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1.
Synthetic pyrope crystals up to 0.5 mm in diameter, substituted by titanium or by titanium plus iron, were grown under defined conditions of P, T, $f_{O_2 }$ in the presence of water using a piston-cylinder device. The crystals were characterized by X-ray and microprobe techniques. Their single-crystal optical absorption spectra were measured by means of a microscope-spectrometer. Two absorption bands at 16100 and 22300 cm{cm-1} in the spectra of pale-blue Fe-free Ti-bearing pyropes, grown under reduced conditions, were identified as originating from spin-allowed transitions, derived from 2 T 2g → 2 E g of octahedral Ti3+ ions. The splitting value of the excited 2E g state, 6200 cm-1, and the crystal field parameter of Ti3+ in pyrope Δ 0 = 19 200 cm-1 are both in agreement with literature data. In spectra of brown Fe, Ti-bearing garnets, a broad band at 23000 cm-1 was interpreted as a Fe2+[8] → Ti4+[6] charge-transfer band. The spectral position and width of this band agree with those observed for a FeTi charge transfer band in natural garnets. Fe, Ti-containing garnets synthesized at relatively high oxygen fugacity (10-11,0 atm), which permits a fraction of Fe3+ to enter the garnet, show an additional Fe2+[8] → Fe3+[6] charge transfer band at 19800 cm-1.  相似文献   

2.
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   

3.
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   

4.
Despite a large number of studies of iron spin state in silicate perovskite at high pressure and high temperature, there is still disagreement regarding the type and P–T conditions of the transition, and whether Fe2+ or Fe3+ or both iron cations are involved. Recently, our group published results of a Mössbauer spectroscopy study of the iron behaviour in (Mg,Fe)(Si,Al)O3 perovskite at pressures up to 110 GPa (McCammon et al. 2008), where we suggested stabilization of the intermediate spin state for 8- to 12-fold coordinated ferrous iron ([8–12]Fe2+) in silicate perovskite above 30 GPa. In order to explore the behaviour in related systems, we performed a comparative Mössbauer spectroscopic study of silicate perovskite (Fe0.12Mg0.88SiO3) and majorite (with two compositions—Fe0.18Mg0.82SiO3 and Fe0.11Mg0.88SiO3) at pressures up to 81 GPa in the temperature range 296–800 K, which was mainly motivated by the fact that the oxygen environment of ferrous iron in majorite is quite similar to that in silicate perovskite. The [8–12]Fe2+ component, dominating the Mössbauer spectra of majorites, shows high quadrupole splitting (QS) values, about 3.6 mm s?1, in the entire studied P–T region (pressures to 58 GPa and 296–800 K). Decrease of the QS of this component with temperature at constant pressure can be described by the Huggins model with the energy splitting between low-energy e g levels of [8–12]Fe2+ equal to 1,500 (50) cm?1 for Fe0.18Mg0.82SiO3 and to 1,680 (70) cm?1 for Fe0.11Mg0.88SiO3. In contrast, for the silicate perovskite dominating Mössbauer component associated with [8–12]Fe2+ suggests the gradual change of the electronic properties. Namely, an additional spectral component with central shift close to that for high-spin [8–12]Fe2+ and QS about 3.7 mm s?1 appeared at ~35 (2) GPa, and the amount of the component increases with both pressure and temperature. The temperature dependence of QS of the component cannot be described in the framework of the Huggins model. Observed differences in the high-pressure high-temperature behaviour of [8–12]Fe2+ in the silicate perovskite and majorite phases provide additional arguments in favour of the gradual high-spin—intermediate-spin crossover in lower mantle perovskite, previously reported by McCammon et al. (2008) and Lin et al. (2008).  相似文献   

5.
Synthetic ringwoodite γ-(Mg1?x Fe x )2SiO4 of 0.4 ≤ x ≤ 1.0 compositions and variously colored micro-grains of natural ringwoodite in shock metamorphism veins of thin sections of two S6-type chondrites were studied by means of microprobe analysis, TEM and optical absorption spectroscopy. Three synthetic samples were studied in addition with Mössbauer spectroscopy. The Mössbauer spectra consist of two doublets caused by VIFe2+ and VIFe3+, with IS and QS parameters close to those established elsewhere (e.g., O’Neill et al. in Am Mineral 78:456–460, 1993). The Fe3+/Fetotal ratio evaluated by curve resolution of the spectra, ranges from 0.04 to 0.1. Optical absorption spectra of all synthetic samples studied are qualitatively very similar as they are directly related to the iron content. They differ mostly in the intensity of the observed absorption features. The spectra consist of a very strong high-energy absorption edge and a series of absorption bands of different width and intensity. The three strongest and broadest absorptions of them are attributed to splitting of electronic spin-allowed 5 T 2g → 5 E g transitions of VIFe2+ and intervalence charge-transfer (IVCT) transition between ferrous and ferric ions in adjacent octahedral sites of the ringwoodite structure. The spin-allowed bands at ca. 8,000 and 11,500 cm?1 weakly depend on temperature, whilst the Fe2+/Fe3+ IVCT band at ~16,400 cm?1 displays very strong temperature dependence: i.e., with increasing temperature it decreases and practically disappears at about 497 K, a behavior typical for bands of this type. With increasing pressure the absorption edge shifts to lower energies while the spin-allowed bands shift to higher energy and strongly decreases in intensity. The IVCT band also strongly weakens and vanishes at about 9 GPa. We assigned this effect to pressure-induced reduction of Fe3+ in ringwoodite. By analogy with synthetic samples three broad bands in spectra of natural (meteoritic) blue ringwoodite are assigned to electronic spin-allowed transitions of VIFe2+ (the bands at ~8,600 and ~12,700 cm?1) and Fe2+/Fe3+ IVCT transition (~18,100 cm?1), respectively. Spectra of colorless ringwoodite of the same composition consist of a single broad band at ca. 12,000 cm?1. It is assumed that such ringwoodite grains are inverse (Fe, Mg)2SiO4-spinels and that the single band is caused by the split spin-allowed 5 E → 5 T 2 transition of IVFe2+. Ringwoodite of intermediate color variations between dark-blue and colorless are assumed to be partly inversed ringwoodite. No glassy material between the grain boundaries in the natural colored ringwoodite aggregates was found in our samples and disprove the cause of the coloration to be due to light scattering effect (Lingemann and Stöffler in Lunar Planet Sci 29(1308), 1998).  相似文献   

6.
The energy levels of MnO 6 9? clusters, with D 4h approximated and C 2v actual symmetry of the M 1 site of Mn3+-bearing andalusite, are calculated using the multiple scattering method. The energies of the electronic d-d transition of Mn3+ in the clusters with D 4h symmetry are calculated to be 6,000–7,000 cm?1 (5 B 1g → 5 A 1g ), ～18,000 cm?1 (5 B 1g → 5 B 2g ) and ～19,000 cm?1 (5 B 1g → 5 E g ). Apart from a splitting of the 5 E g -level into two levels separated by 300–350 cm?1, no significant changes of these transition energies are noted for the corresponding cluster with C 2v symmetry. The calculated transition energies give a good fit to the structure of the optical absorption spectra of Mn3+-bearing andalusites and support recent assignments of the major absorption bands observed in these spectra.  相似文献   

7.
Violet, non-pleochroic and greenish-blue, pleochroic chromium-substituted sapphirines were found in corundum-bearing spinel-websterite xenolites from the Yakutian kimberlite pipes Noyabrskaya (N) and Sludyanka (Sl), respectively. The crystallochemical formulae of sapphirine crystals from such xenolites were determined by EMP to be (Mg3.40Fe0.23Al3.25Cr0.16)[6] Al 1.00 [6] [O2/Al4.53Si1.47O18] (N) and (Mg2.53Fe0.55 Mn0.04Ti 0.03 4+ Al3.55Cr 0.08 3+ )[6]Al 1.00 [16] [O2/Al4.28Si1.73O18] (Sl). Single crystal spectra in the range 35000–6000 cm1- showed a slightly polarization dependent absorption edge near 3200 cm1- (N) or 30000 cm1- (Sl) and unpolarized bands at 25300 and 17300 cm1-, typical of spin-allowed transitions, derived from 4A2g→4T1g and 4A2g→4T2g, of Cr3+ in octahedral sites, with point symmetry C1, of the structure. Another weak band at 23000 cm?1 in the sapphirine-N spectra is attributed to low symmetry splitting of the excited 4T1 (F)-State of Cr3+. These assignments lead to crystal field parameters Dq=1730cm?1 and B= 685cm?1 of Cr3+ in sapphirine. Crystallochemical and spectroscopic arguments suggest that Cr3+ subsitutes for Al in the M(1) or M(8) sites of the sapphirine structure. In addition to Cr3+-transitions, spectra of Sl exhibit weak dd-bands of Fe2+ at 10000 and 7700 cm1-, which are unpolarized in consistency with the C1 site symmetry of the octahedra in the structure. Spectra of Sl show also prominent, broad bands (Δv1/2～-5000 cm1-) at 15000 and 11000 cm1-, which occur in E//Y(//b) and E//Z(//c=12°) only and exhibit an intensity ratio αY∶αz close to 1∶3. This result, the large half width, as well as band energy — MM distance considerations suggest that these bands originate from Fe2+[6]-Fe3+[6] charge-transfer transitions in wall octahedra M(1)M(2), M(6)M(7) etc., forming MM vectors of 30° with the c-axis. The lack of Fe2+-Fe3+ charge-transfer bands in sapphirine N might indicate a lower oxygen fugacity during the formation of the websterite from the Noyabrskaya pipe compared to that from the Sludyanka pipe.  相似文献   

8.
The band positions of three partially overlapping Fe2+ spin-allowed transitions located between 4000 and 9000 cm–1 in almandine-pyrope and almandinespessartine garnets solid solutions were measured using near-infrared (NIR) spectroscopy. The crystal field stabilization energies (CFSE) along both binaries were calculated assuming a splitting of 1100 cm–1 for the lower orbitals. The CFSE show a slight increase along the almandine-spessartine binary from 3730 to 3810 cm–1 and a larger increase from 3730 to 3970 cm–1 for the almandine-pyrope binary. Dodecahedral Fe2+-site distortion increases with an increase in spessartine component and decreases with increasing pyrope component, in agreement with average dodecahedral site distortions determined from diffraction experiments. The excess CFSE's along both joins are negative. For the almandinespessartine binary they are small, but are about 3.5 times larger in magnitude along the join almandine-pyrope, where an interaction parameter of W= -2.9 KJ/mole has been derived from a symmetric mixing model. The excess CFSE are relatively small compared to the magnitudes of the excess enthalpies of mixing that have been assigned to garnet solid solutions. Moreover, they give no indication which could support the positive and asymmetric excess enthalpies of mixing that have been proposed for almandine-pyrope solid solutions.  相似文献   

9.
Polarized optical absorption spectra of Mn(IV) in octahedral crystal fields of Mn(SeO3)2 have been studied by means of microscope-spectrometry in the range 40000-4000 cm?1 and at temperatures between 113 K and 293 K. Intense charge-transfer absorptions (linear absorption coefficient α ? 30000 cm?1) completely mask the d-d transitions in the UV and VIS region above ≈23000 cm?1. The optical electronegativity χ opt of Mn(IV) in Mn(SeO3)2 is estimated to be 2.7. In accordance with the d 3 configuration of tetravalent manganese three d-d bands observed at ambient temperatures at 13250, 14137 (α≈50 cm?1) and ≈18500 cm?1 (α≈500–800 cm?1) are assigned to the spin forbidden 4 A 2g →2 E g and 4 A 2g →2 T 1g transitions as well as to the first spin allowed 4 A 2g →4 T 2g transition, respectively. These assignments allow the calculation of the following ligand field parameters: Dq ≈ 1850 cm?1, B 55 = 869 cm?1 (β 55 = 0.82), and C = 2346 cm?1 (293 K).  相似文献   

10.
The local structural heterogeneity and energetic properties of 22 natural Mg–Fe cordierites, ideal formula (Mg,Fe)2Al4Si5O18·x(H2O,CO2), were investigated at length scales given by powder infrared spectroscopy (IR) and also by published electronic absorption spectra. The studied samples have iron mole fractions from XFe = 0.06 to 0.82 and cover most of the Mg–Fe cordierite binary. Variations in wavenumbers and line widths of the IR bands were determined as a function of composition. Most modes shift linearly to lower wavenumbers with increasing XFe, except those at high wavenumbers located between 900 and 1,200 cm-1. They are vibrations that have a large internal (Si,Al)O4 character and are not greatly affected by Mg–Fe exchange on the octahedral site. The lower wavenumber modes can be best characterized as lattice vibrations having mixed character. The systematics of the wavenumber shifts suggest small continuous variations in the "average" cordierite structure with Mg–Fe exchange and are consistent with an ideal volume of mixing, Vmix= 0, behavior (Boberski and Schreyer 1990). IR line broadening was measured using the autocorrelation function for three wavenumber regions in order to determine the range of structural heterogeneity between roughly 2 and 100 Å (0.2–10.0 nm) in the solid solution. In order to do this, an empirical correction was first made to account for the effect that small amounts of channel Na have on the phonon systematics. The results show that between 1,200 and 540 cm-1 the line widths of the IR bands broaden slightly and linearly with increasing XFe. Between 350 and 125 cm-1 nonlinear behavior was observed and it may be related to dynamic effects. These results suggest minimal excess elastic enthalpies of mixing for Mg–Fe cordierite solid solutions. Channel Na should affect measurably the thermodynamic properties of natural cordierites as evidenced by variations in the IR spectra of Na-containing samples. Occluded H2O (Class I) and CO2 should have little interaction with the framework and can be considered nearly "free" molecules. They should not give rise to measurable structural heterogeneity in the framework. The contribution of the crystal field stabilization energy (CFSE) of octahedral Fe2+ to the energetics of Mg–Fe cordierites was also investigated using published electronic absorption spectra (Khomenko et al. 2001). Two bands are observed between 8,000 and 10,500 cm-1 and they represent electronic dd-excitations of octahedral Fe2+ derived from the 5T2g 5Eg transition. They shift to higher wavenumbers with increasing XMg in cordierite. An analysis gives slightly asymmetric excess -CFSE across the Mg–Fe cordierite join with a maximum of about –550 J/mole towards iron-rich compositions.Editorial responsibility: J. Hoefs  相似文献   

11.
The paper proposes pioneering data on the polarized optical absorption spectra of Li-Fe micas: intermediate members of the siderophyllite-zinnwaldite-polylithionite and annite-protolithionite-zinnwaldite-trilithionite series with variable Fe and Li proportions and Li- and Fe-bearing muscovite (phengite). Based on the analysis of structural data, the complicated structure of the Fe2+ → Fe3+ charge transfer band in the mica structures is explained, and arguments are presented to justify the ascribing of its shortwave component (CTB-1, ν = 17200-14900 cm?1) to charge transfer in the pair Fe2+(M2) → Fe3+(M2) and the longwave component (CTB-2, ν = 14200-13600 cm?1) to charge transfer in the pair Fe2+(M1) Fe3+(M2). It is demonstrated that the anomalous shift of the superposition of two-component CTB toward the shortwave region, to 17000 cm?1, results from a decrease in the length of oxygen edges between adjacent M2M2 and M1M2 tetrahedrons when Li is accommodated in the mica structure. The first data are presented on the spectrum of Fe2+ ions in large distorted M3(M1) tetrahedrons (OAC Fe2+ II) in hetero-octahedral Li micas (zinnwaldite), and the behavior of the corresponding absorption bands at 11400 and 8000 cm?1 is determined. It is proved that characteristics of the optical spectra of Fe2+ ions can be used as an indicator of the structure of the octahedral layer in the mica structures. Results of the comparative analyses of spectral parameters of the Fe2+ → Fe3+ charge transfer, crystal field spectra of Fe2+ ions, and the crystal-chemical characteristics controlling them in micas of the polylithionite-siderophyllite series are completely consistent with the character of cation ordering in the crystal structures of these micas determined by X-ray diffraction analysis.  相似文献   

12.
Natural Fe2+, Fe3+-bearing spinel solid solutions from the spinel s.s.-hercynite and gahnite-hercynite series were analyzed and studied by electronic absorption spectroscopy in the spectral range 30000–3500 cm–1 in the temperature and pressure ranges 77 TK 600 and 10–4 PGPa 11.0. Two crystals were light-violet in color (type I) and six green or bluish-green (type II). The spectra of both types of spinels are dominated by an UV-absorption edge near 28000 to 24000 cm–1, depending on the iron contents, and a very intense band system in the NIR centered around 5000 cm–1, which is caused by spin-allowed dd-transition of tetrahedral Fe2+, derived from 5 E5 T2. The strong band is in all spinels studied, split into four sub-bands, which can only be observed in very thin platelets. Between the UV-edge and the high-energy wing of the NIR-band there occur a number of very weak bands in type I spinels while the green type II spinels show some of these with significantly enhanced intensity. The intensity of the very weak bands is nearly independent from temperature. Such bands are attributed to spin-forbidden electronic transitions of IVFe2+. Temperature and pressure dependence of the intensity enhanced bands of spinels type II indicate that they are caused by IVFe2+ and VIFe3+. They are attributed to spin-forbidden transitions 6A1g4A1g, 4Eg, 4T2g and 4T1g of VIFe3+, the two latter being strongly intensified by exchange-coupling interaction with adjacent IVFe2+. The pressure dependence of IVFe2+ dd-band system in the NIR caused by spin-allowed 5 E5 T2 transition noticeably differs from that of octahedral Fe2+, an effect which is attributed to a dynamic Jahn-Teller effect of IVFe2+ in the spinel structure.
Monika Koch-MüllerEmail: Phone: +49-331-288-1492/1402Fax: +49-331-288-1492/1402
  相似文献   

13.
Blanfordite (I), winchite (II), and juddite (III), all showing vivid colors and pleochroism, from highly oxidized parageneses of Indian gondites were studied by microprobe, Mössbauer, and microscope-spectrophotometric techniques and by X-ray structure refinements. The compositions of the Mn-bearing minerals were close to diopsideacmite (I) and magnesio-arfvedsonite to magnesio-riebeckite (II and III). Transition metal ions are located inM(1)-octahedra (I) or predominantlyM(2)-octahedra (II, III). Mössbauer spectra of57Fe(IS, ΔE Q) are typical of octahedral Fe3+ only. Polarized absorption spectra in the UV/VIS/NIR ranges explain color and pleochroism of the minerals. The position of the UV-“edge” is correlated with Fe3+-contents of the minerals, except for judditeE ∥Z, where the edge shows an unusual low energy position. This is most likely due to Mie-scattering of submicroscopic inclusions of braunite with nearly uniform dimensions. In the VIS range, the spectra are dominated by a complex band system between 15,000 and 20,000 cm?1. Energies and ?-values of component bands are compatible with those of Mn3+ d-d transitions in other Mn3+-bearing silicates. The polarization behavior of component bands can best be explained by aC 2(C2″) symmetry of the crystal field. The Jahn-Teller splitting (<9,000 cm?1) of the5 E g ground state of Mn3+ inO h crystal fields is appreciably smaller than in other Mn3+-silicates. Crystal field parameters 10Dq, (I) 13,650, (II) ca. 11,640, and (III) 11,925 cm?1, are near to that in piemontite. The crystal field stabilization energy of Mn3+, (I) 146, (II) ca. 140, (III) 142 \({{{\text{kJ}}} \mathord{\left/ {\vphantom {{{\text{kJ}}} {\text{g}}}} \right. \kern-0em} {\text{g}}}{\text{ - atom}}_{{\text{Mn}}^{{\text{3 + }}} } \) , is appreciably smaller than that found in other Mn3+-silicates (piemontites and manganian andalusites, viridines and kanonaite).  相似文献   

14.
A crystallographic and m?ssbauer spectroscopy study of Fe   总被引：1，自引：0，他引：1  
The crystal chemistry of garnet solid solutions on the Fe 3 2+ Al2Si3O12-Fe 3 2+ Fe 2 3+ Si3O12 (almandine-“skiagite”) and Ca3Fe 2 3+ Si3O12-Fe 3 2+ Fe 2 3+ Si3O12 (andradite-“skiagite”) joins have been investigated by single-crystal X-ray structure refinements and M?ssbauer spectroscopy. Together, these two solid solution series encompass the complete range in Fe3+/ΣFe from 0.0 to 1.0. All garnets are isotropic and were re0fined in the Ia d space group. Small excess volumes of mixing are observed in andradite-“skiagite” solid solutions (W v =1.0±0.2 cm3 mol-1) and along the almandine-“skiagite” join (W v =-0.77±0.17cm3 mol-1). The octahedral (Al, Fe3+)-O bond lengths show a much greater variation across the almandine-skiagite join compared to the andradite-skiagite garnets. The dodecahedral (X)-O bond lengths show the opposite behaviour. In andradite-“skiagite” solid solutions, the octahedral site passes from being flattened to elongated parallel to the 3 axis of symmetry with increasing “skiagite” content. A perfect octahedron occurs in a composition of ≈35 mol% “skiagite”. The occupancy of the neighboring dodecahedral sites has the greatest effect on octahedral distortion and vice versa. The M?ssbauer hyperfine parameters of Fe2+remain constant in both solid solutions. The hyperfine parameters of Fe3+ (at room temperature: centre shift=0.32–0.40 mm/sec, quadrupole splitting (QS)≈0.21–0.55 mm/ sec) indicate that all Fe3+ is in octahedral coordination. The Fe3+ parameters are nearly constant in almandine-“skiagite” solid solutions, but vary significantly across the andradite-“skiagite” join. The structural unit that contributes to the electric field gradient of the octahedral site is different from that of the coordinating oxygen polyhedron, probably involving the neighboring dodeca-hedral sites.  相似文献   

15.
Room temperature and low temperature Mössbauer and optical absorption spectroscopic data on six natural chloritoids characterized by means of electron microprobe and X-ray powder diffraction techniques are presented. Two narrow quadrupole doublets with widths of 0.25–0.29 mm/s assigned to Fe2+ in a relatively large octahedral site and Fe3+ in a smaller octahedral site, are observed in the Mössbauer spectra. Polarized optical absorption spectra reveal three main absorption bands. A broad absorption band at 16,300 cm?1, which is strongly polarized in E‖X and E‖Y and shows a linear increase in integral absorption with increasing [Fe2+] [Fe3+] concentration product, is assigned to a Fe2++Fe3+→Fe3++Fe2+ charge transfer transition. This band displays also a temperature dependence different from that of single ion d?d transitions. Two absorption bands at 10,900 cm?1 and 8,000 cm?1 are, on the basis of compositional dependence and energy, assigned to Fe2+ in the large M(1B) octahedra of the brucite-type layer in chloritoid. Combined spectroscopic evidence and structural and chemical considerations support a distribution scheme for ferrous and ferric iron which orders the Fe2+ ions in the M(1B) octahedra and the Fe3+ ions in the small M(1A) octahedral sites. Both types of octahedra are found in the brucite type layer of chloritoid.  相似文献   

16.
X-ray absorption spectroscopy, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques, have been used to determine the structure and speciation of complexes for Fe2+ and Fe3+ chloride solutions at a variety of pH's, ionic strengths, and chloride/iron ratios.Low intensity K-edge transition features and analysis of modified pair correlation functions, derived from Fourier transformation of EXAFS spectra, show a regular octahedral coordination of Fe(II) by water molecules with a first-shell Fe2+-O bond distance, closely matching octahedral Fe2+-O bonds obtained from solid oxide model compounds. Solution Fe2+-O bond distances decrease with chloride/iron ratio, pH, and total FeCl2 concentration. A slight intensification of the 1s → 3d transition with increasing FeCl2 concentration suggests that chloride may begin to mix with water as a nearest-neighbor octahedral ligand. Fe3+ solutions show a pronounced increase in the 1s → 3d transition intensities between 1.0 M FeCl3/7.8 M Cl? to 1.0 M FeCl3/ 15 M Cl?, indicating a coordination change from octahedral to tetrahedral complexes. EXAFS analyses of these solutions show an increase in first-shell Fe3+-ligand distances despite this apparent reduction in coordination number. This can be best explained by a change from regular octahedral complexes of ferric iron (either Fe(H2O)63+ or trans-Fe(H2O)4Cl2 or both; Fe3+-O bond distances of 2.10 Å) to tetra-chloro complexes [Fe3+-Cl bond distances of 2.25 Å].  相似文献   

17.
The Mössbauer spectra of several blue beryls have been obtained in the temperature range of 4.2–500 K. A common feature observed in all room-temperature spectra is the presence of an asymmetric Fe2+ doublet (ΔE Q ?～?2.7?mm?s?1, δ?～?1.1?mm?s?1), with a very broad low-velocity peak. This asymmetry seems to be related to a relaxation process involving ferrous ions and water molecules in the structural channels, as suggested by Price et?al. (1976). Surprisingly, the spectrum at 500?K also shows a broad, but symmetrical, doublet, with a clear splitting of the lines indicating the presence of at least two Fe2+ components. The room-temperature spectrum obtained after the 500?K run shows the same features as prior to the heating. At 4.2?K the spectrum of a deep blue beryl was well fitted with four symmetrical doublets, one of which could be related to Fe2+ in the structural channels. Ferrous ion was also found to occupy the octahedral and tetrahedral sites, whereas ferric ion is most probably located in the octahedral site. A meaningful fit of the room-temperature spectrum, as well as an explanation for the temperature dependence of the Mössbauer spectra, are discussed. Finally, it is believed that the color in beryl will be dictated by the relative proportions of Fe3+ in the octahedral sites and of Fe2+ in the channels.  相似文献   

18.
A pyralspite garnet from an anomalously magnetic concentrate of a pegmatitic cassiterite ore has been investigated using 57Fe nuclear gamma-ray resonance spectroscopy. The quadrupole splitting and isomer shift values of 3.6 mm/s and 1.4 mm/s, respectively, are among the largest observed for Fe2+ ions and indicate a very low covalency of the dodecahedral Fe2+ — O2-bonds. These data support the more recent and lower value (10.2–10.1 kcal/ mole) of White and Moore (1972) for the CFSE of the dodecahedral Fe2+ ion and suggest that the CFSE should be a useful approximation to the site preference energy of Fe2+ for this site.  相似文献   

19.
The electron paramagnetic resonance (EPR) spectra of Fe3+ in a well cristallized kaolinite from Decazeville in France are well resolved. It is shown that in this sample there are mainly two slightly different spectra, well separated at low temperature and characterized at -150° C by the constants B 2 0 = 0.112 cm?1, B 2 2 = 0.0688 cm?1 for one and B 2 0 = 0.116 cm?1, B 2 2 = 0.0766 cm?1 for the second. These two spectra arise from Fe3+ substituted for Al3+ at the two octahedral positions in equal amounts. The temperature dependence of EPR spectra was studied and was explained by a modification of the octahedral sites.  相似文献   

20.
A new single beam microtechnique has been developed for measuring the polarized absorption spectra in the region 44,000-4,000 cm?1. Spectra of a natural garnet (Spess70Alm30), measured by the microtechnique and by conventional macrotechniques, are consistent and thus prove the applicability of the microtechnique described. It is possible to obtain well resolved spectra down to about 13,000 cm?1 with crystals as small as about 10 μm. Thus spectra of crystals obtained in routine high-pressure high-temperature silicate syntheses can be measured. The polarized spectra of Mn3+, Fe3+, Fe2+, and Cr3+ in the following synthetic silicate minerals are presented: piemontite (I), acmite (II), orthoferrosilite (III), and kyanite (IV) or uvarovite (V), respectively. O-Cr3+, O-Mn3+, and O-Fe2+ charge transfer band maxima in the UV region are identified at 38,700 cm?1, in V; at 33,200, 35,300, and 39,000 cm?1, in I; and at 32,800, 35,200, and 37,300 cm?1, in III, respectively. Bands in the region ≦25,000 cm?1 are assigned to spin-allowed and spin-forbidden dd transitions as predicted from crystal field theoretical considerations for the foregoing ions in the respective structures.  相似文献   

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