Identifying characteristics of charge transfer transitions in minerals     

Stephanie M. Mattson  George R. Rossman 《Physics and Chemistry of Minerals》1987,14(1):94-99

Criteria used to identify Fe²⁺-Fe³⁺ and Fe²⁺-Ti⁴⁺ intervalence charge transfer absorption bands in electronic spectra are reviewed and compared to the characteristics of unperturbed Fe²⁺ crystal field bands and those that are intensified by interaction with Fe³⁺. Band energy is the least definitive diagnostic criterion. Changes in band intensity with temperature are also of limited value. Large widths are the most reliable characteristic of charge transfer bands. New optical absorption spectra are presented for euclase, as well as 80 K spectra of rockbridgeite, babingtonite and lazulite. Comparison of optical spectra to magnetic susceptibility measurements for rockbridgeite and babingtonite provides support for recent theories regarding the effect of magnetic coupling on the variation of charge transfer intensity with temperature.  相似文献   

Molecular orbital (SCF-Xα-SW) theory of metal-metal charge transfer processes in minerals     

David M. Sherman 《Physics and Chemistry of Minerals》1987,14(4):355-363

A number of mixed valence iron oxides and silicates (e.g., magnetite, ilvaite) exhibit thermally induced electron delocalization between adjacent Fe²⁺ and Fe³⁺ ions and optically induced electronic transitions which are assigned to Fe²⁺→Fe³⁺ intervalence charge transfer. In this paper, the mechanism of electron delocalization (i.e., polarons versus itinerant electrons) and the nature of optically induced intervalence charge-transfer in minerals are investigated using molecular orbital theory. SCF-Xα-SW molecular orbital calculations were done for several mixed-valence (Fe₂O₁₀)^15? clusters corresponding to edgesharing Fe²⁺ and Fe³⁺ coordination polyhedra. A spinunrestricted formalism was used so that the effect of ferromagnetic versus antiferromagnetic coupling of adjacent Fe²⁺ and Fe³⁺ cations could be determined. The molecular orbital results can be related to the polaron theory of solid state physics and the perturbation theory formalism used by Robin and Day (1967) and others to describe electron transfer in mixed valence compounds. Intervalence charge-transfer results from the overlap of Fe(3d) orbitals across the shared edges of adjacent FeO₆ polyhedra to give weak Fe-Fe bonds. Electron delocalization, however, requires that adjacent Fe cations be ferromagnetically coupled. Antiferromagnetic coupling results in distinguishable Fe²⁺ and Fe³⁺ cations. Electronic transitions between the Fe-Fe bonding and Fe-Fe antibonding orbitals results in the optically-induced intervalence charge transfer bands observed in the electronic spectra of mixed valence minerals. Such transitions are predicted to be polarized along the metal-metal bond direction, in agreement with experimental observations.  相似文献   

A reassignment of the optical absorption bands in biotites     

W. Kliem  G. Lehmann 《Physics and Chemistry of Minerals》1979,4(1):65-75

The electronic absorption spectra of three biotites with largely differing Fe²⁺/Fe³⁺ ratios were studied before and after thermal dehydration and oxidation of divalent iron. Three absorption bands near 17,100, 20,500 and 24,100 cm^?1 and an absorption edge at slightly higher energies are assigned to trivalent iron present in clusters of strongly interacting ions. The presence of additional broad absorption bands due to intervalence transfer between Fe²⁺ and Fe³⁺ or Ti⁴⁺ in this region cannot be excluded for biotites with high Fe²⁺ concentrations. Three bands at lower energies show a satisfactory correlation with concentration of divalent iron and decrease in the same proportions with oxidation. We therefore assign them to split components of the spin-allowed ligand field transition of Fe²⁺ at the M ₁ and M ₂ sites. This contradicts the assignment of one of these bands to an intervalence charge transfer between Fe²⁺ and Fe³⁺ by previous authors. It is shown that there is no indisputable evidence against our assignment.  相似文献   

Ilvaite: A study of temperature dependent electron delocalization by the mössbauer effect     

D. A. Nolet  R. G. Burns 《Physics and Chemistry of Minerals》1979,4(3):221-234

The mixed valence iron silicate ilvaite, CaFe ₂ ²⁺ Fe³⁺Si₂O₈(OH), displays electron delocalization associated with Fe²⁺→Fe³⁺ charge transfer as observed by Mössbauer spectroscopy. Previous studies report the observation of an ‘electron hopping phenomenon’ with resolution of discrete valence states below 320 K. Mössbauer spectra of a suite of naturally occurring ilvaites were recorded over a temperature range, 80 K to 575 K. Five quadrupole doublets were resolved by computer fitting and assigned to Fe²⁺(A), Fe²⁺(B), Fe³⁺(A), and Fe²⁺(A)→Fe³⁺(A)‖c and ⊥c. Contrary to prior work, doublets associated with electron delocalization are resolved at 80 K and preclude the use of a Verwey-type order-disorder model. We propose a thermal activation model and discuss its criteria from molecular orbital and mineralogical viewpoints.  相似文献   

Fe2+-Fe3+ interactions in tourmaline     

Stephanie M. Mattson  George R. Rossman 《Physics and Chemistry of Minerals》1987,14(2):163-171

The color and spectroscopic properties of ironbearing tourmalines (elbaite, dravite, uvite, schorl) do not vary smoothly with iron concentration. Such behavior has often been ascribed to intervalence charge transfer between Fe²⁺ and Fe³⁺ which produces a new, intense absorption band in the visible portion of the spectrum. In the case of tourmaline, an entirely different manifestation of the interaction between Fe²⁺ and Fe³⁺ occurs in which the Fe²⁺ bands are intensified without an intense, new absorption band. At low iron concentrations, the intensity of light absorption from Fe²⁺ is about the same for E∥c and E⊥c polarizations, but at high iron concentrations, the intensity of the E⊥c polarization increases more than ten times as much as E∥c. This difference is related to intensification of Fe²⁺ absorption by adjacent Fe³⁺. Extrapolations indicate that pairs of Fe²⁺-Fe³⁺ have Fe²⁺ absorption intensity ～200 times as great as isolated Fe²⁺. Enhanced Fe²⁺ absorption bands are recognized in tourmaline by their intensity increase at 78 K of up to 50%. Enhancement of Fe²⁺ absorption intensity provides a severe limitration on the accuracy of determinations of Fe²⁺ concentration and site occupancy by optical spectroscopic methods. Details of the assignment of tourmaline spectra in the optical region are reconsidered.  相似文献   

Fe2+-Ti4+ charge transfer in stoichiometric Fe2+,Ti4+-minerals     

Stephanie M. Mattson  George R. Rossman 《Physics and Chemistry of Minerals》1988,16(1):78-82

Optical absorption spectra are presented for taramellite, traskite and neptunite, all of which have both Fe²⁺ and Ti⁴⁺ as major elements. The spectra of each of these minerals are dominated by a single, intense absorption band in the 415 to 460 nm region with 7000 to 9000 cm^?1 halfwidth. These transitions, assigned to Fe²⁺-Ti⁴⁺ intervalence charge transfer, showed little difference in intensity at 80 and 300 K and have molar absorptivities which range from ～100 to ～1300 M^?1 cm^?1. The Fe²⁺-Ti⁴⁺ absorptions in these standards generally compare well to other mineral spectra in which Fe²⁺ — Ti⁴⁺ intervalence absorption has previously been proposed with the exception of the most cited example, blue corundum.  相似文献   

Polarized single crystal absorption spectroscopy of the Pnam-P2 1/a transition of Ilvaite Ca(Fe2+, Fe3+)Fe2+Si2O8(OH) as measured between 300 K and 450 K     

B. Güttler  E. Salje  S. Ghose 《Physics and Chemistry of Minerals》1989,16(6):606-613

The temperature dependence of the absorption spectra of ilvaite, Ca(Fe²⁺,Fe³⁺)Fe²⁺Si₂O₈(OH), shows strongly one dimensional transport behaviour with no singularity at the Pnam-P2₁/a phase transition point near 335 K. Polarized single crystal transmission measurements were carried out between 300 K and 450 K in a frequency range between 600 and 23 000 cm⁻¹. No Drude —absorption at low energies was found at any temperature. A macroscopic, thermodynamic model based on Landau-Ginzburg theory is given which accounts for the observed macroscopic properties of the structural phase transition and its coupling with the Fe²⁺-Fe³⁺ ordering. This ordering scheme is discussed on an atomistic level and compared with the behaviour of magnetite and trans-(CH) _x .  相似文献   

Thermally activated electron delocalization in deerite     

G. Amthauer  K. Langer  M. Schliestedt 《Physics and Chemistry of Minerals》1980,6(1):19-30

The ⁵⁷Fe Mössbauer spectra of deerites of different chemical composition, taken at several temperatures, show that Fe²⁺ and Fe³⁺ occupy all the six-coordinated lattice sites with a preference of Fe³⁺ probably for the M(1) to M(3) positions, and a preference of Fe²⁺ probably for the M(4) to M(6) and the M(7) to M(9) sites. The room and high temperature spectra reveal absorption patterns due to thermally activated Fe²⁺ → Fe³⁺ electron delocalization. The extent of electron delocalization is dependent on the chemical composition, e.g., the amount of ions (Mg, Mn, Al) substituting for Fe.  相似文献   

Channel constituents in beryl     

S. Don Goldman  George R. Rossman  Kathleen M. Parkin 《Physics and Chemistry of Minerals》1978,3(3):225-235

The electronic absorption spectra of Fe²⁺ in non-chromium beryls are examined. Fe²⁺ in the Al-rich six-coordinate site produces absorption bands at about 820 nm and 970 nm polarizedE‖c. Fe²⁺ in the channel produces bands at 820 nm (⊥c) and 2100 nm (‖c). Some blue beryls which are more intensely colored than most aquamarines, have an absorption band at ～700 nm (‖c) which is suggested to arise from an Fe²⁺/Fe³⁺ intervalence interaction. Fe²⁺ in both the six-coordinate site and the channel is identified in the Mössbauer spectra. The Mössbauer spectra of deep blue beryls are unusual and have not been satisfactorily explained. Color changes which accompany heating and irradiation are strongly influenced by the channel iron.  相似文献   

Electron delocalization in ilvaite,a reinterpretation of its 57Fe Mössbauer spectrum     

F. J. Litterst  G. Amthauer 《Physics and Chemistry of Minerals》1984,10(6):250-255

The Mössbauer spectra of ilvaite CaFe ₂ ²⁺ Fe³⁺[Si₂O₇/O/OH] and their temperature dependence between 298 K and 455 K can be satisfactorily least-squares fitted by a superposition of the resonances for Fe²⁺(8d), Fe³⁺(8d) and Fe²⁺(4c). The relative areas under the three resonances are nearly equal and vary only weakly with temperature. No additional resonances or line broadenings have to be introduced, if we assume that the hyperfine interactions of Fe²⁺(8d) and Fe³⁺(8d) fluctuate between their values due to electron hopping between the iron ions at the 8d sites. Hopping can be assumed to occur homogeneously among nearly equivalent sites. The fluctuation rate is described by an Arrhenius law with a pre-exponent of about 9 × 10⁸ s^?1 and an activation energy of 0.11 eV indicating non-adiabatic hopping. In addition to the intersite hopping process, the strong decrease of the quadrupole splitting and the isomer shift of Fe²⁺(8d) between 298 K and 360 K suggests the occurrence of intrinsic charge delocalization from Fe²⁺(8d) which does not involve the neighbouring Fe³⁺(8d) ions.  相似文献   

Influence of cation distribution on the optical absorption spectra of Fe3+-bearing spinel s.s.–hercynite crystals: evidence for electron transitions in VIFe2+–VIFe3+ clusters     

U. Hålenius  H. Skogby  G. B. Andreozzi 《Physics and Chemistry of Minerals》2002,29(5):319-330

Flux-grown Fe³⁺-bearing spinel s.s.–hercynite solid-solution crystals, (Mg _1-y Fe²⁺ _y )Al₂O₄ (0 < y≤ 1), have been investigated by means of electron microprobe technique and Mössbauer and electronic spectroscopy. Obtained results show that different electronic processes cause intense optical absorption bands in the near-infrared spectral region. In addition to an electronic d–d transition in single-ion ^IVFe²⁺, observed at 5200 cm^?1, intense and broad bands at 9500 and 14 500 cm^?1 are assigned to exchange-coupled pair (ECP) and intervalence charge-transfer (IVCT) transitions in ^VI Fe ^{2+ VI} Fe³⁺clusters, respectively. The net linear extinction coefficients of these bands (α) were calibrated against Fe²⁺ and Fe³⁺ concentrations and site distributions previously defined by combined microchemical, Mössbauer, and XRD structural refinement data. The following expressions were obtained: where α is measured in cm^?1 and concentrations are expressed in mol?l^?1. The present results show that optical absorption spectroscopy may be used as a probe to obtain high spatial resolution (?～ 10 μm) information on Fe²⁺ ordering as well as on Fe³⁺ concentrations in minerals belonging to the spinel group.  相似文献   

Optical absorption spectra and Fe distribution in the structures of Li-Fe micas     

A. N. Platonov  V. M. Khomenko  T. N. Shuriga 《Geochemistry International》2009,47(2):174-185

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   

The structural behavior of ferric and ferrous iron in aluminosilicate glass near meta-aluminosilicate joins     

Bjorn O. Mysen 《Geochimica et cosmochimica acta》2006,70(9):2337-2353

Iron-57 resonant absorption Mössbauer spectroscopy was used to describe the redox relations and structural roles of Fe³⁺ and Fe²⁺ in meta-aluminosilicate glasses. Melts were formed at 1500 °C in equilibrium with air and quenched to glass in liquid H₂O with quenching rates exceeding 200 °C/s. The aluminosilicate compositions were NaAlSi₂O₆, Ca_0.5AlSi₂O₆, and Mg_0.5AlSi₂O₆. Iron oxide was added in the form of Fe₂O₃, NaFeO₂, CaFe₂O₄, and MgFe₂O₄ with total iron oxide content in the range ∼0.9 to ∼5.6 mol% as Fe₂O₃. The Mössbauer spectra, which were deconvoluted by assuming Gaussian distributions of the hyperfine field, are consistent with one absorption doublet of Fe²⁺ and one of Fe³⁺. From the area ratios of the Fe²⁺ and Fe³⁺ absorption doublets, with corrections for differences in recoil-fractions of Fe³⁺ and Fe²⁺, the Fe³⁺/ΣFe is positively correlated with increasing total iron content and with decreasing ionization potential of the alkali and alkaline earth cation. There is a distribution of hyperfine parameters from the Mössbauer spectra of these glasses. The maximum in the isomer shift distribution function of Fe³⁺, δ_Fe³⁺, ranges from about 0.25 to 0.49 mm/s (at 298 K relative to Fe metal) with the quadrupole splitting maximum, Δ_Fe³⁺, ranging from ∼1.2 to ∼1.6 mm/s. Both δ_Fe³⁺ and δ_Fe²⁺ are negatively correlated with total iron oxide content and Fe³⁺/ΣFe. The dominant oxygen coordination number Fe³⁺ changes from 4 to 6 with decreasing Fe³⁺/ΣFe. The distortion of the Fe³⁺-O polyhedra of the quenched melts (glasses) decreases as the Fe³⁺/ΣFe increases. These polyhedra do, however, coexist with lesser proportions of polyhedra with different oxygen coordination numbers. The δ_Fe²⁺ and Δ_Fe²⁺ distribution maxima at 298 K range from ∼0.95 to 1.15 mm/s and 1.9 to 2.0 mm/s, respectively, and decrease with increasing Fe³⁺/ΣFe. We suggest that these hyperfine parameter values for the most part are more consistent with Fe²⁺ in a range of coordination states from 4- to 6-fold. The lower δ_Fe²⁺-values for the most oxidized melts are consistent with a larger proportion of Fe²⁺ in 4-fold coordination compared with more reduced glasses and melts.  相似文献   

Crystallographic,Mössbauer and electrokinetic study of synthetic lipscombite     

R. Vochten  E. De Grave 《Physics and Chemistry of Minerals》1981,7(5):197-203

The transformation of vivianite and the direct synthesis starting from pure chemicals lead to the formation of lipscombite {Fe _x ²⁺ Fe _3?x ³⁺ [(OH)_3?x/(PO₄)₂]} with varying Fe²⁺/Fe³⁺ molar ratios. The influence of this ratio on the Mössbauer spectra, solubility, electrokinetic potential and infrared spectra has been studied. By means of Mössbauer spectroscopy, the distribution of the Fe²⁺ and Fe³⁺ ions between the octahedral sites I and II has been investigated. The unit cell dimensions have been determined from Guinier-Hägg X-ray diffraction patterns. The crystal system is tetragonal for synthetic lipscombite with a=5.3020±0.0005 Å and c=12.8800±0.0005 Å. Lipscombite has been found to show a negative and time-dependent zeta-potential which, moreover, is influenced by the pH of the suspension and the Fe²⁺/Fe³⁺ molar ratio. An explanation of the time-dependence of the zeta-potential on variations of solubility is proposed. Infrared absorption spectrum only is characterized by two absorption bands: v _OH(3,500 cm^?1) and v _P?O(1,100-960 cm^?1). The density at 25° C is determined in toluene as 3.36±0.01 g·cm^?3.  相似文献   

On the origin of natural sapphire color     

L. V. Nikolskaya  V. M. Terekhova  M. I. Samoilovich 《Physics and Chemistry of Minerals》1978,3(3):213-224

An attempt has been made to interpret natural sapphire optical absorption spectra with regard to the exchange-coupled pairs of iron impurities in their ferric and ferrous states. Level schemes have been calculated for Fe³⁺-Fe³⁺ and Fe²⁺-Fe²⁺ pairs of all the orders, their symmetry being observed. The selection rules are discussed. EPR and infrared spectroscopy information has been used to assist the optical spectroscopy data. The sapphire color has been interpreted as a function of Fe²⁺/Fe³⁺ ratios and of the presence of their pairs in the stone. The various types of charge compensation in Fe²⁺→Al³⁺ isomorphism are discussed.  相似文献   

Synthetic and natural chromium-bearing spinels: an optical spectroscopy study     

M. N. Taran  F. Parisi  D. Lenaz  A. A. Vishnevskyy 《Physics and Chemistry of Minerals》2014,41(8):593-602

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 O^2? → 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   

Crystal chemical properties of synthetic lazulite–scorzalite solid-solution series     

P. Schmid-Beurmann  St. Knitter  L. Cemič 《Physics and Chemistry of Minerals》1999,26(6):496-505

Members of the lazulite–scorzalite (MgAl₂- (PO₄)₂(OH)₂-FeAl₂(PO₄)₂(OH)₂) solid-solution series were synthesized in compositional steps of 12.5?mol% at T?=?485?°C and P?=?0.3?GPa under hydrothermal conditions and controlled oxygen fugacities of the Ni/NiO-buffer. X-ray powder diffraction and ⁵⁷Fe-Mössbauer studies show that under these conditions a complete solid-solution series is formed which is characterized by the substitution of Mg²⁺ and Fe²⁺ on the octahedral Me ²⁺ site. The ⁵⁷Fe-Mössbauer spectra which reveal the presence of both ferrous and ferric iron and the compositional data were interpreted in terms of a defect model with a distribution of the ferric ions over both the Me ²⁺ and the Al³⁺ positions and vacancies on the Me ²⁺ site. The ⁵⁷Fe-Mössbauer parameters of the synthetic compounds correspond to those of natural lazulites except for the total absorption ratio of the ferric iron A(Fe³⁺)/(A(Fe³⁺)+A(Fe²⁺)), which is significantly higher in natural lazulites of the same composition. The total absorption ratio of the ferric iron increases from 4% in pure scorzalite to 15% in a Mg-rich solid-solution with x _Fe ?=?12(1)%  相似文献   

Spectroscopic studies on natural chloritoids     

Ulf Hålenius  Hans Annersten  Klaus Langer 《Physics and Chemistry of Minerals》1981,7(3):117-123

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   

Structural variation of babingtonite depending on cation distribution at the octahedral sites     

Mariko Nagashima  Keiko Mitani  Masahide Akasaka 《Mineralogy and Petrology》2014,108(2):287-301

Babingtonite, Ca₂Fe²⁺Fe³⁺[Si₅O₁₄(OH)] (Z?=?2, space group $ P\overline{1} $ ) from Yakuki mine (Japan), Grönsjöberget (Sweden), Kandivali Quarry (India), Baveno Quarry (Italy), Bråstad Mine (Norway), and Kouragahana (Japan), and manganbabingtonite, Ca₂(Mn²⁺, Fe²⁺)Fe³⁺[Si₅O₁₄(OH)], from Iron Cap mine (USA) were studied using electron-microprobe analysis (EMPA), ⁵⁷Fe Mössbauer analysis and single-crystal X-ray diffraction methods to determine the cation distribution at M1 and M2 and to analyze its effect on the crystal structure of babingtonite. Although all studied babingtonite crystals are relatively homogeneous, chemical zonation due to mainly Fe ? Mn substitution is observed in manganbabingtonite. Mössbauer spectra consist of two doublets with isomer shift (I.S.)?=?1.16–1.22 mm/s and quadrupole splitting (Q.S.)?=?2.33–2.50 mm/s and with I.S.?=?0.38–0.42 mm/s and Q.S.?=?0.82–0.90 mm/s, assigned to Fe²⁺ and Fe³⁺ at the M1 and M2 octahedral sites, respectively. The determined ratio of Fe²⁺/total Fe in manganbabingtonite (0.26) was smaller than that in the others (0.35–0.44) because of high Mn²⁺ content instead of Fe²⁺. The unit-cell parameters of babingtonite are a?=?7.466–7.478, b?=?11.624–11.642, c?=?6.681–6.690 Å, α?=?91.53–91.59, β?=?93.86–93.94, γ?=?104.20–104.34º, and V?=?560.2–562.3 Å³, and those of manganbabingtonite are a?=?7.4967(3), b?=?11.6632(4), c?=?6.7014(2) Å, α?=?91.602(2), β?=?93.989(2), γ?=?104.574(3)º, and V =565.09(5) ų. Structural refinements converged to R ₁ values of 1.64–3.16 %. The <M1-O> distance was lengthened due to the substitution of large octahedral cations such as Mn²⁺ for Fe²⁺. The increase of the M1-O8, M1-O8’ and M1-O13 lengths with mean ionic radii is slightly more pronounced than of the other M1-Oi lengths. The lengthened M1-O13 distance leads the positive correlation between Si5-O15-Si1 angle and M1-O13 distance. The increase of Si2-O3-Si1 and Si5-O12-Si4 angles due to the increase of mean ionic radius of M2 is also observed.  相似文献   

Structure and physical property relations of Mn ilvaite     

E. Schmidbauer  Th. Fehr  R. Hochleitner  J. Schneider 《Physics and Chemistry of Minerals》2005,32(5-6):388-399

Structural and compositional data as well as ⁵⁷Fe Mössbauer parameters were determined on a natural Mn-rich monoclinic ilvaite crystal (ideal composition CaFe ₂ ²⁺ Fe³⁺Si₂O₈(OH)) which was used for electrical conductivity and thermopower measurements (part 2 of this paper). A zonar structure was found by electron microprobe analysis with a strong decrease in Mn concentration from the rim to the centre of the crystal in a plane perpendicular to the [001] direction. X-ray powder diffraction analysis of the most Mn-rich composition was performed. Mn²⁺ cations populate preferentially M2 sites of the ilvaite unit cell (space group P21/a), to a lower extent they reside on M1 and a reduced part is on Ca sites. The monoclinic angle was determined to β=90.178(4)°. The structural results are compared to literature data for other natural Mn-rich as well as low-impurity ilvaites; this concerns in particular the lattice b parameter and the undecided issue of the varying β angle. In the literature, the order parameter σ, which describes the varying degree of ordering of Fe²⁺–Fe³⁺ pairs on M11 and M12 sites in chains running parallel to the [001] direction, and structural defects are thought to be related to β. The interrelationship between β and σ with respect to a possible twin domain structure is discussed. Various ⁵⁷Fe Mössbauer spectra were recorded between 151 K and 327 K. Mössbauer parameters and Fe²⁺/Fe³⁺ concentration ratios were determined from the fits to the spectra. Fitting of subspectra was accomplished with the idea to find assignments of Fe²⁺ and Fe³⁺ doublets in agreement with X-ray results. The fraction of Mn²⁺ substituting Fe²⁺ on M1 sites could be estimated.  相似文献