The MgCr2O4–MgFe2O4 solid solution series: effects of octahedrally coordinated Fe3+ on T–O bond lengths     

Davide Lenaz  Henrik Skogby  Francesco Princivalle  Ulf Hålenius 《Physics and Chemistry of Minerals》2006,33(7):465-474

The influence on the spinel structure of Fe³⁺ → Cr substitution was studied in flux-grown synthetic single crystals of the magnesiochromite–magnesioferrite (MgCr₂O₄–MgFe₂O₄) solid solution series. Samples were analysed by single-crystal X-ray diffraction, electron microprobe analyses, optical absorption and Mössbauer spectroscopy. With the exception of iron-poor samples (3–12 mol-% MgFe₂O₄), optical absorption and Mössbauer spectra show that iron occurs almost exclusively as trivalent Fe in the present samples. A very intense and broad absorption band at ca 7,800 cm^?1 dominates the optical absorption spectra of samples with higher Fe-contents. The appearance of this band is related to a distinct structural disorder of Fe³⁺ and a development of magnetic ordering as demonstrated by Mössbauer spectra. Profound composition-related changes are observed in the Mössbauer spectra, which are magnetically unsplit in the range 2–41 mol-% magnesioferrite, but become magnetically split in the range 59–100 mol-% magnesioferrite. Structural parameters a ₀ and M–O increase with magnesioferrite content and inversion degree, while u and T–O decrease. Our study confirms the previously reported (Lavina et al. 2002) influence of Fe³⁺ at the M site on T–O bond lengths in the spinel structure.  相似文献   

Mössbauer and ELNES spectroscopy of (Mg,Fe)(Si,Al)O3 perovskite: a highly oxidised component of the lower mantle     

S. Lauterbach  C. A. McCammon  P. van Aken  F. Langenhorst  F. Seifert 《Contributions to Mineralogy and Petrology》2000,138(1):17-26

(Mg,Fe)(Si,Al)O₃ perovskite samples with varying Fe and Al concentration were synthesised at high pressure and temperature at varying conditions of oxygen fugacity using a multianvil press, and were characterised using ex?situ X-ray diffraction, electron microprobe, Mössbauer spectroscopy and analytical transmission electron microscopy. The Fe³⁺/ΣFe ratio was determined from Mössbauer spectra recorded at 293 and 80?K, and shows a nearly linear dependence of Fe³⁺/ΣFe with Al composition of (Mg,Fe)(Si,Al)O₃ perovskite. The Fe³⁺/ΣFe values were obtained for selected samples of (Mg,Fe)(Si,Al)O₃ perovskite using electron energy-loss near-edge structure (ELNES) spectroscopy, and are in excellent agreement with Mössbauer data, demonstrating that Fe³⁺/ΣFe can be determined with a spatial resolution on the order of nm. Oxygen concentrations were determined by combining bulk chemical data with Fe³⁺/ΣFe data determined by Mössbauer spectroscopy, and show a significant concentration of oxygen vacancies in (Mg,Fe)(Si,Al)O₃ perovskite.  相似文献   

Determination of Fe<Superscript>3+</Superscript>/ΣFe ratios in chrome spinels using a combined Mössbauer and single-crystal X-ray approach: application to chromitites from the mantle section of the Oman ophiolite     

Davide?LenazEmail author  Jacob?Adetunji  Hugh?Rollinson 《Contributions to Mineralogy and Petrology》2014,167(1):958

We present the results of a comparative study in which we have measured Fe³⁺/ΣFe ratios in chromites from mantle chromitites in the Oman ophiolite using Mössbauer spectroscopy and single-crystal X-ray diffraction. We have compared these results with ratios calculated from mineral stoichiometry and find that mineral stoichiometry calculations do not accurately reflect the measured Fe³⁺/ΣFe ratios. We have identified three groups of samples. The majority preserve Fe³⁺/ΣFe ratios which are thought to be magmatic, whereas a few samples are highly oxidized and have high Fe³⁺/ΣFe ratios. There is also a group of partially oxidized samples. The oxidized chromites show anomalously low cell edge (a ₀) values and their oxygen positional parameters among the lowest ever found for chromites. Site occupancy calculations show that some chromites are non-stoichiometric and contain vacancies in their structure randomly distributed between both the T and M sites. The field relationships suggest that the oxidation of the magmatic chromitites took place in association with a ductile shear zone in mantle harzburgites. Primary magmatic Fe³⁺/ΣFe ratios measured for the Oman mantle chromitites are between 0.193–0.285 (X-ray data) and 0.164–0.270 (Mössbauer data) and preserve a range of Fe³⁺/ΣFe ratios which we propose is real and reflects differences in the composition of the magmas parental to the chromitites. The range of values extends from those MORB melts (0.16 ± 0.1) to those for arc basalts (0.22–0.28).  相似文献   

A comparison of iron redox ratios in silicate glasses determined by wet-chemical and 57Fe Mössbauer resonant absorption methods     

B. O. Mysen  I. S. E. Carmichael  D. Virgo 《Contributions to Mineralogy and Petrology》1985,90(2-3):101-106

The Fe³⁺/Σ Fe of twenty-nine experimentally formed, iron-bearing silicate glasses has been determined by wet-chemical and Mössbauer spectroscopic methods from 5–10 mg individual splits of 20–40 mg experimental run products. The wet-chemical and Mössbauer analyses were conducted in two separate laboratories (University of California, Berkeley, and the Geophysical Laboratory, respectively). The Fe³⁺/Σ Fe ranges from less than 0.2 to 0.96, and the total iron oxide content of the samples, from 2.2 to 34.7 wt %, added as Fe₂O₃. The interlaboratory comparison shows 70% of the Fe³⁺/ΣFe analyses from the two methods within the quoted uncertainties (±1 σ) of each other and 83% of the analyses within ±2 σ of each other. Replicate analyses in the current data set result in variations within ±1 σ. These uncertainties are similar to those obtained from several hundred Fe³⁺/Σ Fe analyses of reequilibrated natural rock and simple system compositions carried out with identical analytical methods in the two laboratories. There is no systematic bias in the results from either of the two techniques. The Fe³⁺/Σ Fe of silicate glasses can be analyzed, therefore, with equal confidence by either the wet-chemical or the Mössbauer spectroscopic method.  相似文献   

Ferric iron content of mineral inclusions in diamonds from George Creek, Colorado determined using Mössbauer spectroscopy     

C. A. McCammon  I. L. Chinn  J. J. Gurney  M. E. McCallum 《Contributions to Mineralogy and Petrology》1998,133(1-2):30-37

The Mössbauer milliprobe allows the determination of Fe³⁺/ΣFe in samples as small as 50?μm. For the first time this technique is applied to a suite of diamonds of eclogitic paragenesis, where three garnet and five clinopyroxene inclusions in diamonds from George Creek, Colorado have been analysed. For garnet Fe³⁺/ΣFe ranges from 0–7%, while values for clinopyroxene range from 8–14%. These results are consistent with the low oxygen fugacity conditions implied by the presence of the inclusions in diamond.  相似文献   

Pressure effect on the electronic structure of iron in (Mg,Fe)(Si,Al)O3 perovskite: a combined synchrotron Mössbauer and X-ray emission spectroscopy study up to 100 GPa     

J. Li  W. Sturhahn  J. M. Jackson  V. V. Struzhkin  J. F. Lin  J. Zhao  H. K. Mao  G. Shen 《Physics and Chemistry of Minerals》2006,33(8-9):575-585

We investigated the valence state and spin state of iron in an Al-bearing ferromagnesian silicate perovskite sample with the composition (Mg_0.88Fe_0.09)(Si_0.94Al_0.10)O₃ between 1 bar and 100 GPa and at 300 K, using diamond cells and synchrotron Mössbauer spectroscopy techniques. At pressures below 12 GPa, our Mössbauer spectra can be sufficiently fitted by a “two-doublet” model, which assumes one ferrous Fe²⁺-like site and one ferric Fe³⁺-like site with distinct hyperfine parameters. The simplest interpretation that is consistent with both the Mössbauer data and previous X-ray emission data on the same sample is that the Fe²⁺-like site is high-spin Fe²⁺, and the Fe³⁺-like site is high-spin Fe³⁺. At 12 GPa and higher pressures, a “three-doublet” model is necessary and sufficient to fit the Mössbauer spectra. This model assumes two Fe²⁺-like sites and one Fe³⁺-like site distinguished by their hyperfine parameters. Between 12 and 20 GPa, the fraction of the Fe³⁺-like site, Fe³⁺/∑Fe, changes abruptly from about 50 to 70%, possibly due to a spin crossover in six-coordinate Fe²⁺. At pressures above 20 GPa, the fractions of all three sites remain unchanged to the highest pressure, indicating a fixed valence state of iron within this pressure range. From 20 to 100 GPa, the isomer shift between the Fe³⁺-like and Fe²⁺-like sites increases slightly, while the values and widths of the quadruple splitting of all three sites remain essentially constant. In conjunction with the previous X-ray emission data, the Mössbauer data suggest that Fe²⁺ alone, or concurrently with Fe³⁺, undergoes pressure-induced spin crossover between 20 and 100 GPa.  相似文献   

Pressure-induced phase transition in Mg0.8Fe0.2O ferropericlase     

Innokenty Kantor  Leonid Dubrovinsky  Catherine McCammon  Anastasia Kantor  Sakura Pascarelli  Giuliana Aquilanti  Wilson Crichton  Maurizio Mattesini  Rajeev Ahuja  Jailton Almeida  Vadim Urusov 《Physics and Chemistry of Minerals》2006,33(1):35-44

Combined X-ray powder diffraction, Mössbauer, and XANES spectroscopy in situ experiments revealed the transformation of cubic (Mg_0.8Fe_0.2)O ferropericlase to a rhombohedrally distorted phase at 35(1) GPa and room temperature. The Mössbauer spectroscopy results show that the rhombohedral distortion does not involve magnetic ordering. Combined with data from the literature, our results imply that the cubic to rhombodedral transition occurs in (Mg,Fe)O under conditions of non-hydrostatic stress over a wide range of composition (0.2≤x _Fe≤1).  相似文献   

The crystal chemistry and mössbauer study of schorlomite     

Wu Gongbao  Mu Baolei 《Physics and Chemistry of Minerals》1986,13(3):198-205

Six schorlomite samples with TiO₂ contents varying between 9.70 and 15.34 weight percent were studied by means of Mössbauer spectroscopy and chemical analysis. The measured Mössbauer spectra have complex shapes. The spectra of these samples were fitted with six doublets, which can be assigned to ^VIIIFe²⁺, ^VIFe²⁺, ^VIFe³⁺, ^IVFe³⁺ and two electron delocalizations, ^IVFe³⁺ ? ^VIIIFe²⁺ and ^IVFe³⁺ ? ^VIFe²⁺, respectively. The assignment of iron absorption doublets and their Mössbauer parameters are discussed in terms of the single crystal structure data of one of the samples studied in this work. Cation distributions are also given. The occupancies of cations at the tetrahedral (Z) site are Fe³⁺>Al³⁺, Ti⁴⁺, and the relative enrichments at Z site are always Fe³⁺>Ti⁴⁺. Most of the six samples contain Ti³⁺. Ti³⁺/ΣTi ratios range from 1.43 percent to 6.40 percent. Fe²⁺/ΣFe ratios vary from 8.84 percent to 11.31 percent. Four types of substitution must be considered for Ti entering the garnet structure.  相似文献   

Crystal chemistry of Fe3+-bearing (Mg,Fe)SiO3 perovskite: a single-crystal X-ray diffraction study     

Ryosuke Sinmyo  Elena Bykova  Catherine McCammon  Ilya Kupenko  Vasily Potapkin  Leonid Dubrovinsky 《Physics and Chemistry of Minerals》2014,41(6):409-417

Magnesium silicate perovskite is the predominant phase in the Earth’s lower mantle, and it is well known that incorporation of iron has a strong effect on its crystal structure and physical properties. To constrain the crystal chemistry of (Mg, Fe)SiO₃ perovskite more accurately, we synthesized single crystals of Mg_0.946(17)Fe_0.056(12)Si_0.997(16)O₃ perovskite at 26 GPa and 2,073 K using a multianvil press and investigated its crystal structure, oxidation state and iron-site occupancy using single-crystal X-ray diffraction and energy-domain Synchrotron Mössbauer Source spectroscopy. Single-crystal refinements indicate that all iron (Fe²⁺ and Fe³⁺) substitutes on the A-site only, where \( {\text{Fe}}^{ 3+ } /\Upsigma {\text{Fe}}\sim 20\,\% \) based on Mössbauer spectroscopy. Charge balance likely occurs through a small number of cation vacancies on either the A- or the B-site. The octahedral tilt angle (Φ) calculated for our sample from the refined atomic coordinates is 20.3°, which is 2° higher than the value calculated from the unit-cell parameters (a = 4.7877 Å, b = 4.9480 Å, c = 6.915 Å) which assumes undistorted octahedra. A compilation of all available single-crystal data (atomic coordinates) for (Mg, Fe)(Si, Al)O₃ perovskite from the literature shows a smooth increase of Φ with composition that is independent of the nature of cation substitution (e.g., \( {\text{Mg}}^{ 2+ } - {\text{Fe}}^{ 2+ } \) or \( {\text{Mg}}^{ 2+ } {\text{Si}}^{ 4+ } - {\text{Fe}}^{ 3+ } {\text{Al}}^{ 3+ } \) substitution mechanism), contrary to previous observations based on unit-cell parameter calculations.  相似文献   

Electrical resistivity and 57Fe Mössbauer spectra of Fe-bearing calcic amphiboles     

E. Schmidbauer  Th. Kunzmann  Th. Fehr  R. Hochleitner 《Physics and Chemistry of Minerals》2000,27(5):347-356

Electrical resistivity and ⁵⁷Fe Mössbauer spectra are reported for three calcic amphiboles with different Fe concentrations. AC measurements (20?Hz–1?MHz) were performed, applying impedance spectroscopy between 100 and 785?°C in an N₂ gas atmosphere. It was found that up to three semiconducting charge transport processes can be distinguished, which in part changed slightly when several runs were carried out to higher temperatures. The extrapolated DC resistivity is much smaller for an amphibole with high Fe content than for the two with lower Fe concentrations. The derived activation energies are between ～0.48 and ～1.06?eV. For temperatures ≤600?°C the results are compatible with a charge transport mechanism due to electron hopping between Fe²⁺ and Fe³⁺. Above 600?°C, dehydrogenation and/or beginning amphibole decomposition obviously alter the conduction mechanism. From Mössbauer spectra it was established that in all amphibole samples Fe²⁺ and Fe³⁺ are simultaneously present. Mössbauer parameters were derived by fitting the observed spectra to models taking the occupation of various M sites into account.  相似文献   

Mössbauer milliprobe studies of small mineral samples with a silicon drift detector     

Todor Ruskov  Ivan Spirov  Harry W. Green II  Daniela Kovacheva  Peter Tzvetkov  Milena Georgieva  Larissa Dobrzhinetskaya 《Physics and Chemistry of Minerals》2008,35(9):485-491

Analysis of ⁵⁷Fe transmission Mössbauer spectra collected on a system where the proportional counter has been replaced with a silicon drift detector (SDD) to test milliprobing of mineral samples is described. In the region of the 14.4 keV Mössbauer line the detector has about 70% efficiency and is capable of delivering spectroscopic information with a high energy resolution and high counting rate. Satisfactory results are obtained from a phase analysis of mixtures of olivine and ilmenite in the proportion 97:3, 99:1 wt%, where in the latter case 2.4 μg of Fe³⁺ in the form of hematite was found in the ilmenite. New perovskite-type minerals (Pb_1.33Ba_0.67Fe₂O₅, Pb_1.33Sr_0.67Fe₂O₅ and Pb_1.33Ba_0.33Sr_0.33Fe₂O₅), synthesised by a combustion method, were studied by X-ray diffraction and Mössbauer spectroscopy as well. The advantage of the system with SDD compared to a conventional Mössbauer spectrometer equipped with a proportional counter as a detector is demonstrated for the perovskite samples. The Mössbauer set-up with the silicon drift detector may be successfully used for a wide range of materials containing a negligible amount of iron.  相似文献   

Crystal chemistry of ferric iron in (Mg,Fe)(Si,Al)O<Subscript>3</Subscript> majorite with implications for the transition zone     

C. A. McCammon  N. L. Ross 《Physics and Chemistry of Minerals》2003,30(4):206-216

Fifteen samples of (Mg,Fe)SiO₃ majorite with varying Fe/Mg composition and one sample of (Mg,Fe)(Si,Al)O₃ majorite were synthesized at high pressure and temperature under different conditions of oxygen fugacity using a multianvil press, and examined ex situ using X-ray diffraction and Mössbauer and optical absorption spectroscopy. The relative concentration of Fe³⁺ increases both with total iron content and increasing oxygen fugacity, but not with Al concentration. Optical absorption spectra indicate the presence of Fe²⁺–Fe³⁺ charge transfer, where band intensity increases with increasing Fe³⁺ concentration. Mössbauer data were used in conjunction with electron microprobe analyses to determine the site distribution of all cations. Both Al and Fe³⁺ substitute on the octahedral site, and charge balance occurs through the removal of Si. The degree of Mg/Si ordering on the octahedral sites in (Mg,Fe)SiO₃ majorite, which affects both the c/a ratio and the unit cell volume, is influenced by the thermal history of the sample. The Fe³⁺ concentration of (Mg,Fe)(Si,Al)O₃ majorite in the mantle will reflect prevailing redox conditions, which are believed to be relatively reducing in the transition zone. Exchange of material across the transition boundary to (Mg,Fe) (Si,Al)O₃ perovskite would then require a mechanism to oxidize sufficient iron to satisfy crystal-chemical requirements of the lower-mantle perovskite phase.  相似文献   

Cordierite III: the site occupation and concentration of Fe3+     

Charles A. Geiger  Helmut Rager  Michael Czank 《Contributions to Mineralogy and Petrology》2000,140(3):344-352

Cordierite has the ideal formula (Mg,Fe)₂Al₄Si₅O₁₈ ^.x(H₂O,CO₂), but it must contain some Fe³⁺ to account for its blue color and strong pleochroism. The site occupation and concentration of Fe³⁺ in two Mg-rich natural cordierites have been investigated by EPR and ⁵⁷Fe Mössbauer spectroscopy. In addition, powder IR spectroscopy, X-ray diffraction, and TEM examination were used to characterize the samples. Single-crystal and powder EPR spectra indicate that Fe³⁺ is located on T₁1 in natural cordierites and not in the channels. The amount in Mg-rich cordierites is very small with an upper limit set by Mössbauer spectroscopy giving less than 0.004 cations per formula unit (pfu). Fe³⁺ in cordierite can, therefore, be considered insignificant for most petrologic calculations. Heat-treating cordierite in air at 1,000?°C for 2?days causes an oxidation and/or loss of Fe²⁺ on T₁1, together with an expulsion of Na⁺ from the channels, whereas heating at the Fe–FeO buffer produces little Fe³⁺ in cordierite. Heating at 1,000?°C removes all class I H₂O, but small amounts of class II H₂O remain as shown by the IR measurements. No evidence for channel Fe²⁺ or Fe³⁺ in the heat-treated samples was found. The blue color in cordierite arises from a broad absorption band (E//b and weaker with E//a) around 18,000?cm^?1 originating from charge-transfer between Fe²⁺ in the octahedron and Fe³⁺ in the edge-shared T₁1 tetrahedron. It therefore appears that all natural cordierites contain some tetrahedral Fe³⁺. The brown color of samples heated in air may be due to the formation of very small amounts of submicroscopic magnetite and possibly hematite. These inclusions in cordierite can only be identified through TEM study.  相似文献   

The crystal chemistry of ferric iron in Fe0.05Mg0.95SiO3 perovskite as determined by Mössbauer spectroscopy in the temperature range 80–293 K     

C. A. McCammon 《Physics and Chemistry of Minerals》1998,25(4):292-300

Mössbauer spectra were recorded at multiple temperatures between 80 and 293 K to study the nature of Fe³⁺ in Fe_0.05Mg_0.95SiO₃ perovskite that had been synthesised in a multianvil press at 1650 °C and 25 GPa at its mimimum stability limit. The Mössbauer data were fitted to a model with quadrupole splitting distributions (Fe²⁺) and Lorentzian lineshapes (Fe³⁺ and Feⁿ⁺). The centre shift data were fitted to a Debye model with the following results: Θ_M (Fe²⁺)=365±52 K and Θ_M (Fe³⁺)=476±96 K. Hyperfine parameter data for Fe³⁺ suggest occupation of the octahedral site only. The average valence seen by the Mössbauer effect in rapid electron exchange that occurs between Fe²⁺ and Fe³⁺ is calculated from the hyperfine parameters to be 2.50±0.07. Correction of area fractions for site-dependent recoil-free fractions gives a value for Fe³⁺/∑Fe of 9.4±1.4%, which is independent of temperature. A perovskite phase of similar composition synthesised in the multianvil press at higher oxygen fugacity gives a value for Fe³⁺/∑Fe of 16±3%, where Fe³⁺ appears to occupy both sites in the perovskite structure.  相似文献   

The binary systems FeS-MgS and FeS-MnS: Mössbauer spectroscopy of the B1 solid solutions and high-pressure phase equilibria     

C. A. McCammon  I. Jackson  A. E. Ringwood  J. D. Cashion 《Physics and Chemistry of Minerals》1984,11(4):182-193

(Fe, Mn)S and (Fe, Mg)S solid solutions are examined to study and compare the properties of Fe²⁺ in two different B1-structured hosts, and also to study the relative stability of the B1 (NaCl) and B8 (NiAs) structures at high pressure. The Mössbauer spectra of (Fe, Mn)S and (Fe, Mg)S B1 solid solutions are quadrupole doublets at 298 K with parameters which vary smoothly with Fe²⁺ concentration. At 4.2 K the Mössbauer spectra of (Fe, Mn)S and Fe-rich (Fe, Mg)S B1 solid solutions are magnetically split into eight lines, but the spectra of Mg-rich (Fe, Mg)S solid solutions are quadrupole doublets. The line widths of the magnetic spectra are broad, consistent with a multiaxial spin arrangement. Some properties of the hypothetical phase FeS(B1) are calculated from the solid solution data; the phase is inferred to be relatively ionic compared to FeS(B8) and has a molar volume that is 7.2 percent larger than the B8 phase at 298 K. The large inferred volume difference between FeS(B1) and FeS(B8) should cause exsolution of a B8-structured phase from (Fe, Mn)S and (Fe, Mg)S B1 solid solutions at high pressure. This behaviour is confirmed experimentally at high pressure using X-ray diffraction and Mössbauer spectroscopy, and the results are correlated with thermodynamic calculations of the phase boundaries based on estimates of the volume and free energy differences between the B1 and B8 phases of FeS derived from atmospheric pressure data. The absence of an increase in solubility of Mg and Mn in the B8 phase with pressure suggests that any polymorphism in MnS and MgS at high pressure is unlikely to involve the B8 phase. Shock wave data for MgO and Fe_0.94O reported in the literature suggest similar behaviour in the system FeO-MgO at high pressure, namely exsolution of essentially pure FeO(hpp) from (Fe, Mg)O B1 solid solutions.  相似文献   

Iron isotopic systematics of UHP eclogites respond to oxidizing fluid during exhumation   总被引：1，自引：0，他引：1       下载免费PDF全文

D.‐Y. Li  Y. L. Xiao  W.‐Y. Li  X. Zhu  H. M. Williams  Y.‐L. Li 《Journal of Metamorphic Geology》2016,34(9):987-997

The iron stable isotope compositions (δ⁵⁶Fe) and iron valence states of ultrahigh‐pressure eclogites from Bixiling in the Dabie orogen belt, China, were measured to trace the changes of geochemical conditions during vertical transportation of earth materials, for example, oxygen fugacity. The bulk Fe³⁺/ΣFe ratios of retrograde eclogites, determined by Mössbauer spectroscopy, are consistently higher than those of fresh eclogites, suggesting oxidation during retrograde metamorphism and fluid infiltration. The studied eclogites (five samples) display limited mid‐ocean ridge basalts (MORB)‐like (~0.10‰) δ⁵⁶Fe values, which are indistinguishable from their protoliths, that is, gabbro cumulates formed through differentiation of mantle‐derived basaltic magma. This suggests that Fe isotope fractionation during continental subduction is limited. Garnet separates display limited δ⁵⁶Fe variation ranging from ?0.08 ± 0.07 ‰ to 0.02 ± 0.07‰, whereas coexisting omphacite displays a large variation of δ⁵⁶Fe values from 0.15 ± 0.07‰ to 0.47 ± 0.07‰. Omphacite also has highly variable Fe³⁺/ΣFe ratios from 0.367 ± 0.025 to 0.598 ± 0.024, indicating modification after peak metamorphism. Omphacite from retrograde eclogites has elevated Fe³⁺/ΣFe ratios (0.54–0.60) compared to that from fresh eclogites (~0.37), whereas garnet displays a narrow range of ferric iron content with Fe³⁺/ΣFe ratios from 0.039 ± 0.013 to 0.065 ± 0.022. The homogenous δ⁵⁶Fe values and Fe³⁺/ΣFe ratios of garnet suggest that it survived the retrograde metamorphism and preserved its Fe‐isotopic features and ferric contents of peak metamorphism. Because of similar diffusion rates of Fe and Mg in garnet and omphacite, and constant Δ²⁶Mg_{omphacite‐garnet} values (1.14 ± 0.04‰), equilibrium iron isotope fractionation between garnet and omphacite was probably achieved during peak metamorphism. Elevated Fe³⁺/ΣFe ratios of omphacite from retrograde eclogites and variant Δ⁵⁶Fe_{omphacite‐garnet} values of the studied eclogites (0.13 ± 0.10‰ to 0.48 ± 0.10‰) indicate that oxidized geofluid infiltration resulted in the elevation of δ⁵⁶Fe values of omphacite during retrograde metamorphism.  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   

<Superscript>57</Superscript>Fe Mössbauer spectroscopy,X-ray single-crystal diffractometry,and electronic structure calculations on natural sinhalites     

Sven-Ulf Weber  Michael Grodzicki  Werner Lottermoser  Günther J. Redhammer  Dan Topa  Gerold Tippelt  Georg Amthauer 《Physics and Chemistry of Minerals》2009,36(5):259-269

Natural sinhalites, MgAlBO_4, from the Ratnapura District, Sri Lanka, and from Bodnar Quarry near Hamburg, Sussex Co., New Jersey, USA, have been characterized by ⁵⁷Fe Mössbauer spectroscopy, electron microprobe, X-ray single-crystal diffractometry and by electronic structure calculations in order to determine the oxidation state and site occupancy of iron in the sinhalite structure. The samples contain about 3.35 and 1.46 wt% of total iron oxide, respectively. The structure refinement is successful and reproduces the total iron content provided that the substitution of Mg²⁺ by Fe²⁺ on the M2 position only is assumed. The ⁵⁷Fe Mössbauer spectra at 77, 293, 573 and 773 K can be resolved into two doublets with hyperfine parameters common for octahedrally coordinated high-spin Fe²⁺. There is no evidence for iron in the tetrahedral site. Electronic structure calculations in local spin density approximation yield hyperfine parameters for Fe²⁺ on the M2-site at 0, 293, 573 and 773 K in quantitative agreement with experiments. Calculated spectroscopic properties for Fe²⁺ on the M1-site are at variance with the experimental data and, thus, indicate that substitution of Al³⁺ by Fe²⁺, if occurring at all, must be accompanied by considerable local expansion and distortion of the M1-octahedron.  相似文献   

<Superscript>57</Superscript>Fe Mössbauer measurements and electronic structure calculations on natural lawsonites     

S.-U. Weber  M. Grodzicki  C. A. Geiger  W. Lottermoser  G. Tippelt  G. J. Redhammer  M. Bernroider  G. Amthauer 《Physics and Chemistry of Minerals》2007,34(1):1-9

Three natural lawsonites from Syke Rock, Mendocino Co., Reed Ranch, Marin Co., and Blake Gardens, Sonoma Co., all from the Coast Range Region in California, were studied by ⁵⁷Fe Mössbauer spectroscopy, electron microprobe analysis, and X-ray powder diffraction. The samples contain about 0.6, 1.0, and 1.4 wt% of total iron oxide, respectively. ⁵⁷Fe Mössbauer spectra are consistent with the assumption that high-spin Fe³⁺ substitutes for Al in the octahedrally coordinated site. The Mössbauer spectrum of lawsonite from Syke Rock exhibits a second doublet with ⁵⁷Fe hyperfine parameters typical for octahedrally coordinated high-spin Fe²⁺. Electronic structure calculations in the local spin density approximation yield quadrupole splittings for Fe³⁺ in quantitative agreement with experiment indicating, however, that substitution of Al by Fe³⁺ must be accompanied by local distortion around the octahedral site. Model calculations also reproduce the room temperature hyperfine parameters of ferrous high-spin iron assuming the substitution of Ca by Fe²⁺. However, it cannot be excluded that Fe²⁺ may occupy a more asymmetric site within the microstructural cavity occupied by Ca and a H₂O molecule.  相似文献   

Mössbauer spectra and magnetic features of ilvaites     

Takamitsu Yamanaka  Yoshio Takéuchi 《Physics and Chemistry of Minerals》1979,4(2):149-159

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