首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
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.  相似文献   

2.
Crystals of the olivine minerals, tephroite (Mn2SiO4) and fayalite (Fe2SiO4) containing manganese(II) and iron (II and trace of III), respectively, were synthesized. Glasses were prepared from these crystalline materials by a splat-quench technique. Measurement of electron paramagnetic resonance (EPR) of all these powdered samples at room temperature show that the g-factors of Mn2+ in both glassy and crystalline environments (geff = 2.004) are the same, although the EPR linewidths (for glass, ΔHpp = 200 G; for crystals ΔHpp = 287 G) suggest less clustering of paramagnetic Mn2+ ions in the glass. Mn2+ probably occupies a distorted octahedral site in the tephroite crystal structure, although a four-fold coordination is suggested from other spectroscopic investigation on this glass. The EPR parameters of Fe3+ in synthetic fayalite glass (geff = 2.01 and 6.00; ΔHpp=150 and 1375 G, respectively, for the high and low field resonances) and powdered crystals (geff = 3.31 and ΔHpp = 900 G) indicated that Fe3+ ion in the crystals, is probably located in a distorted tetragonal site M2 and an axial environment has been proposed in the glassy system.  相似文献   

3.
Practically identical Mössbauer spectra have been obtained for 40 ferromanganese nodules from a wide variety of marine and fresh-water locations. None of the nodules examined contains more than one weight percent Fe2+, so no more than a few percent of the total iron in these nodules can be Fe2+. Most of the iron is present as Fe3+ in paramagnetic or superparamagnetic oxide phases, although hysteresis loops show the presence of small amounts of ferromagnetic phases not detected by the Mössbauer technique.  相似文献   

4.
The use of ultrasonically modulated electron resonance (UMER) to study S-state ions in substitutional sites of mineral single crystals is discussed. Mn2+ and Fe3+ in natural single crystals of tremolite are used as examples. Combined electron paramagnetic resonance (EPR) and UMER measurements establish almost certainly that Mn2+ enters predominantly into the distorted M4 sites occupied by Ca2+ in the ideal tremolite structure and only to a minor extent into the M1, M2 and M3 sites normally occupied by Mg2+. Fe3+ in tremolite gives rise to the well known high spin resonance with g eff?4.3 but there is considerable uncertainty as to the site of the impurity ion.  相似文献   

5.
Piemontite- and thulite-bearing assemblages from highly oxidized metapelitic and metacalcareous schists associated with braunite quartzites at Vitali, Andros island, Greece, were chemically investigated. The Mn-rich metasediments are intercalated in a series of metapelitic quartzose schists, marbles, and basic metavolcanites which were affected by a regional metamorphism of the highP/T type (T=400–500° C,P>9 kb) and a later Barrovian-type greenschist metamorphism (T=400–500° C,P～-5–6 kb). Texturally and chemically two generations of piemontite (I and II) can be distinguished which may show complex compositional zoning. Piemontite I coexisted at highP/T conditions with braunite, manganian phengite (alurgite), Mn3+-Mn2+-bearing Na-pyroxene (violan), carbonate, quartz, hollandite, and hematite. Zoned grains generally exhibit a decreasing Mn3+ and an increasing Fe3+ and Al content towards the rim. Chemical compositions of piemontite I range from 2.0 to 32.1 mole % Mn3+, 0 to 25.6 mole % Fe3+, and 60.2 to 81.2 mole % Al. Up to 12.5 mole % Ca on the A(2) site can be substituted by Sr. Piemontites formed in contact or close to braunite (±hematite) attained maximum (Mn3++Fe3+)Al?1 substitution corrresponding to about 33 mole % Mn3++Fe3+ in lowiron compositions and up to about 39 mole % Mn3++ Fe3+ at intermediate Fe3+/(Fe3++Mn3+) ratios. Piemontite II which discontinuously overgrows piemontite I or occurs as separate grains may have been formed by greenschist facies decomposition of manganian Na-pyroxenes according to the reaction: (1) $$\begin{gathered} {\text{Mn}}^{{\text{3 + }}} - Mn^{2 + } - bearing omphacite/chloromelanite \hfill \\ + CO_2 + H_2 O + HCl \pm hermatite \hfill \\ = piemontite + tremolite + albite + chlorite \hfill \\ + calcite + quartz + NaCl \pm O_2 . \hfill \\ \end{gathered} $$ Thulites crystallized in coexistence with Al-rich piemontite II. All thulites analysed are low-Fe3+ manganian orthozoisites with Mntot～-Mn3+ substituting for Al on the M(3) site. Their compositions range from 2.9 to 7.2 mole % Mn3+, 0 to 1.2 mole % Fe3+, and 91.8 to 96.7 mole % Al. Piemontites II in thulite-bearing assemblages range from 5.8 to 15.9 mole % Mn3+, 0 to 3.7 mole % Fe3+, and 83.7 to 93.6 mole % Al. By contrast, piemontites II in thulite-free assemblages are similarly enriched in Mn3+ + Fe3+ — and partially in Sr2+ — as core compositions of piemontite I (21.1 to 29.6 mole % Mn3+, 2.0 to 16.5 mole % Fe3+, 60.6 to 68.4 mole % Al, 0 to 29.4 mole % Sr in the A(2) site). The analytical data presented in this paper document for the first time a continuous low-Fe3+ piemontite solid solution series from 5.8 to 32.1 mole % Mn3+. Aluminous piemontite II is enriched by about 3 mole % Mn3++Fe3+ relative to coexisting thulite in Fe3+-poor samples and by about 6 mole % Mn3++Fe3+ in more Fe3+-rich samples. Mineral pairs from different samples form a continuous compositional loop. Compositional shift of mineral pairs is attributed to the effect of a variable fluid composition at constantP fluid andT on the continuous reaction: (2) $$\begin{gathered} piemontite + CO_2 \hfill \\ = thulite + calcite + quartz \hfill \\ + Mn^{2 + } Ca_{ - 1} [calcite] + H{_2} O + O{_2} \hfill \\ \end{gathered} $$ Further evidence for a variable \(x_{H_2 O} \) and/or \(f_{O_2 } \) possibly resulting from fluid infiltration and local buffering during the greenschist metamorphism is derived from the local decomposition of piemontite, braunite, and rutile to form spessartine, calcite, titanite, and hematite by the reactions: (3) $$\begin{gathered} piemontite + braunite + CO_2 \hfill \\ = sperssartine + calcite + quartz \pm hermatite \hfill \\ + H{_2} O + O{_2} \hfill \\ \end{gathered} $$ and more rarely: (4) $$\begin{gathered} piemontite + quartz + rutile + braunite \hfill \\ = spessartine + titanite + hematite + H{_2} O + O{_2} . \hfill \\ \end{gathered} $$   相似文献   

6.
Only one part of the EPR lines of a kaolinite spectrum of structural Fe3+ is clearly observable because of the overlapping of other lines with other spectra. For this reason, to determine the structural Fe3+ concentration we used the line near g=9, although it is not intense. A standard is needed: powders of ZnS containing given concentrations of Mn2+ (isoelectronic to Fe3+) were used for this purpose. Using the simulations of the EPR spectra, the concentration (number of Fe3+ per Al3+) is determined; it is in the range 10?5 to 10?4 for our samples. Considering that the crystal-field disorder around Fe3+ is responsible for line broadening, we looked for a possible effect of the broadening on the intensity of the EPR spectra. This effect is taken as a distribution of the parameter λ=B22/B20. The influence of the parameter λ and its statistical distribution on the position, shape, width and intensity of the EPR line has been calculated using simulation procedures. The correction due to the disorder on the calculated concentration is of the same order of magnitude as the precision measurement. This method can be applied for other kaolinites by comparing the area of their g=9 lines with known ones.  相似文献   

7.
Ilvaite samples from six different localities in Japan are found to be members of a solid-solution series varying from Ca(Fe2+,Fe3+)2Fe2+(OH)O Si2O7 to approaximately Ca(Fe2+,Fe3+)2Fe 0.5 2+ Mn 0.5 2+ (OH)O Si2O7, and have been studied by Mössbauer spectrometry and magnetic measurements. The variation in intensity of Mössbauer doublets confirms that Mn substitutes for Fe2+ in the M(B) cation site. An temperatures decreasing from 300 K to 4K, an abrupt change in the reciprocal mass magnetic susceptibility, 1/x g, occurs about 120 K; 1/x g depends linearly upon temperature above 120 K. This change, which is characterized by an unusual mode of decrease in 1/x g, has been interpreted based on Mössbauer spectra at 80 K: the spectra of Fe2+ and Fe3+ in the M(A) site show Zeeman splitting, whereas those of Fe2+ in the M(B) site do not show the effect. This Mössbauer evidence suggests that magnetic spins of Fe in M(A) are in an ordered state, very likely of antiparallel coupling, whereas those of Fe in M(B) are randomly oriented, showing that below 120 K ilvaite has two different magnetic states for Fe ions. As there is a line of evidence that the spins of Fe in M(B) would take an ordered state at extremely low temperatures, ilvaite magnetism may be regarded as basically antiferromagnetic. The magnetic spins of Fe in M(A) and M(B) undergo magnetic transitions at different specific temperatures, thus giving as a whole unusual features of magnetism.  相似文献   

8.
Natural blue and colorless rare-gem mineral specimens of euclase from Brazil are investigated by electron paramagnetic resonance (EPR). Angular dependences of Fe3+ EPR spectra in three mutually perpendicular crystal planes are analyzed revealing g and D tensors with significant low-symmetry effects, as for example, the high asymmetry parameter E/D = 0.28. Fourth-order degree Stevens parameters are also included in analysis. The anisotropy of both g and D tensors is consistent with Fe3+ substituting for Al3+ ions in strongly distorted AlO5(OH) octahedra in which the oxygen distances range from 1.85 to 1.98 Å. Fe3+ is not responsible for the blue color because colorless and blue euclase show nearly the same Fe3+ concentration as measured by EPR. However, total iron content in blue sample is much higher than in the colorless one suggesting that the existing model that Fe2+–Fe3+ intervalence charge transfer transition may explain the blue color of euclase.  相似文献   

9.
  相似文献   

10.
A natural sample of clinochlore from the Longitudinal Valley area of northeastern Taiwan has been characterized by using the powder X-ray diffraction (XRD), differential thermal analysis and electron paramagnetic resonance (EPR) spectroscopic techniques. The lattice parameters of the monoclinic (IIb) clinochlore with the composition (Mg2.988 Al1.196 Fe1.6845 Mn0.026)5.8945 (Si2.559 Al1.441)4 O10 (OH)8 have been calculated from the powder XRD data and are found to be a = 5.347 Å, b = 9.223 Å, c = 14.250 Å, β = 97.2° and Z = 2. The thermal behaviour of the sample showed the typical behaviour of clinochlore with a hydroxyl content of 12.5 wt%. The EPR spectrum at room temperature exhibits two resonance signals centred at g ≈ 2.0 and g ≈ 8.0. The signal at g ≈ 2.0 shows a six-line hyperfine structure which is a characteristic of Mn2+ ions in octahedral symmetry. The resonance signal at g ≈ 8.0 is a characteristic of Fe3+ ions. The EPR spectra have also been recorded at different temperatures (123–295 K). The population of spin levels (N) has been calculated for g ≈ 2.0 and g ≈ 8.0 resonance signals. It is observed that N increases with decreasing temperature. From EPR spectra, the spin-Hamiltonian parameters have been evaluated. The zero-field splitting parameter (D) is found to be temperature dependent. The peak-to-peak width of the g ≈ 8.0 resonance signal is found to increase with decrease in temperature.  相似文献   

11.
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.  相似文献   

12.
Summary ?Rocks containing braunite from the Ossa-Morena central belt (Iberian Massif, SW Spain) have been studied; these include nodules and layers of braunite (association I), Mn-slates (association II) and Mn-metatuffs (associations III and IV). Geochemical features of braunite nodules such as Mn/Fe ratios around 2, positive Ce-anomalies and good correlations among Mn, Fe, Co, Cu and REE contents indicate that the protolith of the braunite-nodules was precipitated from oxidising sea water. Greenschist facies Hercynian metamorphism reduced initial Mn4+ to Mn3+ and Mn2+. High initial fO2 of oxide beds (association I) limited reduction to the formation of braunite. Reduction continued until the formation of garnet + piemontite (associations II and III), and pyroxmangite + pyrophanite (association IV). Ti-rich braunites (up to 6.8% of TiO2) occur in slates and metatuffs in which the (Mn + Fe)/Ti ratio of the whole rock is lower than 30, while braunites have lower Ti contents in slates and metatuffs with (Mn + Fe)/Ti ratios around 90. Fe-rich braunite crystallized in rocks with Mn2+ oxide and silicate where low Mn3+/Mn2+ in the whole rock facilitated substitution of Fe3+ for Mn3+. Received January 30, 2002; revised version accepted May 7, 2002 Published online November 22, 2002  相似文献   

13.
Single crystals of the garnet Mn2+ 3Mn3+ 2[SiO4]3 and coesite were synthesised from MnO2-SiO2 oxide mixtures at 1000°C and 9 GPa in a multianvil press. The crystal structure of the garnet [space group Ia3¯d, a=11.801(2) Å] was refined at room temperature and 100 K from single-crystal X-ray data to R1=2.36% and R1=2.71%, respectively. In contrast to tetragonal Ca3Mn3+ 2[GeO4]3 (space group I41/a), the high-pressure garnet is cubic and does not display an ordered Jahn-Teller distortion of octahedral Mn3+. A disordered Jahn-Teller distortion either dynamic or static is evidenced by unusual high anisotropic displacement parameters. The room temperature structure is characterised by following bond lengths: Si-O=1.636(4) Å (tetrahedron), Mn3+-O=1.995 (4) Å (octahedron), Mn2+-O=2.280(5) and 2.409(4) Å (dodecahedron). The cubic structure was preserved upon cooling to 100 K [a=11.788(2) Å] and upon compressing up to 11.8 GPa in a diamond-anvil cell. Pressure variation of the unit cell parameter expressed by a third-order Birch-Murnaghan equation of state led to a bulk modulus K 0=151.6(8) GPa and its pressure derivatives K′=6.38(19). The peak positions of the Raman spectrum recorded for Mn2+ 3Mn3+ 2[SiO4]3 were assigned based on a calderite Mn2+ 3Fe3+ 2[SiO4]3 model extrapolated from andradite and grossular literature data.  相似文献   

14.
The dependence of the electronic and the crystallographic structure on temperature of synthetic Mnbearing ilvaites CaFe2+ 2-xMn2+ xFe3+ [Si2O7/O/OH] with 0≤x≤0.19 has been investigated. The change of the electronic structure was studied by 57Fe Mössbauer spectroscopy. The spectra show an increasing valence fluctuation rate between Fe2+ and Fe3+ in the double chain of edge-sharing octahedra with increasing temperature resulting in a mixed valent state of iron. The valence fluctuation rate is distinctly increased by the Mnsubstitution. The temperature of the crystallographic phase transition T x as studied by a high temperature Guinier method is distinctly lowered by the Mn-substitution (x = 0.0, T x=390K; x = 0.12, T x =370K; x = 0.19, T x=295K). The reasons for this behaviour are discussed in terms of Fe2 +, Fe3 + cation order-disorder, electronic relaxation rate, and relaxation of the lattice. In the monoclinic phase there is electron hopping between Fe2 +, Fe3 + pairs whereas in the orthorhombic phase there is extended electron delocalization via a narrow, d-band mechanism.  相似文献   

15.
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.  相似文献   

16.
Four samples of synthetic chromium-bearing spinels of (Mg, Fe2+)(Cr, Fe3+)2O4 composition and four samples of natural spinels of predominantly (Mg, Fe2+)(Al, Cr)2O4 composition were studied at ambient conditions by means of optical absorption spectroscopy. Synthetic end-member MgCr2O4 spinel was also studied at pressures up to ca. 10 GPa. In both synthetic and natural samples, chromium is present predominantly as octahedral Cr3+ seen in the spectra as two broad intense absorption bands in the visible range caused by the electronic spin-allowed 4 A 2g  → 4 T 2g and 4 A 2g  → 4 T 1g transitions (U- and Y-band, respectively). A distinct doublet structure of the Y-band in both synthetic and natural spinels is related to trigonal distortion of the octahedral site in the spinel structure. A small, if any, splitting of the U-band can only be resolved at curve-fitting analysis. In all synthetic high-chromium spinels, a couple of relatively narrow and weak bands of the spin-allowed transitions 4 A 2g  → 2 E g and 4 A 2g  → 2 T 1g of Cr3+, intensified by exchange-coupled interaction between Cr3+ and Fe3+ at neighboring octahedral sites of the structure, appear at ~14,400 and ~15,100 cm?1. A vague broad band in the range from ca. 15,000 to 12,000 cm?1 in synthetic spinels is tentatively attributed to IVCr2+ + VICr3+ → IVCr3+ + VICr2+ intervalence charge-transfer transition. Iron, mainly as octahedral Fe3+, causes intense high-energy absorption edge in near UV-range (ligand–metal charge-transfer O2? → Fe3+, Fe2+ transitions). As tetrahedral Fe2+, it appears as a strong infrared absorption band at around 4,850 cm?1 caused by electronic spin-allowed 5 E → 5 T 2 transitions of IVFe2+. From the composition shift of the U-band in natural and synthetic MgCr2O4 spinels, the coefficient of local structural relaxation around Cr3+ in spinel MgAl2O4–MgCr2O4 system was evaluated as ~0.56(4), one of the lowest among (Al, Cr)O6 polyhedra known so far. The octahedral modulus of Cr3+ in MgCr2O4, derived from pressure-induced shift of the U-band of Cr3+, is ~313 (50) GPa, which is nearly the same as in natural low-chromium Mg, Al-spinel reported by Langer et al. (1997). Calculated from the results of the curve-fitting analysis, the Racah parameter B of Cr3+ in natural and synthetic MgCr2O4 spinels indicates that Cr–O-bonding in octahedral sites of MgCr2O4 has more covalent character than in the diluted natural samples. Within the uncertainty of determination in synthetic MgAl2O4 spinel, B does not much depend on pressure.  相似文献   

17.
Redox reactions between Fe2+ in solution and Mn-oxides are proposed as a mechanism for concentration of Mn in sediments both during weathering and diagenesis in marine sediments, e.g. the formation of Mn-nodules.If such a mechanism is to be effective, then reaction rates between Fe2+ and Mn-oxides should be fast. The kinetics and stoichiometry of the reaction between dissolved Fe2+ and synthetically prepared birnessite (Mn7O13·5H2O) were studied experimentally in the pH range 3–6.Results show a stoichiometry which at pH < 4 conforms to a simple reaction between Fe2+ and birnessite, releasing Mn2+ and Fe3+ to the solution. At pH > 4 FeOOH is precipitated and excess Fe2+ consumption compared to the theoretical stoichiometry is observed. The excess Fe2+ consumption is not due to a formation of a quantitative MnOOH layer but rather to adsorption.Reaction kinetics are very fast at pH < 4 and change at pH 4 to a slower mechanism. At pH > 4 the reaction is fast initially until 17% of the bimessite has dissolved and changes then to a slower stage. The later stage can be described by the equation: J = km0(H+)?0.45[Fe2+]γ(mm0)β where J is the overall rate of Mn2+ release, m0 and m the mass of birnessite at time t = 0 and t > 0, β = 6.76?0.94 pH and γ has values of 0.76 at pH 5 and 0.39 at pH 6. The rate constant k is 7.2·10?7 moles s?1 g?1 (moles/1)?0.31 at pH 5 and 9.6·10?8 moles s?1 g?1 (moles/1)0.06 at pH 6.Diffusion calculations show that the rate is controlled by surface reaction and it is tentatively proposed that the availability of vacancies in octahedral [MnO6]sheets of the birnessite surface could be rate controlling. It is concluded that reactions between Fe(II) and birnessite, and probably other Mn-oxides, are fast enough to be important in natural environments at the earth surface.  相似文献   

18.
The crystal and magnetic structures of ilvaite Ca(Fe2+, Fe3+)Fe2+Si2O7O(OH) have been obtained by profile refinement of high resolution neutron powder data from a natural sample from Seriphos, Greece. Below about 400 K an electronic transition from an itinerant to an ordered state is expected, with the structure changing from orthorhombic to monoclinic. The structure remains monoclinic P2 1/a down to 5 K, with Fe2+ almost completely ordered on one of the A-sites and Fe3+ on the other: the ordering may increase with decreasing temperature. The B-site contains Fe2+ plus a small amount of Mn2+ impurity. There are two magnetic transitions, at 116 K and 40 K: at 80 K the Fe2+ and Fe3+ spins on the A-sites along one infinite c-axis chain are parallel, and antiparallel to those along the adjoined edge-sharing centrosymmetrically related chain. The spin vectors are all perpendicular to the plane of these chains, i.e. almost parallel to the crystallographic b-axis. At 5 K, this order is maintained, but the Fe2+ spins on the B-sites order antiferromagnetically as well, again almost along the b-axis. These results explain the earlier Mössbauer and magnetisation measurements.  相似文献   

19.
Mössbauer and polarized optical absorption spectra of the kyanite-related mineral yoderite were recorded. Mössbauer spectra of the purple (PY) and green yoderite (GY) from Mautia Hill, Tanzania, show that the bulk of the iron is Fe3+ in both varieties, with Fe2+/(Fe2++Fe3+) ratios near 0.05. Combining this result with new microprobe data for PY and with literature data for GY gives the crystallochemical formulae: $$\begin{gathered} ({\text{Mg}}_{{\text{1}}{\text{.95}}} {\text{Fe}}_{{\text{0}}{\text{.02}}}^{{\text{2 + }}} {\text{Mn}}_{{\text{0}}{\text{.01}}}^{{\text{2 + }}} {\text{Fe}}_{{\text{0}}{\text{.34}}}^{{\text{3 + }}} {\text{Mn}}_{{\text{0}}{\text{.07}}}^{{\text{3 + }}} {\text{Ti}}_{{\text{0}}{\text{.01}}} {\text{Al}}_{{\text{3}}{\text{.57}}} )_{5.97}^{[5,6]} \hfill \\ {\text{Al}}_{{\text{2}}{\text{.00}}}^{{\text{[5]}}} [({\text{Si}}_{{\text{3}}{\text{.98}}} {\text{P}}_{{\text{0}}{\text{.03}}} ){\text{O}}_{{\text{18}}{\text{.02}}} ({\text{OH)}}_{{\text{1}}{\text{.98}}} ] \hfill \\ \end{gathered}$$ and PY and $$\begin{gathered} ({\text{Mg}}_{{\text{1}}{\text{.98}}} {\text{Fe}}_{{\text{0}}{\text{.02}}}^{{\text{2 + }}} {\text{Mn}}_{{\text{< 0}}{\text{.001}}}^{{\text{2 + }}} {\text{Fe}}_{{\text{0}}{\text{.45}}}^{{\text{3 + }}} {\text{Ti}}_{{\text{0}}{\text{.01}}} {\text{Al}}_{{\text{3}}{\text{.56}}} )_{6.02}^{[5,6]} \hfill \\ {\text{Al}}_{{\text{2}}{\text{.00}}}^{{\text{[5]}}} [({\text{Si}}_{{\text{3}}{\text{.91}}} {\text{O}}_{{\text{17}}{\text{.73}}} {\text{(OH)}}_{{\text{2}}{\text{.27}}} ] \hfill \\ \end{gathered}$$ for GY. The Mössbauer spectra at room temperature contain one main doublet with isomer shifts and quadrupole splittings of 0.36 (PY), 0.38 (GY) and 1.00 (PY), 0.92 (GY) mm s?1, respectively. These values correspond to Fe3+ in six or five-fold coordination. The doublet components have anomalously large half widths indicating either accomodation of Fe3+ in more than one position (e.g., octahedraA1 and five coordinatedA2) or the yet unresolved superstructure. Besides strong absorption in the ultraviolet (UV) starting from about 25,000 cm?1, the polarized optical absorption spectra are dominated by strong bands around 16,500 and 21,000 cm?1 (PY) and a medium strong band at around 13,800 cm?1 (GY). Position and polarization of these bands, in combination with the UV absorption, explain the colour and pleochroism of the two varieties. The bands in question are assigned to homonuclear metal-to-metal charge transfer transitions: Mn2+(A1) Mn3+(A1′) ? Mn3+(A1) Mn2+(A1′) and Mn2+(A1) Mn3+(A2 ? Mn3+(A1) Mn2+(A2) in PY and Fe2+(A1) Fe3+(A1′) ? Fe3+(A1) Fe2+(A1′) in GY. The evidence for homonuclear Mn2+ Mn3+ charge transfer (CTF) is not quite clear and needs further study. Heteronuclear FeTi CTF does not contribute to the spectra. In PY, additional weak bands were resolved at energies around 17,700, 18,700, 21,000, and 21,900 cm?1 and assigned to Mn3+ in two positions. Weak bands around 10,000 cm?1 in both varieties are assigned to Fe2+ spin-alloweddd-transitions. Very weak and sharp bands, around 15,400, 16,400, 21,300, 22,100, 23,800, and 25,000 cm?1 are identified in GY and assigned to Fe3+ spin-forbiddendd-transitions.  相似文献   

20.
Two synthetic series of spinels, MgCr2O4–Fe2+Cr2O4 and MgCr2O4–MgFe2 3+O4 have been studied by Raman spectroscopy to investigate the effects of Fe2+ and Fe3+ on their structure. In the first case, where Fe2+ substitutes Mg within the tetrahedral site, there is a continuous and monotonic shift of the Raman modes A1g and Eg toward lower wavenumbers with the increase of the chromite component into the spinel, while the F2g modes remain nearly in the same position. In the second series, for low Mg-ferrite content, Fe3+ substitutes for Cr in the octahedral site; when the Mg-ferrite content nears 40 %, a drastic change in the Raman spectra occurs as Fe3+ starts entering the tetrahedral site as well, consequently pushing Mg to occupy the octahedral one. The Raman spectral region between 620 and 700 cm?1 is associated to the octahedral site, where three peaks are present and it is possible to observe the Cr–Fe3+ substitution and the effects of order–disorder in the tetrahedral site. The spectral range at 500–620 cm?1 region shows that there is a shift of modes toward lower values with the increase of the Mg-ferrite content. The peaks in the region at 200–500 cm?1, when observed, show little or negligible Raman shift.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号