Experimental determination of the diffusion coefficient for calcium in olivine between 900°C and 1500°C     

L.A. Coogan  A. Hain  S. Chakraborty 《Geochimica et cosmochimica acta》2005,69(14):3683-3694

Experiments have been carried out to determine the temperature, oxygen fugacity (fO₂) and compositional dependence of the tracer diffusion coefficient (D) of calcium in olivine. These data constrain the diffusion coefficient over the temperature range 900 to 1500°C for the three principal crystallographic axes. Well constrained linear relationships between the reciprocal of the absolute temperature and log(D) exist at any given oxygen fugacity. There is a strong dependence of the diffusion coefficient on oxygen fugacity with D ∝ fO₂^(1/3). This makes a knowledge of the T-fO₂ path followed by geological samples a prerequisite for modelling Ca diffusion in olivine. The best fitting preexponential factor (Do) and activation energy (E) to the Arrhenius equation log (D) = log [Do exp(−E/RT)] + 0.31Δ log fO₂ for Ca diffusion in olivine at a given oxygen fugacity (fO₂*) are given by:diffusion along [100]: log [Do (m²/s)] = −10.78 ± 0.43; E = 193 ± 11 kJ/moldiffusion along [010]: log [Do (m²/s)] = −10.46 ± 0.37; E = 201 ± 10 kJ/moldiffusion along [001]: log [Do (m²/s)] = −10.02 ± 0.29; E = 207 ± 8 kJ/molwhere Δ log fO₂ = log[fO₂*] − log[10⁻¹²] with fO₂* in units of bars. There is no measurable compositional dependence of the diffusion coefficient between Fo₈₃ and Fo₉₂. Diffusion in Fo₁₀₀ has a much higher activation energy than in Fe-bearing olivine and has a weaker fO₂ dependence.  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   

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   

Quantification of the ferric/ferrous iron ratio in silicates by scanning transmission X-ray microscopy at the Fe L2,3 edges     

Franck Bourdelle  Karim Benzerara  Olivier Beyssac  Julie Cosmidis  Daniel R. Neuville  Gordon E. Brown Jr.  Erwan Paineau 《Contributions to Mineralogy and Petrology》2013,166(2):423-434

Estimation of Fe³⁺/ΣFe ratios in materials at the submicrometre scale has been a long-standing challenge in the Earth and environmental sciences because of the usefulness of this ratio in estimating redox conditions as well as for geothermometry. To date, few quantitative methods with submicrometric resolution have been developed for this purpose, and most of them have used electron energy-loss spectroscopy carried out in the ultra-high vacuum environment of a transmission electron microscope (TEM). Scanning transmission X-ray microscopy (STXM) is a relatively new technique complementary to TEM and is increasingly being used in the Earth sciences. Here, we detail an analytical procedure to quantify the Fe³⁺/ΣFe ratio in silicates using Fe L_2,3-edge X-ray absorption near edge structure (XANES) spectra obtained by STXM, and we discuss its advantages and limitations. Two different methods for retrieving Fe³⁺/ΣFe ratios from XANES spectra are calibrated using reference samples with known Fe³⁺ content by independent approaches. The first method uses the intensity ratio of the two major peaks at the L₃-edge. This method allows mapping of Fe³⁺/ΣFe ratios at a spatial scale better than 50 nm by the acquisition of 5 images only. The second method employs a 2-eV-wide integration window centred on the L₂ maximum for Fe³⁺, which is compared to the total integral intensity of the Fe L₂-edge. These two approaches are applied to metapelites from the Glarus massif (Switzerland), containing micrometre-sized chlorite and illite grains and prepared as ultrathin foils by focused ion beam milling. Nanometre-scale mapping of iron redox in these samples is presented and shows evidence of compositional zonation. The existence of such zonation has crucial implications for geothermometry and illustrates the importance of being able to measure Fe³⁺/ΣFe ratios at the submicrometre scale in geological samples.  相似文献   

The stability of primary alluaudites in granitic pegmatites: an experimental investigation of the Na2(Mn2−2x Fe1+2x )(PO4)3 system     

Frédéric Hatert  André-Mathieu Fransolet  Walter V. Maresch 《Contributions to Mineralogy and Petrology》2006,152(4):399-419

In order to assess the geothermometric potential of the Na₂(Mn_2−2x Fe_1+2x )(PO₄)₃ system (x = 0–1), which represents the compositions of natural weakly oxidized alluaudites, we performed hydrothermal experiments between 400 and 800°C, at 1 kbar, under an oxygen fugacity (f(O₂)) controlled by the Ni–NiO (NNO), Fe₂O₃–Fe₃O₄ (HM), Cu₂O–CuO (CT), and Fe–Fe₃O₄ (MI) buffers. When f(O₂) is controlled by NNO, single-phase alluaudites crystallize at 400 and 500°C, whereas the association alluaudite + marićite appears between 500 and 700°C. The limit between these two fields corresponds to the maximum temperature that can be reached by alluaudites in granitic pegmatites, because marićite has never been observed in these geological environments. Because alluaudites are very sensitive to variations of oxygen fugacity, the field of hagendorfite, Na₂MnFe²⁺Fe³⁺(PO₄)₃, has been positioned in the f(O₂)–T diagram, and provides a tool that can be used to estimate the oxygen fugacity conditions that prevailed in granitic pegmatites during the crystallization of this phosphate.  相似文献