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   

Electric conductivity of Fe2SiO4–Fe3O4 spinel solid solutions     

T. Yamanaka  H. Shimazu  K. Ota 《Physics and Chemistry of Minerals》2001,28(2):110-118

 The spinel solid solution was found to exist in the whole range between Fe₃O₄ and γ-Fe₂SiO₄ at over 10 GPa. The resistivity of Fe_{3− x} Si _x O₄ (0.0<x<0.288) was measured in the temperature range of 80∼300 K by the AC impedance method. Electron hopping between Fe³⁺ and Fe²⁺ in the octahedral site of iron-rich phases gives a large electric conductivity at room temperature. The activation energy of the electron hopping becomes larger with increasing γ-Fe₂SiO₄ component. A nonlinear change in electric conductivity is not simply caused by the statistical probability of Fe³⁺–Fe²⁺ electron hopping with increasing the total Si content. This is probably because a large number of Si⁴⁺ ions occupies the octahedral site and the adjacent Fe²⁺ keeping the local electric neutrality around Si⁴⁺ makes a cluster, which generates a local deformation by Si substitution. The temperature dependence of the conductivity of solid solutions indicates the Verwey transition temperature, which decreases from 124(±2) K at x=0 (Fe₃O₄) to 102(±5) K at x=0.288, and the electric conductivity gap at the transition temperature decreases with Si⁴⁺ substitution. Received: 15 March 2000 / Accepted: 4 September 2000  相似文献   

An internally consistent model for the thermodynamic properties of Fe−Mg-titanomagnetite-aluminate spinels   总被引：1，自引：0，他引：1  

Richard O. Sack  Mark S. Ghiorso 《Contributions to Mineralogy and Petrology》1991,106(4):474-505

A model is developed for the thermodynamic properties of Fe²⁺–Mg²⁺-aluminate-titanate-ferrite spinels of space group Fd3m. The model incorporates an expression for the configurational entropy of mixing which accounts for long-range order over tetrahedral and octahedral sites. Short-range order or departures from cubic symmetry are not considered. The non-configurational Gibbs energy is formulated as a second degree Taylor expansion in six linearly independent composition and ordering variables. The model parameters are calibrated to reproduce miscibility gap constraints, order-disorder phenomena in MgAl₂O₄ and MgFe₂O₄, and Fe²⁺–Mg²⁺ partitioning data between olivine and: (1) aluminate spinels; (2) ferrite spinels; (3) titanate spinels; (4) mixed aluminate-ferrite spinels. This calibration is achieved without invoking non-configurational excess entropies of mixing. The model predicts that the ordering state of FeAl₂O₄ is more normal than that of MgAl₂O₄. It also successfully accounts for heat of solution measurements and activity-composition relations in the constituent binaries. Phase equilibrium constraints require that the structure of Fe₃O₄ is more inverse than random at all temperatures and that Mg²⁺ has a strong tetrahedral site preference with respect to that of Fe²⁺. The analysis suggests that in the titanates short range order on octahedral sites may be significant at temperatures as high as 1300° C. Constraints developed from calibrating the thermodynamic properties of Fe²⁺–Mg²⁺-aluminatetitanate-ferrite spinel solid solutions permit extension of the database of Berman (1988) to include estimates of the end-member properties of hercynite (FeAl₂O₄), ulvöspinel (Fe₂TiO₄), MgFe₂O₄ and cubic Mg₂TiO₄. In constructing these estimates, provision is made for low-temperature magnetic entropy contributions and the energetic consequences of disordering the aluminates and the ferrites. These estimates are consistent with all of the available low-temperature adiabatic calorimetry, high-temperature heat content, and heat of solution measurements on the end-members. The analysis implies that there is a substantial heat capacity anomaly in the range 300°–900° C associated with disordering of the MgAl₂O₄ structure while that in FeAl₂O₄ becomes significant at temperatures above 700° C. The same heat capacity response in the ferrites indicates that the order/disorder transformation is coupled to the antiferromagnetic-paramagnetic transition in MgFe₂O₄ but takes place well above the ferrimagnetic-paramagnetic transition in magnetite. The proposed model is internally consistent with solution theory reported elsewhere for Fe²⁺–Mg²⁺ olivines and orthopyroxenes (Sack and Ghiorso 1989), rhombohedral oxides (Ghiorso 1990a) and the remaining end-member properties of Berman (1988).  相似文献   

H T -XRD study of synthetic ferrian magnesian spodumene: the effect of site dimension on the P 2₁/ c → C 2/ c phase transition     

F. Cámara  G. Iezzi  R. Oberti 《Physics and Chemistry of Minerals》2003,30(1):20-30

 Ferrian magnesian spodumene was synthesized in the MLFSH system at P=0.4 GPa, T=700 °C, fO₂=NNO+2.3. The space group at room T is P2₁/c [a=9.638(3) ?, b=8.709(2) ?, c=5.258(2) ?, β=109.83(3)^∘, V=415.2 ?³]. The structure is topologically equivalent to that of ferrian spodumene, LiFeSi₂O₆, and has two symmetrically independent tetrahedral chains, A and B, and two independent octahedral sites, M1 and M2. The crystal-chemical composition was determined combining EMP, SIMS and single-crystal XRD analysis, yielding ^M2(Li_0.85Mg_0.09Fe²⁺ _0.06) ^M1(Fe³⁺ _0.85Mg_0.15)Si₂O₆. Li is ordered at the M2 site and Fe³⁺ is ordered at the M1 site, whereas Mg (and Fe²⁺) distribute over both octahedral sites. Structure refinements done at different temperatures (25, 70, 95, 125, 150 and 200 °C) allowed characterization of a reversible displacive P2₁/c→C2/c transition at 106 °C. Previous HT-XRD studies of Li-clinopyroxenes had shown that the transition temperature is inversely related to the size of the M1 cation. For the crystal of this work, the aggregate ionic radius at M1 is longer than that of ferrian spodumene, for which the transition temperature is −44 °C. The higher transition temperature observed can only be explained on the basis of the shorter aggregate radius at the M2 site (due to the presence of Mg substituting after Li), in keeping with the results obtained for ferromagnesian P2₁/c pyroxenes. The effects of all the chemical substitutions must be considered when modelling transition temperatures and thermodynamic behaviour in clinopyroxenes. Received: 7 May 2002 / Accepted: 23 October 2002  相似文献   

Phase transitions in ilvaite,a mixed-valence iron silicate     

Subrata Ghose  Katsuhiro Tsukimura  Dorian M. Hatch 《Physics and Chemistry of Minerals》1989,16(5):483-496

Ilvaite, Ca(Fe²⁺, Fe³⁺) Fe²⁺Si₂O₇O(OH), a mixed-valence iron silicate shows an insulator-semimetal transition with a band gap of 0.13 eV due to thermally induced charge delocalization between Fe²⁺ and Fe³⁺ ions (A sites) in double octahedral chains. The charge delocalization induces a second order crystallographic phase transition on heating from monoclinic (P2₁/a) to orthorhombic (Pnam) symmetry at 346 K. The unit cell dimensions within the 295–420 K range and the crystal structures at 295, 320, 340, 360, 380 and 400 K have been determined by high temperature single crystal X-ray diffraction. The degree of charge delocalization determined from the sizes of the Fe(Ao) and Fe(Am) octahedra is the primary order parameter, Q which couples linearly with the spontaneous strain component, ₁₃. The order parameter coupling and the associated free energy expression is given. The calculated normal modes of the space group symmetry change are consistent with the experimentally observed atomic displacements, which are parallel and antiparallel to c. Formation of antiphase lamellar twin domains parallel to (001) in the monoclinic phase is predicted to occur as a result of the phase transition. Above T_c (= 346 K), the slow asymptotic decrease of ₁₃ attaining a zero value at 380 K indicates the presence of fluctuating precursor clusters with considerable short-range order above T_c. A peak in the specific heat (C_p) measurements coincides with the onset of longrange order at 380 K, whereas ⁵⁷Fe Mössbauer measurements indicate the onset of charge localization at a considerbly higher temperature (470 K). The coupling of the d⁶ electron of the Fe²⁺ (A) ion with a longitudinal optic phonon with the polarization vector along c ^* is the likely mechanism to drive the phase transition. The electronphonon coupling also provides a charge conduction mechanism through electron hopping, whereby the short-bonded Fe²⁺-Fe³⁺ pair containing the d⁶ electron (intermediate polaron) will break up and re-form, thereby propagating the electron one step along the c axis.  相似文献   

Effects of Fe and Al incorporations on the bridgmanite–postperovskite coexistence domain     

Xianlong Wang  Taku Tsuchiya  Zhi Zeng 《Comptes Rendus Geoscience》2019,351(2-3):141-146

The postperovskite phase transition of Fe and Al-bearing MgSiO₃ bridgmanite, the most aboundant mineral in the Earth's lower mantle, is believed to be a key to understanding seismological observations in the D″ layer, e.g., the discontinuous changes in seismic wave velocities. Experimentally reported phase transition boundaries of Fe and Al-bearing bridgmanite are currently largely controversial and generally suggest wide two-phase coexistence domains. Theoretical simulations ignoring temperature effects cannot evaluate correctly two-phase coexistence domains under high-temperature. We show high-pressure and high-temperature phase transition boundaries for various compositions with geophysically relevant impurities of Fe²⁺SiO₃, Fe³⁺Fe³⁺O₃, Fe³⁺Al³⁺O₃, and Al³⁺Al³⁺O₃ derived from the ab initio finite-temperature free energies calculated combining the internally consistent LSDA + U method and a lattice dynamics approach. We found that at ～ 2500 K, incorporations accompanied by Fe³⁺ expand the two-phase coexistence domains distinctly, implying that D″ seismic discontinuities likely arise from the phase transition of Fe²⁺-bearing bridgmanite.  相似文献   

Polarized X-ray absorption spectroscopy of metal ions in minerals     

G. A. Waychunas  G. E. Brown Jr. 《Physics and Chemistry of Minerals》1990,17(5):420-430

We have made use of the nearly complete linear polarization of synchrotron radiation to study the polarization dependence of X-ray absorption near-edge structure (XANES) and extended fine structure (EXAFS) in oriented single crystals of gillespite (BaFe²⁺ Si₄O₁₀; Fe^{2 +} in square-planar coordination, point symmetry C ₄), anatase (TiO₂; Ti⁴⁺ in octahedral coordination, point symmetry D _2d), and epidote (Ca₂(Al, Fe³⁺)₃SiO₄)₃(OH); Fe³⁺ in distorted octahedral coordination, point symmetry (C _s). For gillespite, the Fe K-XANES spectrum varies strongly with E-vector orientation of the incident X-ray beam. When the E-vector lies in the plane of the FeO₄ group (i.e., perpendicular to the c-axis), multiple-scattering features at 7127 and 7131 eV intensify, whereas when the E-vector is perpendicular to the plane of the FeO₄ group (i.e., parallel to the c-axis), a strongly-polarized 1s to 4p bound state transition occurs at 7116 eV and a localized continuum resonance occurs at 7122 eV. The Fe-K-EXAFS spectrum of gillespite is also highly polarization dependent. When the E-vector is perpendicular to c, all four nearest-neighbor oxygens around Fe²⁺ contribute to the EXAFS signal; when E is parallel to c, the EXAFS signal from nearest-neighbors is reduced by at least 86%. The unpolarized Ti K-XANES spectrum of anatase has three relatively strong pre-edge features at 4967.1, 4969.9, and 4972.7 eV which have resisted definitive interpretation in past studies. The lowest energy feature has a strong xy polarization dependence, suggesting a large amount of 4p _x,y character, and it is also very sharp, indicating a well-defined transition energy. Both of these observations are consistent with an excitonic state with a binding energy of 2.8 eV. The two higher energy features, which are characteristic of octahedrally-coordinated Ti⁴⁺, show little polarization dependence and are probably due to 1s to 3d bound-state transitions, with a small degree of np character in the final state wavefunction. Interpretation of the polarization dependence of Fe K-XANES spectra for epidote is not as straightforward due to the lower space group symmetry (P2₁/m) relative to gillespite (P4/ncc) and anatase (I4₁/amd) and the lower point group symmetry (C _s) of the M(3) site which contains most of the Fe³⁺ in the epidote structure. However, the presence of a shoulder at 7121 eV in the E parallel to b spectrum and its absence in the E normal to bc spectrum are consistent with it being a 1s to 4p _z bound-state transition. Strong, weakly x, y polarized features near 7126 eV in both spectra are most likely due to localized continuum transitions. Also, the 1s to 3d pre-edge intensity varies in intensity with E-vector orientation which is consistent with displacement of Fe³⁺ from the center of the M(3) octahedral site. Analysis of EXAFS spectra of epidote in these two polarizations yields bond distances which are within 0.04 Å of previous single-crystal X-ray diffraction analysis. This study demonstrates the utility of polarized X-ray absorption spectroscopy in quantifying the energies and orbital compositions of final state wavefunctions associated with various X-ray induced transitions in transition-metal containing minerals. It also shows that reasonably accurate M-O distances can be obtained for individual bonds oriented in crystallographically non-equivalent directions.  相似文献   

Behaviour of Fe<Subscript>4</Subscript>O<Subscript>5</Subscript>–Mg<Subscript>2</Subscript>Fe<Subscript>2</Subscript>O<Subscript>5</Subscript> solid solutions and their relation to coexisting Mg–Fe silicates and oxide phases     

Laura?Uenver-ThieleEmail author  Alan?B.?Woodland  Nobuyoshi?Miyajima  Tiziana?Boffa?Ballaran  Daniel?J.?Frost 《Contributions to Mineralogy and Petrology》2018,173(3):20

Experiments at high pressures and temperatures were carried out (1) to investigate the crystal-chemical behaviour of Fe₄O₅–Mg₂Fe₂O₅ solid solutions and (2) to explore the phase relations involving (Mg,Fe)₂Fe₂O₅ (denoted as O₅-phase) and Mg–Fe silicates. Multi-anvil experiments were performed at 11–20 GPa and 1100–1600 °C using different starting compositions including two that were Si-bearing. In Si-free experiments the O₅-phase coexists with Fe₂O₃, hp-(Mg,Fe)Fe₂O₄, (Mg,Fe)₃Fe₄O₉ or an unquenchable phase of different stoichiometry. Si-bearing experiments yielded phase assemblages consisting of the O₅-phase together with olivine, wadsleyite or ringwoodite, majoritic garnet or Fe³⁺-bearing phase B. However, (Mg,Fe)₂Fe₂O₅ does not incorporate Si. Electron microprobe analyses revealed that phase B incorporates significant amounts of Fe²⁺ and Fe³⁺ (at least ~?1.0 cations Fe per formula unit). Fe-L_2,3-edge energy-loss near-edge structure spectra confirm the presence of ferric iron [Fe³⁺/Fe_tot?=?~?0.41(4)] and indicate substitution according to the following charge-balanced exchange: [4]Si⁴⁺?+?[6]Mg²⁺?=?2Fe³⁺. The ability to accommodate Fe²⁺ and Fe³⁺ makes this potential “water-storing” mineral interesting since such substitutions should enlarge its stability field. The thermodynamic properties of Mg₂Fe₂O₅ have been refined, yielding H°_1bar,298?=???1981.5 kJ mol^??1. Solid solution is complete across the Fe₄O₅–Mg₂Fe₂O₅ binary. Molar volume decreases essentially linearly with increasing Mg content, consistent with ideal mixing behaviour. The partitioning of Mg and Fe²⁺ with silicates indicates that (Mg,Fe)₂Fe₂O₅ has a strong preference for Fe²⁺. Modelling of partitioning with olivine is consistent with the O₅-phase exhibiting ideal mixing behaviour. Mg–Fe²⁺ partitioning between (Mg,Fe)₂Fe₂O₅ and ringwoodite or wadsleyite is influenced by the presence of Fe³⁺ and OH incorporation in the silicate phases.  相似文献   

A neutron powder diffraction study of the crystal and magnetic structures of ilvaite from 305 K to 5 K — a mixed valence iron silicate with an electronic transition     

Subrata Ghose  A. W. Hewat  M. Marezio 《Physics and Chemistry of Minerals》1984,11(2):67-74

The crystal and magnetic structures of ilvaite Ca(Fe²⁺, Fe³⁺)Fe²⁺Si₂O₇O(OH) have been obtained by profile refinement of high resolution neutron powder data from a natural sample from Seriphos, Greece. Below about 400 K an electronic transition from an itinerant to an ordered state is expected, with the structure changing from orthorhombic to monoclinic. The structure remains monoclinic P2 ₁/a down to 5 K, with Fe²⁺ almost completely ordered on one of the A-sites and Fe³⁺ on the other: the ordering may increase with decreasing temperature. The B-site contains Fe²⁺ plus a small amount of Mn²⁺ impurity. There are two magnetic transitions, at 116 K and 40 K: at 80 K the Fe²⁺ and Fe³⁺ spins on the A-sites along one infinite c-axis chain are parallel, and antiparallel to those along the adjoined edge-sharing centrosymmetrically related chain. The spin vectors are all perpendicular to the plane of these chains, i.e. almost parallel to the crystallographic b-axis. At 5 K, this order is maintained, but the Fe²⁺ spins on the B-sites order antiferromagnetically as well, again almost along the b-axis. These results explain the earlier Mössbauer and magnetisation measurements.  相似文献   

Fe4O5 and its solid solutions in several simple systems     

A. B. Woodland  K. Schollenbruch  M. Koch  T. Boffa Ballaran  R. J. Angel  D. J. Frost 《Contributions to Mineralogy and Petrology》2013,166(6):1677-1686

Experiments at high pressures and temperatures reveal the stability of a Fe₄O₅-type structured phase in several simple chemical systems. On the one hand, the Fe₄O₅ end-member is stable in the presence of SiO₂-rich phases, including stishovite, but contains ≤0.01 Si cations per formula unit. This indicates that Si is essentially excluded from this phase. On the other hand, the Fe₄O₅ phase can form solid solutions with Mg and Cr and can coexist with silicate phases at the high P–T conditions expected in the transition zone of the mantle (i.e. >~9 GPa). It can coexist with both wadsleyite and Mg-rich ringwoodite and can contain at least 25 mol% Mg₂Fe₂O₅ component. The Fe₄O₅ phase always contains the least amount of Mg in any given mineral assemblage. Cr-bearing Fe₄O₅ has been synthesised with up to 46 mol% Fe₂Cr₂O₅ component and can coexist with spinel and/or hematite-eskolatite solid solutions. Substitution of Mg and Cr for Fe²⁺ and Fe³⁺, respectively, leads to variations in Fe³⁺/∑Fe from the ideal value of 0.5 for the Fe₄O₅ end-member composition, which can influence its redox stability. These cations also have contrasting effects on the unit-cell parameters, which indicate that they substitute into different sites. This initial study suggests that Fe₄O₅-type structured phases may be stable over a range of P–T–fO₂ conditions and bulk compositions, and can be important in understanding the post-spinel phase relations in a number of chemical systems relevant to the Earth’s transition zone. Thus, the presence of even small amounts of Fe³⁺ could alter the expected phase relations in peridotitic bulk compositions by stabilising this additional phase.  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   

Magnetite activities across the MgAl2O4-Fe3O4 spinel join,with application to thermobarometric estimates of upper mantle oxygen fugacity     

Glen S. Mattioli  Bernard J. Wood 《Contributions to Mineralogy and Petrology》1988,98(2):148-162

The activity of Fe₃O₄ component in MgAl₂O₄-Fe₃O₄ spinels has been measured at 900° and 1000° C and 1 atm total pressure using a zirconia oxygen electrolyte. As previously reported for the dilute Fe₃O₄ concentration region (Mattioli and Wood 1986a), magnetite activity at 1000° C is greater than at 900° C at constant Fe₃O₄ mole fraction, for compositions across the MgAl₂O₄-Fe₃O₄ join between 20 and 80 mol% Fe₃O₄ component. The 1-atm solvus crest lies between 900° and 1000° C and, at 900° C the limbs are at Fe₃O₄ mole fractions of 0.2 and 0.6 approximately.Application of the O'Neill and Navrotsky (1983, 1984) cation distribution model indicates that the unusual activity — composition behavior of Fe₃O₄ is caused by changes in the equilibrium state of disorder of mixed MgAl₂O₄-Fe₃O₄ spinels relative to the disordered Fe₃O₄ standard state. In addition, both stoichiometric volumes (Mattioli et al. 1987) and activities across the MgAl₂O₄-Fe₃O₄ join suggest that short range order is significant for this binary. Excess free energy terms must be added to ideal Fe₃O₄ activities formulated from equilibrium cation distributions in complex MgAl₂O₄-Fe₃O₄ spinels in order to increase Fe₃O₄ activities to values consistent with observation and to generate the apparent region of immiscibility at 900° C.We have applied our activity data to the estimation of upper mantle spinel-lherzolite oxygen fugacities. We calculated that minimum 's are about 2 log units below the synthetic QFM buffer at 15 kbar total pressure for Fe₃O₄ concentration of 2 mol%, in a Cr-free spinel phase. If a preliminary calibration of an additional 25 mol% Fe²⁺-substitution as FeCr₂O₄ or FeAl₂O₄ component is incorporated into Fe₃O₄ activity, then olivine-orthopyroxene-spinel assemblages of depleted-Type 1-spinel-lherzolite xenoliths indicate 's close to QFM at 15 kbar. This is in good agreement with previous thermobarometric estimates and in sharp contrast to 1 atm intrinsic measurements near IW.  相似文献   

Thermodynamic analysis of the effect of magnetic ordering on miscibility gaps in the Fe−Ti cubic and rhombohedral oxide minerals and the Fe−Ti oxide geothermometer     

M. S. Ghiorso 《Physics and Chemistry of Minerals》1997,25(1):28-38

  相似文献   

Phase transitions in ilvaite,a mixed-valence iron silicate I. A 57Fe Mössbauer study of magnetic order and spin frustration     

Kan Xuemin  Subrata Ghose  B. D. Dunlap 《Physics and Chemistry of Minerals》1988,16(1):55-60

Ilvaite, Ca(Fe²⁺,Fe³⁺)Fe²⁺Si₂O₈(OH) shows two magnetic phase transitions, which have been studied by Mössbauer spectroscopy within the temperature range 120–4 K. The continued charge localization between Fe²⁺ and Fe³⁺ ions in octahedral A-sites causes the Fe²⁺-Fe³⁺ interaction to be ferromagnetic, although the overall magnetic order is antiferromagnetic. The thermal evolution of the hyperfine fields at the Fe²⁺ (A) and Fe³⁺ (A) sites indicates B _hf: 328 and 523 kOe respectively at 0 K and T _N1= 116K. The corresponding values for Fe²⁺ (B) site are: B _hf 186 kOe and T _N2=36K. An additional hyperfine field exists at the Fe²⁺(B) site within the temperature range 116–36K due to short-range order induced by the spin ordering in A sites. The considerable difference between the two magnetic transition temperatures is due to spin frustration, because the Fe²⁺ (B) site occurs on a corner common between two triangles with respect to two sets of Fe²⁺ (A) and Fe³⁺ (A) sites with opposite spin directions.  相似文献   

The transfer of oxygen isotopic signals from precipitation to drip water and modern calcite on the seasonal time scale in Yongxing Cave,central China     

Quan Wang  Yongjin Wang  Kan Zhao  Shitao Chen  Dianbing Liu  Zhenqiu Zhang  Wei Huang  Shaohua Yang  Yijia Liang 《Environmental Earth Sciences》2018,77(12):474

Stable isotope data of precipitation (δ¹⁸O_p and deuterium excess), drip water (δ¹⁸O_d), and modern calcite precipitates (δ¹⁸O_c and δ¹³C_c) from Yongxing Cave, central China, are presented, with monthly sampling intervals from June 2013 to September 2016. Moderate correlations between the monthly variation of δ¹⁸O_p values (from ??11.5 to ??0.7‰) and precipitation amount (r = ??0.59, n?=?34, p?<?0.01) and deuterium excess (r?=?0.39, n?=?31, p?<?0.01) imply a combined effect of changes in precipitation amount and atmospheric circulation. At five drip sites, the δ¹⁸O_d values have a much smaller variability (from ??9.1 to ??7.5‰), without seasonal signals, probably a consequence of the mixing in the karst reservoir with a deep aquifer. The mean δ¹⁸O_d value (??8.4‰) for all drip waters is significantly more negative than the mean δ¹⁸O_p value (??6.9‰) weighted by precipitation amount, but close to the wet season (May to September) mean value (??8.3‰), suggesting that a threshold of precipitation amount must be exceeded to provide recharge. Calculation based on the equilibrium fractionation factor indicates that the δ¹⁸O_c values are not in isotopic equilibrium with their corresponding drip waters, with a range of disequilibrium effects from 0.4 to 1.4‰. The δ¹⁸O_c and δ¹³C_c values generally increase progressively away from the locus of precipitation on glass plates. The disequilibrium effects in the cave are likely caused by progressive calcite precipitation and CO₂ degassing related to a high gradient of CO₂ concentration between drip waters and cave air. Our study provides an important reference to interpret δ¹⁸O_c records from the monsoon region of China.  相似文献   

Synthesis and characterization of themixed valent iron silicate ilvaite,CaFe3[Si2O7/O/(OH)]     

B. Ghazi-Bayat'  G. Amthauer  K. Schürmann'  E. Hellner' 《Mineralogy and Petrology》1987,37(2):97-108

Summary The mixed valent iron silicate ilvaite CaFe ₂ ^{2 +}Fe³⁺ [Si₂O₇/O/(OH)] has been synthesized under hydrothermal conditions at temperatures between 300 and 500°C, pressures between 1.5 and 4 Kbars and oxygen fugacities controlled by the solid state buffers Fe₃O₄/Fe₂O₃, Fe/Fe₃O₄ and Ni/NiO. All these ilvaites are monoclinic (P2₁/a with cell parameters a₀ = 13.0065 (9) Å, b₀ = 8.8073 (7) Å, c₀ = 5.8580 (4) Å, and = 90.332 (6)°. The quality of the samples has been checked by Mössbauer spectroscopy. Scanning electron microscopic pictures show small euhedral crystals with a size up to 30 .

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Synthese und Charakterisierung des gemischt valenten Eisensilikates Ilvait, CaFe₃ [Si₂O₇/ 0/(OH)]

Zusammenfassung Das gemischt valente Eisensilikat Ilvait CaFe ₂ ^{2 +}Fe³⁺ [Si₂O₇/0/(OH)] wurde unter hydrothermalen Bedingungen bei Temperaturen zwischen 300 und 500 °C, Drucken zwischen 1,5 und 4,0 Kbar und bei Sauerstoff-Fugazitäten, die durch Festkörperpuffer (Fe₃O₄/Fe₂O₃, Fe/Fe₃O₄ and Ni/NiO) kontrolliert wurden, hergestellt. Diese Ilvaite sind alle monoklin mit den Zellparameters a₀=13,0065 (9) Å, b₀ = 8,8073 (7) Å, c₀ = 5,8580 (4) Å und = 90,332 (6)°. Die Qualität der Proben wurde mit Mössbauer Spektroskopie überprüft. Rasterelektronenmikroskopische Aufnahmen zeigten idiomorphe Kristalle mit einer Größe bis 30 .

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With 6 Figures  相似文献   

Electron delocalization and magnetic behavior in a single crystal of ilvaite,a mixed valence iron silicate     

D. Ghosh  T. Kundu  S. Dasgupta  Subralā Ghose 《Physics and Chemistry of Minerals》1987,14(2):151-155

Ilvaite, Ca(Fe²⁺, Fe³⁺)Fe²⁺Si₂O₈(OH), a black mixed valence iron silicate shows considerable Fe²⁺?Fe³⁺ electron delocalization above 400 K, reminiscent of magnetite. A crystallographic phase transition from orthorhombic (Pnam) to monoclinic (P2 ₁/a) symmetry takes place on cooling at 343 K induced by electron ordering. In both phases, Fe²⁺ and Fe³⁺ occur in double octahedral chains parallel to the c axis. The thermal characteristics of the magnetic susceptibilities and their anisotropies in different crystallographic planes have been measured in the temperature range 400?21 K. Below 343±1K, a continuous rotation of the molar susceptibility K _∥ in the ab plane down to 90±2 K is observed, where the symmetry of the magnetic ellipsoid remains unchanged. X _a, X _b and X _c increase abruptly below 123±0.5 K, although antiferromagnetic ordering of Fe²⁺ and Fe³⁺ spins on A sites was suggested in previous Mössbauer and neutron powder diffraction studies. In addition, 1/X _a shows an antiferromagnetic maximum at 50±3 K, whereas 1/X _b and 1/X _c at first increase sharply below 123 K, followed by antiferromagnetic curvatures in the lowest temperature region. This behavior is consistent with the antiferromagnetic ordering of Fe²⁺ spins in the B sites. The observed magnetic phenomena suggest charge delocatization effects between adjacent Fe²⁺(A)-Fe³⁺(A) pairs not only along c, but also along a and b directions. The negative sign of the molar anisotropy (K _∥-K_⊥) suggests a singlet ground State ⁵A₁ for the Fe²⁺ ions, in agreement with previous Mössbauer studies.  相似文献   

Crystal field and charge transfer spectrum of (Mg, Fe)SiO3 majorite     

Hans Keppler  Catherine A. McCammon 《Physics and Chemistry of Minerals》1996,23(2):94-98

Majorite of bulk composition Mg_0.86Fe_0.15SiO₃ was synthesized at 19 GPa and 1900 °C at an oxygen fugacity close to the Re/ReO₂ buffer. Optical absorption spectra of polycrystalline samples were measured from 4000 to 25000cm^?1. The following features were observed: (1) Three bands at 4554, 6005 and 8093 cm^?1 due to the ⁵E_g → ⁵T_2g transition of Fe²⁺ in a distorted dodecahedral site. (2) A band at 9340 cm^?1 due to the transition ⁵T_2g → ⁵E_g of octahedral Fe²⁺. (3) A band at 22784 cm^?1 resulting from Fe³⁺, probably in an octahedral site (⁶A_1g → ⁴A_1g, ⁴E_g). (4) A very intense system of Fe²⁺ → Fe³⁺ intervalence charge transfer bands which can be modelled by two Gaussian components centered at 16542 and 20128 cm^?1. The existence of two components in the charge transfer spectrum could be related to the fact that the tetragonal majorite structure may contain Fe³⁺ in two different octahedral sites. The crystal field splitting Δ of Fe²⁺ in dodecahedral coordination is 5717 cm^?1. If a splitting of the ground state in the order of 1000 cm^?1 is assumed, this yields a crystal field stabilization energy (CFSE) of 3930 cm^?1, comparable to the CFSE of Fe²⁺ in pyrope-rich garnet. However, the splitting of ⁵T_2g is significantly higher than in pyrope. This would be consistent with Fe²⁺ preferentially occupying the more distorted one of the two dodecahedral sites in the majorite structure. For octahedral Fe²⁺, Δ= 9340 cm^?1 and CFSE=3736 cm^?1, assuming negligible splitting of the ground state.  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   

Cation partitioning versus temperature in spinel: optimization of site occupants     

J. A. Foley  S. E. Wright  J. M. Hughes 《Physics and Chemistry of Minerals》2001,28(3):143-149

 Pavese et al. (1999) examined cation partitioning vs. temperature in a synthetic spinel of composition (Mg_0.70 Fe_0.23 ³⁺) Al_1.97 O₄ using structure data obtained from in situ neutron powder diffraction. After imposing assumptions on the site assignment of vacancies and Fe³⁺, they assigned the remaining cations by applying least-squares minimization to chemical constraints on site-occupancy sums, site-scattering, chemical composition, and thermal expansion of the octahedral site. Their proposed site assignments exhibit a sharp discontinuity in occupancy fractions versus temperature, a necessary consequence of their assumptions on vacancy assignments. In this paper we reexamine the cation partitioning of the same spinel using the constrained least-squares formulation of OccQP (Wright et al. 2000), which optimizes site occupancies without ad hoc assumptions. We obtain strikingly different results, supporting the general view that spinel undergoes a lambda transition at ∼1000 K. For all observed parameters, the residuals obtained with the OccQP assignments are lower than those obtained with the Pavese et al. assignments, in some cases by more than 1 order of magnitude. Received: 05 April 2000 / Accepted: 19 October 2000  相似文献