defects associated with dislocations in the crystal structure of quartz.  相似文献   

4.
Irradiation techniques are often applied to gem minerals for color enhancement purposes. Natural green, blue and colorless specimens of rare gemological quality euclase, BeAlSiO4(OH), from Brazil were irradiated with gamma rays in the dose range from 10 to 500 kGy. Although the colors of the different specimens were not strongly influenced, two different irradiation-induced paramagnetic defect centers were found by electron paramagnetic resonance (EPR). The first one is an O− hole center interacting with one Al neighbor and the second is a Ti3+ electron center. The EPR angular rotation patterns of both irradiation-induced defects were measured and analyzed. The results suggest that O− hole centers are formed by dissociation of the hydroxyl ions, similar as in topaz crystals. In euclase the OH− ions interconnect distorted Al octahedra and Be tetrahedra in O5 positions. During irradiation, the electrons are captured by titanium ions (Ti4+ + e−), leading to the formation of paramagnetic Ti3+ ions. From the EPR rotation patterns it is clear that these ions substitute for Al ions. The spin Hamiltonian parameters of the irradiation-induced defects are analyzed and compared to similar defect centers in other mineral specimens. Thermal annealing experiments show that the O− hole centers and Ti3+ electron centers are directly connected through the radiation process.  相似文献   

5.
Radiation-induced smoky color and associatedelectron paramagnetic resonance (EPR) signals develop only in potassium feldspar (KAlSi3O8) free of structurally bound molecular water. Fluid inclusion water does not influence coloration. The integrated intensity of each of the four bands (11,600, 16,200, 19,100, and 27,200 cm?1) in the optical absorption spectra are linearly correlated with the doubly-integrated intensity of a broad, asymmetric first derivative atg eff=2.027 in EPR spectra. In microcline, the EPR pattern is resolved into an asymmetric six-line pattern atg eff=2.024 and a single derivative atg eff=2.009 which, based on analogy to alkali-silicate glass, are due respectively to [SiO4/K+]2+ and a hole shared between two nonbonding oxygens on Si. We propose that structural water inhibits formation of smoky centers in feldspar by releasing atomic hydrogen during irradiation which destroys centers while diffusing towards a stable site.  相似文献   

6.
Single-crystal electron paramagnetic resonance spectra of electron-irradiated stishovite, measured at temperatures from 3.5 to 294?K, reveal three S?=?1/2 radiation-induced defects: an aluminum-associated oxygen hole center and two nd 1 centers (Ti3+ and W5+). The aluminum-associated oxygen hole center, characterized by an orthorhombic site symmetry, coaxial matrices of the electronic Zeeman g, nuclear hyperfine A(27Al) and nuclear quadrupole P(27Al), and the orientation of the g-minimum axis along an O–O direction and those of the unique A(27Al) and P(27Al) axes perpendicular to the O–O direction, is an Al–O2 3? center, with the unpaired electron equally distributed on two equatorial oxygen atoms of a substitutional Al3+ ion at the octahedral Si site. Fully optimized Al-doped structure, theoretical 27Al nuclear hyperfine and quadrupole coupling constants and directions, and 3D spin densities from periodic hybrid density functional theory calculations provide further support for this structural model. Spin Hamiltonian parameters of the Ti3+ and W5+ centers, which are confirmed by their diagnostic 47Ti, 49Ti and 183W hyperfine structures, arise from electron trapping on substitutional Ti4+ and W6+ ions at the octahedral Si site.  相似文献   

7.
Four new types of paramagnetic centers — NH+ 3, N2?, 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 N2? 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.  相似文献   

8.
Single-crystal electron paramagnetic resonance (EPR) spectra of a natural citrine quartz without any artificial irradiation, measured at W-band frequencies (∼94 GHz) and temperatures of 77, 110 and 298 K, allow better characterization of three previously-reported Centers (#6, #7 and B) and discovery of three new defects (B′, C′ and G′). The W-band EPR spectra reveal that Centers #6 and #7 do not reside on twofold symmetry axes, contrary to results from a previous X-band EPR study. The W-band spectra also show that the previously reported Center B is a mixture of two defects (B and B′) with similar g matrices but different-sized 27Al hyperfine structures. Center C′ has similar principal g values to the previously reported Center C but is distinct from the latter by a larger 27Al hyperfine structure with splittings from 0.10 to 0.22 mT. Also, Center G′ has a similar g matrix to the previously reported Center G but a different 27Al hyperfine structure with splittings from 0.41 to 0.53 mT. These spin-Hamiltonian parameters, together with observed thermal properties and microwave-power dependence, suggest that Centers #6 and #7 probably represent O23− type defects. Centers B and B′ are probably superoxide radicals (O2−) with the unpaired spin localized on the same pair of oxygen atoms around a missing Si atom but linked to a substitutional Al3+ ion each at different neighboring tetrahedral sites. Similarly, Centers G and G′ are most likely superoxide radicals with the unpaired spin localized on another pair of oxygen atoms around a missing Si atom and linked to a substitutional Al3+ ion each at different neighboring tetrahedral sites. Center C′ is probably an ozonide radical associated with a missing Si atom and linked to a substitutional Al3+ ion at the neighboring tetrahedral site. This study exemplifies the value of  high-frequency EPR for discrimination of  similar defect centers and determination of  small local structural distortions that are often difficult to resolve in conventional  X- and Q-band EPR studies.  相似文献   

9.
A natural montmorillonite containing radiation-induced defects was studied with Electron Paramagnetic Resonance (EPR) spectroscopy (X- and Q-band). A first dominant native defect, namely native defect 1, is identified. It gives rise to an orthorhombic spectrum with gx= 2.004 ± 0.005 gy= 2.010 ± 0.003, gz= 2.065 ± 0.002. Simulation of the EPR spectrum at X- and Q-band reveals a second native defect with isotropic spectrum at g= 2.019 ± 0.005. Both are electron holes trapped on oxygen atoms of the structure. The native defect 1 is located on an oxygen-silicon bond or a non-bonding orbital parallel to the c* axis. These defects are annealed at 500°C and the half-life determined for native defect 1 is circa 3,000 years. Irradiations with beta rays produced two additional hole centers of lower stability and distinct EPR parameters. Artificial irradiations show that montmorillonite can be used as a dosimeter in a large dose range.  相似文献   

10.
Experimental determination of the pressure and temperature controls on Ti solubility in quartz provides a calibration of the Ti‐in‐quartz (TitaniQ) geothermometer applicable to geological conditions up to ~ 20 kbar. We present a new method for determining 48Ti mass fractions in quartz by LA‐ICP‐MS at the 1 μg g?1 level, relevant to quartz in HP‐LT terranes. We suggest that natural quartz such as the low‐CL rims of the Bishop Tuff quartz (determined by EPMA; 41 ± 2 μg g?1 Ti, 2s) is more suitable than NIST reference glasses as a reference material for low Ti mass fractions because matrix effects are limited, Ca isobaric interferences are avoided, and polyatomic interferences at mass 48 are insignificant, thus allowing for the use of 48Ti as a normalising mass. Average titanium mass fraction from thirty‐three analyses of low temperature quartz from the Czech Erzgebirge is 0.9 ± 0.2 μg g?1 (2s) using 48Ti as a normalising mass and Bishop Tuff quartz rims as a reference material. The 2s average analytical uncertainty for individual analyses of 48Ti is 8% for 50 μm spots and 7% for 100 μm spots, which offers much greater accuracy than the 21–41% uncertainty (2s) incurred from using 49Ti as an analyte.  相似文献   

11.
The rate of Cd2+ sorption by a calcareous aquifer sand was characterized by two reaction steps, with the first step reaching completion in 24 hours. The second step proceeded at a slow and nearly constant rate for at least seven days. The first step includes a fast adsorption reaction which is followed by diffusive transport into either a disordered surface film of hydrated calcium carbonate or into pore spaces. After 24 hours the rate of Cd2+ sorption was constant and controlled by the rate of surface coprecipitation, as a solid solution of CdCO3 in CaCO3 formed in recrystallizing material. Desorption of Cd2+ from the sand was slow. Clean grains of primary minerals, e.g. quartz and aluminosilicates. sorbed much less Cd2+ than grains which had surface patches of secondary minerals, e.g. carbonates, iron and manganese oxides. Calcite grains sorbed the greatest amount of Cd2+ on a weight-normalized basis despite the greater abundance of quartz. A method is illustrated for determining empirical binding constants for trace metals at in situ pH values without introducing the experimental problem of supersaturation. The binding constants are useful for solute transport models which include a computation of aqueous speciation.  相似文献   

12.
A natural smoky quartz crystal from Shandong province, China, was characterised by laser ablation ICP‐MS, electron probe microanalysis (EPMA) and solution ICP‐MS to determine the concentration of twenty‐four trace and ultra trace elements. Our main focus was on Ti quantification because of the increased use of this element for titanium‐in‐quartz (TitaniQ) thermobarometry. Pieces of a uniform growth zone of 9 mm thickness within the quartz crystal were analysed in four different LA‐ICP‐MS laboratories, three EPMA laboratories and one solution‐ICP‐MS laboratory. The results reveal reproducible concentrations of Ti (57 ± 4 μg g?1), Al (154 ± 15 μg g?1), Li (30 ± 2 μg g?1), Fe (2.2 ± 0.3 μg g?1), Mn (0.34 ± 0.04 μg g?1), Ge (1.7 ± 0.2 μg g?1) and Ga (0.020 ± 0.002 μg g?1) and detectable, but less reproducible, concentrations of Be, B, Na, Cu, Zr, Sn and Pb. Concentrations of K, Ca, Sr, Mo, Ag, Sb, Ba and Au were below the limits of detection of all three techniques. The uncertainties on the average concentration determinations by multiple techniques and laboratories for Ti, Al, Li, Fe, Mn, Ga and Ge are low; hence, this quartz can serve as a reference material or a secondary reference material for microanalytical applications involving the quantification of trace elements in quartz.  相似文献   

13.
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.  相似文献   

14.
The electron paramagnetic resonance (EPR) study of gammaor x-ray-irradiated natural barite and celestite has revealed the presence of a radiation center with principal values of the g tensor and the A tensor [MHz] of hyperfine interaction (from the 33S isotope): gxx=1.9963, gyy=2.0073, gzz=2.0025, Axx=434, Ayy=447, Azz=528 in BaSO4, gxx=1.9990, gyy=2.0075, gzz=2.0027, Axx=426, Ayy=439, Azz=520 in SrSO4. The center has been identified as SO 3- 4 radical. The electron centers SO 4 3- in barite have been found to be produced along with the hole centers SO 4 3- , and maximum concentration of both centers is reached at a gamma-ray-radiation dose of about 5·105 Gy. UV or thermal treatment causes both centers to disappear. The SO 4 3- radicals proved to be more thermally stable than the SO 4 3- radicals: within about 0.5 h the latter disappear at 125° C, whereas the former do so at 180° C.  相似文献   

15.
Single-crystal and powder electron paramagnetic resonance (EPR) spectroscopic studies of natural amethyst quartz, before and after isochronal annealing between 573 and 1,173 K, have been made from 90 to 294 K. Single-crystal EPR spectra confirm the presence of two substitutional Fe3+ centers. Powder EPR spectra are characterized by two broad resonance signals at g = ~10.8 and 4.0 and a sharp signal at g = 2.002. The sharp signal is readily attributed to the well-established oxygen vacancy electron center E 1′. However, the two broad signals do not correspond to any known Fe3+ centers in the quartz lattice, but are most likely attributable to Fe3+ clusters on surfaces. The absolute numbers of spins of the Fe3+ species at g = ~10.8 have been calculated from powder EPR spectra measured at temperatures from 90 to 294 K. These results have been used to extract thermodynamic potentials, including Gibbs energy of activation ΔG, activation energy E a, entropy of activation ΔS and enthalpy of activation ΔH for the Fe3+ species in amethyst. In addition, magnetic susceptibilities (χ) have been calculated from EPR data at different temperatures. A linear relationship between magnetic susceptibility and temperature is consistent with the Curie–Weiss law. Knowledge about the stability and properties of Fe3+ species on the surfaces of quartz is important to better understanding of the reactivity, bioavailability and heath effects of iron in silica particles.  相似文献   

16.
In natural citrines five new hole centers were detected and analyzed by electron paramagnetic resonance. An additional one was observed in rose-colored quartz crystals with radiation defects as the cause of coloration. Characteristic hyperfine patterns due to an adjacent aluminum impurity were resolved in the spectra of three of these centers. Their relations to other hole centers of well-known structure in quartz and fused silica are discussed and possible models for their structures are proposed.  相似文献   

17.
Neutron- and gamma irradiation-induced paramagnetic centers in natural colorless topaz from four different Brazilian localities were studied by electron paramagnetic resonance (EPR) and optical absorption as a function of irradiation dose and thermal treatment. Gamma irradiation doses were applied up to 1,000 kGy with a dose rate of 15 kGy h−1. For the neutron irradiation experiments, a neutron flux of 4 × 1012 cm−2 s−1 was used with an integrated flux of up to 1 × 1018 cm−2. From the experiments, it is concluded that brownish colors are induced by gamma-rays and may be associated with a single broad isotropic EPR line with g = 2.015(2). Both the EPR line and the related optical absorption band at 460 nm (2.7 eV) are lost during thermal treatments between 150 and 200°C. Fast neutrons create the paramagnetic peroxyl radicals O2 − and the paramagnetic O− hole centers. The O− centers have the same thermal stability as the optical absorption band at 620 nm (2.0 eV). It is confirmed that the absorption due to the O− center is responsible for the blue color in topaz. Both color centers and their absorption bands are discussed in the context of O− bound small polarons.  相似文献   

18.
《Applied Geochemistry》1986,1(1):153-160
The biaxial absorption bands in amethyst quartz, with peaks at 2.28 eV and 3.54 eV related to Fe4+ and a peak at 3.02 eV—which is the A3 band related to the [AlO4]° trapped hole center, have orientations of maximum light absorption in the basal plane of Brazil-twinnedr-growth sectors paralleling the planes of Brazil optical twinning. Absorption minima are at 90° to the maxima in all cases. The Brazil twinning planes always parallel thea-axes(1210,etc) of quartz and in many cases also parallel planes perpendicular to ther-faces(1011,etc.). These are directions of channels in the quartz structure. The anisotropy ratio,σmax/σmin, of the Fe4+ band is that of the A2 absorption band in smoky quartz as would be expected if Fe3+ furnishes electrons to quench the trapped holes causing this absorption band. In the absence of the A1 and A2 absorption bands, the A3 absorption band width at half-maximum decreases from 1.43 to 0.36 eV indicating decreased charge-transfer character of the [AlO4]° center in the absence of the other types of Al trapped-hole centers in quartz. The key to the Brazil twinning in α-quartz are the channels which fill with large Fe3+ ions that force twinning to relieve strain in the structure. Amethyst color results only if aluminum is present substitutionally in the quartz as well as the interstitial iron plus ionizing radiation.  相似文献   

19.
 The charge compensation problem for Si−Al substitution in Brazilian natural quartz is investigated in relation to γ-irradiation darkening response and impurity contents evaluated by atomic absorption spectrometry and infrared absorption spectroscopy. Al+3 in as-grown natural quartz is compensated by Li+ and H+ to form Al−Li and Al−OH centers. The content ratios (Al−Li)/(Al−OH) and Li/(broad OH band) depend on the environmental conditions of crystal growth and influence the formation of Al-hole centers due to γ-irradiation. Al−OH centers are the dominant Al-related centers in quartz from hydrothermal origin while Al−Li centers are dominant for that of pegmatitic origin. The formation of Al-hole centers is little in quartz with a high content of broad OH band which is from low temperature hydrothermal origin. Received May 23, 1995 / Revised, accepted May 8, 1996  相似文献   

20.
A model for the formation of supermassive black holes at the center of a cluster of primordial black holes is developed. It is assumed that ～10?3 of the mass of the Universe consists of compact clusters of primordial black holes that arose as a result of phase transitions in the early Universe. These clusters also serve as centers for the condensation of dark matter. The formation of protogalaxies with masses of the order of 2 × 108 M ⊙ at redshift z = 15 containing clusters of black holes is investigated. The nuclei of these protogalaxies contain central black holes with masses ～105 M ⊙, and the protogalaxies themselves resemble dwarf spherical galaxies with their maximum density at their centers. Subsequent merging of these induced protogalaxies with ordinary halos of dark matter leads to the standard picture for the formation of the large-scale structure of the Universe. The merging of the primordial black holes leads to the formation of supermassive black holes in galactic nuclei and produces the observed correlation between the mass of the central black hole and the bulge velocity dispersion.  相似文献   

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