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

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.     

Fe2+–Fe3+ ordered distribution in chromite spinels

Mossbauer measurements have been carried out on three natural chromite minerals from different locations in China over the temperature range 50 to 750 K. The experiments showed these samples to be magnesioferrochromites. The Mossbauer spectra measured could be decomposed into three doublets: two attributed to the tetrahedral T-site Fe²⁺ ions and the third to the octahedral M-site Fe³⁺ ions. Thus for the chromite spinels the results strongly supported the ordered distribution with Fe²⁺ in the T-site and Fe³⁺ in M-site.     

Fe2+-Fe3+ interactions in tourmaline

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.     

The role of Fe2+ and Fe3+ in synthetic Fe-substituted tetrahedrite

Summary Tetrahedrites with the composition between Cu₁₂Sb₄S₁₃ and Cu₁₀Fe₂Sb₄S₁₃ were synthesized at 457 °C and 500 °C from the elements and carefully studied by Mössbauer spectroscopy of⁵⁷Fe. Between Cu₁₂Sb₄S₁₃ and Cu₁₁Fe₁Sb₄S₁₃ iron is predominantly ferric. Between Cu₁₁Fe₁Sb₄S₁₃ and Cu₁₀Fe₂Sb₄S₁₃ iron is predominantly ferrous and occupies the tetrahedral M1-sites.

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Zusammenfassung Die Rolle von Fe²⁺ und Fe³⁺ in synthetischen Tetraedriten mit Fe-Substitution Tetraedrite mit einer Zusammensetzung zwischen Cu₁₂Sb₄S₁₃ and Cu₁₀Fe₂Sb₄S₁₃ wurden bei 457 °C und 500 °C aus den Elementen synthetisiert und sorgfdltig mit Mössbauer-Spektroskopie von⁵⁷Fe untersucht. Zwischen Cu₁₂Sb₄S₁₃ and Cu₁₁Fe₁Sb₄S₁₃ ist Eisen überwiegend dreiwertig. Zwischen Cu₁₁Fe₁Sb₄S₁₃ and Cu₁₁Fe₂Sb₄S₁₃ ist Eisen überwiegend zweiwertig und besetzt die tetraedrisch koordinierten M1-Plätze.

     

Ferromagnetic or antiferromagnetic Fe III spin configurations in sheet silicates

Mössbauer spectra of glauconite and nontronite recorded at temperatures down to 1.3K and in applied fields up to 4.5 T show that Fe III spin configurations are respectively ferromagnetic and antiferromagnetic. It is shown that in a particular material depending on the distribution and concentration of Fe III in the silicate sheet either mode might occur. A new model of competing nearest-neighbour (J ₁) and next-nearest-neighbour (J ₂) magnetic exchange interactions in the triangular lattice is introduced to account for the results. From available magnetic susceptibilities we estimate ∣J ₁∣～6∣J ₂∣. The results lead to the conclusion that the Fe III cations are highly ordered in glauconite and occupy cis sites so as to maximize their mutual separations.     

Sign of the magnetic coupling of Fe2+ and Fe3+ in biotite

Mössbauer spectra of biotite at 4 K are reported. The biotite crystals were oriented with the c-axis parallel to the γ-ray direction and some spectra were recorded with external magnetic fields of 40 kOe applied at right angles to the c-axis. Decrease of the magnetic-hyperfine field of both Fe²⁺ and Fe³⁺ ions on application of the external field shows that both Fe³⁺-Fe³⁺ pairs and Fe²⁺-Fe²⁺ pairs are coupled ferromagnetically.     

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

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

Matrix-independent Fe isotope ratio determination in silicates using UV femtosecond laser ablation

UV femtosecond laser ablation coupled to MC-ICP-MS provides a promising in situ tool to investigate elemental and isotope ratios by non-matrix-matched calibration. In this study, we investigate Fe isotope composition in siliceous matrices including biotite, hornblende, garnet, fayalite and forsterite (San Carlos Olivine), and an oceanic Fe–Mn crust using the iron reference material IRMM-014 for calibration. To test the accuracy of the laser ablation data, Fe isotope compositions were obtained independently by solution ICP-MS after chromatographic separation of Fe. Sample materials with low Cr content, i.e. biotite, hornblende, fayalite and the Fe–Mn crust, reveal δ^56/54Fe and δ^57/54Fe values that agree with those from solution ICP-MS data within the measured precision. For high Cr concentration (⁵⁴Cr/⁵⁴Fe >0.0001), i.e. in the garnet and forsterite sample, δ^56/54Fe and δ^57/54Fe values were derived from ⁵⁷Fe/⁵⁶Fe ratios as correction of the isobaric interference of ⁵⁴Cr on ⁵⁴Fe is unsatisfactory. This approach provides accurate results for both minerals. Moreover, the garnet crystal exhibits isotopic zonation with differences of 0.3‰ in δ^56/54Fe showing that substantial heterogeneities exist in high-temperature metamorphic minerals. Multiple analyses of homogeneous sample materials reveal a repeatability of 0.1‰ (2 SD) for δ^56/54Fe and 0.2‰ (2 SD) for δ^57/54Fe, respectively. This study adds to the observations of Horn et al. (2006) who have shown that the determination of Fe isotope ratios in various matrices including iron alloys, iron oxides and hydroxides, iron sulfide and iron carbonates can be performed with high accuracy and precision at high spatial resolution using UV femtosecond laser ablation ICP-MS. These results demonstrate that femtosecond laser ablation ICP-MS is a largely matrix-independent method, which provides a substantial advantage over commonly employed nanosecond laser ablation systems.     

Determination of Fe3+ and Fe2+ concentrations in feldspar by optical absorption and EPR spectroscopy

Ferrous and ferric iron concentrations in feldspars with low total iron content (<0.32 wt% total Fe) were determined from optical and electron paramagnetic resonance (EPR) spectra to better than ±15 percent of the amount present. Optical spectra indicate that Fe²⁺ occupies two distorted M-sites in plagioclases of intermediate structural state. The linear dependence of the Fe²⁺/Fe total ratio on An content demonstrates that Fe²⁺ substitutes for Ca (not Na) so that the number of Ca-sites is a principal factor in iron partitioning in plagioclase. EPR powder spectra show that the number of sites for Fe³⁺ depends on structural state rather than on plagioclase chemistry. The observed linear correspondence of EPR double-integrated intensities with optical peak areas shows that all Fe³⁺ is tetrahedrally coordinated in both plagioclase and disordered potassium feldspar. Microcline perthites show, in addition to tetrahedral Fe³⁺, a signal due to axially coordinated ferric iron, which we associate with formation of hematite inclusions.     

An EPR study of Mn2+ and Fe3+ in dolomites

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

The distribution of Fe2+ and Fe3+ in iron-bearing tourmalines: A Mössbauer study

Summary Room-temperature Mössbauer spectra of five iron-bearing tourmalines were measured and analyzed. The Fe²⁺/Fe³⁺ ratio and the iron occupancy of the Y and Z positions could be assigned to all samples, with the help of two previously well characterized samples, from Mexico and Madagascar. Ferric or ferrous ions or both partially occupy the Z as well as the Y octahedra. This fact of observation is interpreted as the chemical response, during crystal growth, to the requirement of size matching for the edge-sharing Y and Z oxygen octahedra. It accounts for the inexistence of solid solution between the Mg and (Li, Al) tourmalines.

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Die Verteilung von Fe²⁺ und Fe³⁺ in eisenhaltigen Turmalinen: Eine Mössbauer-Untersuchung

Zusammenfassung Mössbauer-Spektren von fünf eisenhaltigen Turmalinen wurden gemessen und analysiert. Das Verhältnis Fe²⁺/Fe³⁺ und die Eisenverteilung konnten mit Hilfe von zwei gut identifizierten Turmalin-Kristallen von Mexiko und Madagascar für die Y-und Z-Lagen aller Exemplare bestimmt werden. Zweiwertiges sowie dreiwertiges Eisen findet sich sowohl in der Z-als auch in der Y-Lage. Da sich die Y-und Z-Oktaeder in einer gemeinsamen Kante treffen, wird diese Beobachtung als chemische Antwort des Kristalles auf die erforderte Größenanpassung der Y-und Z-Oktaeder während seines Wachstums erklärt. Die Abwesenheit der festen Lösung zwischen Dravit und Elbait kann somit erklärt werden.

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With 4 Figures     

Raman spectroscopic features of Al- Fe3+- poor magnesiochromite and Fe2+- Fe3+- rich ferrian chromite solid solutions

Mineralogy and Petrology - Naturally occurring Al- Fe3 +- poor magnesiochromite and Fe2+- Fe3 +- rich ferrian chromite solid solutions have been analyzed by micro-Raman spectroscopy....     

Partition of Mg and Fe2+ in coexisting pyroxenes

A general theory for the partition of elements between coexisting, multicomponent phases is outlined and applied to data for Ca-rich pyroxene (Cap) — Ca-poor pyroxene (Op) assemblages from the Skaergaard and Bushveld intrusions and from charnockites. The intercrystalline partition of Mg and Fe²⁺ are studied separately rather than through the exchange reaction, MgSiO ₃ ^Cap +FeSiO ₃ ^Op FeSiO ₃ ^Cap +MgSiO ₃ ^Op .The separate distributions for x_MgSiO3> and x_FeSiO3> are quite distinct and demonstrate directly that solutions of both Mg and Fe²⁺ in the two pyroxenes are nonideal.     

Mg,Fe2+ site occupancies in coexisting pyroxenes

The separate distributions for MgSiO₃ and FeSiO₃ in coexisting pyroxenes from the Skaergaard and Bushveld intrusions and charnockites, which were introduced in an earlier communication, indicate directly that significant amounts of both Fe²⁺ and Mg were present in the M(2) site of the Ca-rich pyroxene at the temperature of final intercrystalline equilibration. The calculated Fe²⁺ M(2) site occupancy in the Ca-rich pyroxene increases markedly with decrease in total MgSiO₃ content but the corresponding Mg site occupancy appears largely independent of MgSiO₃. The mean value of the distribution constant for intracrystalline exchange in the Ca-rich pyroxene decreases, away from unity, with decreasing temperature of equilibration. Occupancy of Mg and Fe²⁺ in the M (2) site of the Ca-rich pyroxene effectively compensates for the expected variation in K _D with composition resulting from intracrystalline partition in Ca-poor pyroxene, and this largely accounts for the difference in K _D between igneous and metamorphic pyroxenes. The variation of the augite limb of the pyroxene solvus within the pyroxene quadrilateral is developed as a possible geothermometer.     

Kinetics of Fe2+-Mg distribution in aluminous orthopyroxenes

Kinetic studies of isothermal heating experiments (600–800° C) on aluminous pyroxenes (Mg_0.942Fe _0.880 ²⁺ Fe _0.068 ³⁺ Mn_0.016Ca_0.010Al_0.084) (Si_1.848Al_0.152) permit the determination of rate constant of isothermal disordering as 2.5457 E13(±1.4 E13) min^?1. The activation energy is determined as 278 (±23) kJ/mol. Data on two other aluminous pyroxenes at 700° C indicate that the rate constant decreases significantly with increasing amount of trivalent cations. There is a similar but reverse correlation between the concentration of trivalent cations and the Fe²⁺-Mg equilibrium distribution between sites. The site distribution coefficient increases with increasing concentration of trivalent cations at constant temperature.     

The spin-forbidden absorption spectrum of Fe2+ in orthopyroxene

In this article, we have established the energy matrices of the strong-field terms SΓ of the d ⁴ (d ⁶) electron configuration in a crystal-field with C _2v symmetry. When we used this procedure to calculate the spin-forbidden absorption bands of Fe²⁺ in the M(2) site in orthopyroxene, we substituted the single-electron crystal-field energy levels (determined by the experimental results of the spinallowed spectrum) into the energy matrices instead of the single-electron crystal-field matrix elements. Thus, by means of only one parameter B (C=4B), most of the spin-forbidden bands of Fe²⁺ have been determined. Furthermore, when a similar treatment was made of the M(1) site of O _h symmetry, the entire spin-forbidden spectrum of Fe²⁺ in orthopyroxenes could be semiquantitatively explained. This shows that the method is particularly useful for the calculation of spin-forbidden spectra of complexes with the d ⁴ (d ⁶) configuration in a low-symmetry site.     

57Fe Mössbauer study on compositions of the series Fe3+TaO4-Fe2+Ta2O6

Fe⁵⁷ Mössbauer spectra were measured on compositions of the series Fe_1?x/3Ta_1+x/3O₄, 0≤x≤1. The spectra are characterized by mixed valencies of Fe²⁺ and Fe³⁺ ions for 0<x<1. Starting from x=0 with rutile structure, a trirutile structure forms towards x=1. Quadrupole splitting QS of Fe³⁺ is QS(Fe³⁺)≈0.55 mm/s and isomer shift IS is IS(Fe³⁺)≈0.40 mm/s (referred to Fe); both quantities exhibit minor variations along the series. The Fe²⁺ subspectra for x>0.5 were fitted using one symmetrical doublet; however, for x<0.5 two symmetrical doublets were necessary to describe these patterns. QS(Fe²⁺)=2.0–3.2 mm/s and IS(Fe²⁺)=0.90–1.15 mm/s for all compositions. In the case x<0.5, marked temperature dependent QS values appear to exist. This feature may be related to short range order effects and possibly also in part to intervalence electron transfer betwee Fe²⁺ and Fe³⁺ ions.     

Time-resolved luminescence of Fe3+ and Mn2+ ions in hydrous volcanic glasses

 Time-resolved luminescence spectra of natural and synthetic hydrous volcanic glasses with different colors and different Fe, Mn, and H₂O content were measured, and the implications for the glass structure are discussed. Three luminescence ranges are observed at about 380–460, 500–560, and 700–760 nm. The very short-living (lifetimes less than 40 ns) blue band (380–460 nm) is most probably due to the ⁴T₂(⁴D) →⁶A₁(⁶S) and ⁴A₁(⁴G) →⁶A₁(⁶S) ligand field transitions of Fe³⁺. The green luminescence (500–560 nm) arises from the Mn²⁺ transition ⁴T₁(⁴G) →⁶A₁(⁶S). It shows weak vibronic structure, short lifetimes less than 250 μs, and indicates that Mn²⁺ is tetrahedrally coordinated, occupying sites with similar distortions and ion–oxygen interactions in all samples studied. The red luminescence (700–760 nm) arising from the ⁴T₁(⁴G) →⁶A₁(⁶S) transition of Fe³⁺ has much longer lifetimes of the order of several ms, and indicates that ferric iron is also mainly tetrahedrally coordinated. Increasing the total water content of the glasses leads to quenching of the red luminescence and decrease of the distortions of the Fe³⁺ polyhedra. Received: 30 July 2001 / Accepted: 15 November 2001     

57Fe Mössbauer study of the asbestiform silicates balangeroite and carlosturanite

⁵⁷FeMössbauer spectra of the two silicate minerals balangeroite (BAL) and carlosturanite (CST) have been collected at 80 and 295 K under normal and magic angle geometry. For both minerals the spectra have been fitted with two ferrous and two ferric doublets; Fe²⁺ accounts for 80 and 62% of Fe_tot in Bal and CST, respectively. The number of doublets used to fit the spectra supports the hypotheses that: (i) in the serpentine-like structure of CST iron occupies only octahedra which lie between the tetrahedral silicate strips; (ii) the octahedral framework of BAL (actually monoclinic) is satisfactorily described with an orthorhombic sub-cell.     

Die Kristallstruktur des Voltaits,K2Fe 5 2+ Fe 3 3+ Al[SO4]12·18H2O

Zusammenfassung Die Kristallstruktur von künstlichem Voltait, K₂Fe₅ ²⁺Fe₃ ³⁺Al[SO₄]₁₂· ·18 H₂O, kubisch hexakisoktaedrisch,Fd3c–O _h ⁸,a ₀=27,254 ,Z-16, wurde mittels photographischer Röntgendaten bestimmt. Die Aufklärung der Struktur erfolgte mit Patterson- und Fouriermethoden unter Zuhilfenahme des multiplen isomorphen Ersatzes. Die Verfeinerung nach der Methode der kleinsten Quadrate ergab mit anisotropen Temperaturfaktoren für 726 beobachteteF _hkl R=0,033. Das Hauptmerkmal der Struktur ist ein 3dimensionales Gerüst aus [Fe³⁺O₆]-Oktaedern, [Fe ₅ ^6/2+ Fe ₁ ^6/3+ O₄(H₂O)₂]-Oktaedern und [K⁺O₁₂]-Polyedern, die durch SO₄-Tetraeder verknüpft werden. Hohlräume dieses Gerüstes werden von ungeordnet orientierten [Al(H₂O)₆]-Oktaedern eingenommen. Es wird gezeigt, daß Al als wesentlicher Bestandteil dieses Voltaits angesehen werden muß.

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The crystal structure of voltaite, K₂Fe₅ ²⁺Fe₃ ³⁺Al[SO₄]₁₂·18H₂O

Summary The crystal structure of synthetic voltaite, K₂Fe₅ ²⁺Fe₃ ³⁺Al[SO₄]₁₂· · 18 H₂O, cubic hexakis-octahedral, space groupFd3c–O _h ⁸,a ₀=27.254 ,Z=16, was determined from photographic X-ray data. The structure was solved by Patterson and Fourier-methods with the aid of multiple isomorphic substitution. Least squares refinement with anisotropic temperature factors resulted inR=0.033 for 726 observedF _hkl . The dominant structural feature is a continous framework composed of [Fe³⁺O₆]-octahedra, [Fe ₅ ^6/2+ Fe ₁ ^6/3+ O₄(H₂O)₂]-octahedra and [K⁺O₁₂]-polyhedra linked by SO₄-tetrahedra. The arrangement gives rise to cages occupied by disordered [Al(H₂O)₆]-octahedra. It is shown that Al must be considered to be a essential constituent of such voltaites.

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