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.     

Partitioning of Ni2+, Co2+, Fe2+, Mn2+ and Mg2+ between olivine and silicate melts: compositional dependence of partition coefficient

Partitioning of Ni²⁺, Co²⁺, Fe²⁺, Mn²⁺ and Mg²⁺ between olivine and silicate melts has been determined near the join (Mg_0.5·-Fe_0.5)₂SiO₄-K₂O 4SiO₂ and for seven different basaltic compositions. The experiments were made at 1 atm total pressure, 1500-1100°C, and under moderate to reducing oxygen fugacities. The concentration factor, defined as K^MO = (MO)^ol/(MO)^liq (molar ratio), increases markedly for all the cations studied as the olivine component of the liquid decreases. Much of the increase in K^MO is considered as due to the compositional effect of the coexisting liquid: the temperature effect on K^MO is probably opposite to the compositional effect (K^MO decreases as temperature decreases).The partition coefficient K^MO-MgO = (MO/MgO)^ol/(MO/MgO)^liq for the reaction, M_ol²⁺ + Mg_liq²⁺ = M_liq²⁺ + Mg_ol²⁺. is relatively constant over a wide range of SiO₂ content of the liquid, except in the case of Ni²⁺. The partition coefficients have similar ranges both in synthetic and natural rock systems: K^NiO-MgO = 1.8–3.0, K^CoO-MgO = 0.6–0.8, K^FeO-MgO = 0.27–0.38, and K^MnO-MgO = 0.23–0.32. There is a systematic variation in the partition coefficient K^MO-MgO with the composition of liquid; K^MO-MgO increases with increasing SiO₂ content of melt. The partition coefficients for Co²⁺, Fe²⁺ and Mn²⁺ are useful to test the equilibration of olivine with magma of a wide compositional range.     

Calculated optical absorption spectra of Ni2+-bearing compounds

The transition energies responsible for optical absorption spectra can be obtained by crystal-field analysis, but the transition intensities are notoriously difficult to calculate. This paper examines the basic ingredients of the calculation of optical spectrum intensities. Magnetic dipole and electric quadrupole transitions intensities are evaluated, as well as the direct d(Ni²⁺) to p(O²⁻) electric dipole transitions. All these contributions are shown to be small in the optical range, so that spectral intensities are due to the mixing of odd orbitals with the Ni²⁺ 3d ⁿ states. Received: 11 November 1997 / Revised, accepted: 6 September 1999     

Cordierite III: the site occupation and concentration of Fe3+

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.     

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.     

Olivine-spinel geospeedometry: Analysis of diffusion-controlled Mg-Fe2+ exchange

Subsolidus Mg-Fe²⁺ exchange between olivine and spinel is governed by Mg-Fe²⁺ interdiffusion. Incomplete exchange results in Mg-Fe²⁺ heterogeneity in both olivine and spinel, which provides information on the thermal histories of the host rocks. A composite sphere model has been developed to obtain quantitative cooling rates or heating duration from the Mg-Fe²⁺ heterogeneity. The model assumes that a spherical core of spinel and a surrounding semi-infinite spherical shell of olivine interact by diffusion-controlled exchange of Mg and Fe²⁺. The differential equations describing the model are solved numerically by finite difference approximations. The numerical solution reveals that cooling rates or heating duration can be estimated from the relationship between the grain size of spinel and temperature calculated from the chemical compositions of the core of a spinel grain and of olivine far away from it. The calculated temperature is employed in place of $\text{Mg}\text{(Mg + Fe}{}^{\mathrm{2+}}\text{)}$ at the center of spinel to obtain the absolute temperature of thermal events.This olivine-spinel geospeedometer has been applied to peridotites. gabbro, and picrites from some ophiolite complexes in Japan to estimate their cooling rates. The estimated cooling rates for the peridotites range from 10^?4 to 10^?1 °C/yr, and those for the picrites from 10³ to 10⁴ °C/yr. The geospeedometer has been extended to estimate the heating duration of lherzolite xenoliths in basalt from the Ichinomegata crater, northeast Japan. The estimated heating duration of the xenoliths is less than one day.     

Structural changes in the FeAl2O4–FeCr2O4 solid solution series and their consequences on natural Cr-bearing spinels

The influence of Al–Cr substitution on the spinel structure was studied in synthetic single crystals belonging to the FeCr₂O₄–FeAl₂O₄ series produced by flux growth at 1,000–1,300 °C in controlled atmosphere. Samples were characterized by single-crystal X-ray diffraction, electron microprobe analyses and Mössbauer spectroscopy. Crystals of sufficient size and quality for single-crystal X-ray diffraction were obtained in the ranges Chr_0–0.45 and Chr_70–100 but not for intermediate compositions, possibly due to a reduced stability in this range. The increase in chromite component leads to an increase in the cell edge from 8.1534 (6) to 8.3672 (1) Å and a decrease in the u parameter from 0.2645 (2) to 0.2628 (1). Chemical analyses show that Fe²⁺ is very close to 1 apfu (0.994–1.007), Al is in the range 0.0793–1.981 apfu, Cr between 0 and 1.925 apfu. In some cases, Fe³⁺ is present in amounts up to 0.031 apfu. Spinels with intermediate Cr content (Chr component between 40 and 60) are strongly zoned with Cr-rich cores and Cr-poor rims. Mössbauer analyses on powdered spinels of the runs from which single crystal has been used for X-ray structural data show values of Fe³⁺/Fe_tot consistently larger than that calculated by EMPA on single crystals, presumably due to chemical variation between single crystals from the same runs. The synthesis runs ended at a temperature of 1,000 °C, but it is possible that cation ordering continued in the Cr-poor samples towards lower temperatures, possibly down to 700 °C.     

The incorporation of Mg2+ and Sr2+ into calcite overgrowths: influences of growth rate and solution composition

The concentrations of Mg²⁺ and Sr²⁺ incorporated within calcite overgrowths precipitated from seawater and related solutions, determined at 25°C, were independent of the precipitation rate over approximately an order of magnitude. The saturation states used to produce this range of precipitation rates varied from 3 to 17 depending on the composition of the solution.The amount of Mg²⁺ incorporated in the overgrowths was not directly proportional to $\text{Mg}{}^{\mathrm{2+}}\text{Ca}{}^{\mathrm{2+}}$ in solution over the entire range (1–20) of ratios studied. Below a ratio of 7.5, the overgrowth was enriched in MgCO₃ relative to what is predicted by the constant distribution coefficient measured above a ratio of 7.5. This increased MgCO₃ correlates with the relative enrichment of adsorbed Mg²⁺. Above a ratio of 7.5 the concentration of MgCO₃ in the calcite overgrowths followed a classical thermodynamic behavior characterized by a constant distribution coefficient of 0.0123 (±0.008 std dev).The concentration of SrCO₃ incorporated in the overgrowths was linearly related to the MgCO₃ content of the overgrowths, and is attributed to increased solubility of SrCO₃ in calcite due to the incorporation of the smaller Mg²⁺ ions.The kinetic data indicate that the growth mechanism involves the adsorption of the cations on the surface of the calcite prior to dehydration and final incorporation. It is suggested that dehydration of cations at the surface is the rate controlling step.     

Mg2+ removal in the system Mg2+—amorphous SiO2—H2O by adsorption and Mg-hydroxysilicate precipitation

Two chemical processes can remove Mg²⁺ from suspensions containing amorphous silica (am-SiO₂) at low temperatures: adsorption and precipitation of a Mg-hydroxysilicate resembling sepiolite. Mg²⁺ removal from am-SiO₂ suspensions was investigated, and the relative role of the two removal processes evaluated, as a function of: pH, ionic strength, Mg²⁺ concentration, and temperature.The extent of Mg²⁺ adsorption onto am-SiO₂ decreases with increasing NaCl concentration due to displacement of Mg²⁺ by Na⁺. At NaCl concentrations of 0.05 M and above, adsorption occurs only at pH values above 8.5, where rapid dissolution of am-SiO₂ gives rise to high concentrations of dissolved silica, resulting in supersaturation with respect to sepiolite. Removal of Mg²⁺, at concentrations of 40 to 650 μM, from am-SiO₂ suspensions in 0.70 M NaCl at 25 °C occurs at pH 9.0 and above. Experiments show that under these conditions adsorption and Mg-hydroxysilicate precipitation remove Mg²⁺ at similar rates. For 0.05 M Mg²⁺, at 0.70 M ionic strength and 25 °C, measurable Mg²⁺ removal occurs down to ca. pH 7.5 but is primarily due to Mg-hydroxysilicate precipitation. For the same solution conditions at 5°C, Mg²⁺ removal occurs above pH 8.0 and is primarily due to adsorption.Assuming that increasing pressure does not greatly enhance adsorption, Mg²⁺ adsorption onto am-SiO₂ is an insignificant process in sea water. The surface charge of pristine am-SiO₂ in sea water is primarily controlled by interactions with Na⁺. The principal reaction between Mg²⁺ and am-SiO₂ in marine sediments is sepiolite precipitation.The age distribution of sepiolite in siliceous pelagic sediments is influenced by temperatures of bottom waters and by geothermal gradients.     

A theoretical study of the absorption spectra of Pb+ and Pb3+ in site K+ of microcline: application to the color of amazonite

The blue-green color of amazonite has been assigned by various authors to ions Pb⁺ (6 s)² (6 p) and/or Pb³⁺ (6 s) in site of K⁺ of microcline. Owing to the complex which forms between the ion Pb³⁺ and the lone pairs of the oxygen atoms surrounding it, the peripheral electron of Pb³⁺ passes on the levels (6 p) of the latter, which results in a great similarity of the spectra of Pb⁺ and Pb³⁺ in amazonite (the transition energies are multiplied by a factor greater than 1), whereas, in the isolated state, these spectra are completely different from one another. An analytical development of the crystal field around a site K⁺ is established. Under the effect of the crystal field, the transition ² P _1/2→² P _3/2 (6 p) is split into two double transitions. The lower transition only falls in the visible domain (1.6–1.8 eV for Pb⁺), the second in U−V. The green color would arise from the ion Pb⁺, whereas the blue one would be attributed to the ion Pb³⁺. Received: 23 January 1997 / Revised, accepted: 10 September 1997     

The sorption of toxic elements onto natural zeolite,synthetic goethite and modified powdered block carbon

Inorganic elements analyses of Carapicuíba lake reveal that As, Cr, Pb and Mn are above the recommended drinking water standards. The mean total concentrations of toxic elements in surface water decrease in the order Mn > Cr > Pb > As. At elevated concentrations, toxic elements like Cr can accumulate in soils and enter the food chain, leading to serious health hazards and threatening the long-term sustainability of the local ecosystem. Absorbing materials has often been used to improve water quality. In this investigation three types of material were studied: the natural zeolite (mordenite); synthetic goethite and the powdered block carbon modified. The adsorption of Pb²⁺ and Mn²⁺ onto natural zeolite as a function of their concentrations was studied at 24°C by varying the metal concentration from 100 to 400 mg L⁻¹ while keeping all other parameters constant. The low-cost zeolites removed Pb from water without any pretreatment at pH values <6. The maximum adsorption attained was as follows: Pb²⁺ 78.7% and Mn²⁺ 19.6%. The modified powdered block carbon effectively removed As(V) and Cr(VI) while goethite removed more chromate than arsenate in the pH range 5–6. Results of this study will be used to evaluate the application these materials for the treatment of the Carapicuíba lake’s water.     

Electron paramagnetic resonance and ENDOR studies of Cr3+ - Al3+ pairs in forsterite

The electron paramagnetic resonance (EPR) spectrum of Cr³⁺ in synthetic crystals of forsterite consists primarily of lines of Cr³⁺ “isolated” at the M1 and M2 positions in a “perfect” crystal environment without local charge compensation. In addition it shows two nonequivalent superhyperfine-split sextets with different intensities which are due to strong interaction of the Cr³⁺ electron spin S (S=3/2) with an adjacent nuclear spin I(I=5/2). Electron nuclear double resonance (ENDOR) experiments revealed that the sextets result from Cr³⁺ (M1) - Al³⁺ and Cr³⁺ (M2) - Al³⁺ pairs, Al being located at adjacent tetrahedral Si sites. The g, D, A, and A′ tensor components of the Cr³⁺ - Al³⁺ pairs have been determined at room temperature. The values of the pairs are distinct although they are not very different from the corresponding data of “isolated” Cr³⁺. From the intensities of the EPR spectra the relative concentration of the Cr³⁺ - Al³⁺ pairs with respect to “isolated” Cr³⁺ has been estimated.     

Fe2+ -F avoidance in silicates

Fe²⁺-F avoidance, reported in the literature in micas and amphiboles, can be accounted for by crystal field theory. The crystal field splitting parameter, Δ_O, of Fe²⁺ octahedrally coordinated to F^? is significantly smaller than its value when (OH)^? is the coordinating anion. Thus, the presence of Fe²⁺ is not favored at sites where F^? substitutes for (OH)^? due to smaller crystal field stabilization energy.     

Local relaxations around Fe3+ and Cr3+ in Al sites in minerals

Second-order zero-field splitting (ZFS) parameters from the literature for Fe³⁺ in twelve and for Cr³⁺ in seven minerals substituting for Al were evaluated by application of the superposition model. For Fe³⁺ in monoclinic site symmetries a fair agreement of the observed splitting patterns with those calculated from the crystal structure data was observed in most cases, but the distortions for Fe³⁺ appear to be usually larger than those of the unrelaxed Al sites. In cases of not too large local relaxation the unknown sign of the axialZFS parameterb ₀ ² could be predicted, in two cases a different sign than that reported was postulated. In cordierite and scolecite the reportedEPR spectra could thus be assigned to the sites with larger average bond distances. For Fe³⁺ in beryl the relaxation of the axial site can be deduced within narrow limits. For Cr³⁺ significantly larger differences between observed and calculatedZFS patterns are found suggesting additional relaxations due to the non-spherical electron distribution in the ground state of this ion.     

Study of the symmetry of Fe3+ sites in SnO2 by electron paramagnetic resonance

Electron paramagnetic resonance (EPR) of Fe³⁺ in SnO₂ has been realized in a natural single crystal of cassiterite at 9.55 GHz (X-band) and at 34.40 GHz (Q-band). Spectra show the simultaneous presence of four groups of independent signals, each one typical of the immediate environment of a specific paramagnetic iron. Fe³⁺ always substitutes Sn⁴⁺ in an octahedral site. The four paramagnetic centers are due to four different charge compensation mechanisms. The spin Hamiltonian constant values for the SN center and I1 center confirm the former results of the authors about for these two centers. SN and I1 present a weak deviation from axial symmetry. The first preserves the crystallographic local symmetry of the tin site and the second shows a symmetry deviation of 0.6° probably due to the presence of an OH group in the coordination polyhedron. On the other hand, for the Sd1 center and mostly for the Sd2 center, never previously subjected to single crystal EPR measurements, the study of spectra symmetry and the determination of B ₂ ⁰ and B ₂ ² constants produced new data. The Sd1 center could be due to a relaxation of the lattice together with a non local charge compensation mechanism. The Sd2 center presents a strong deviation from axial symmetry with mm local symmetry coordination due to coupling of Fe³⁺ and Nb⁵⁺. This coupling is proven by EPR studies of synthetic cassiterites doped with iron and niobium.     

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.     

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.     

Evidence for absorption by exchange-coupled Fe2+-Fe3+ pairs in the near infra-red spectra of minerals

Reduction of Fe³⁺ to Fe²⁺ by heating in hydrogen reduces the absorbance of the bands at 9,000 and 13,800 cm^?1 in the E ⊥ c spectrum of tourmaline, and the 9,000 and 11,000 cm^?1 bands in the E ∥ (001) spectrum of biotite. This behaviour is consistent with the presumed d-d origin of these bands (which seems well established) only if they gain much of their intensity from exchange-coupling with neighbouring Fe³⁺ ions. Intensification of spin-forbidden bands in sapphire by Fe³⁺-Fe³⁺ exchange-coupling was recognized by Ferguson and Fielding (1971, 1972), but exchange-coupling has not previously been thought to intensify spin-allowed d-d bands. Spin-allowed exchange-coupled bands resulting from Fe²⁺-Fe³⁺ pairs have features in common with both normal single ion d-d bands, which they resemble in energy, width and pressure dependence, and Fe²⁺+Fe³⁺→Fe³⁺+Fe²⁺ charge transfer bands, which they resemble in temperature-, heat treatment-, composition-, and polarization-dependence. Distinction between normal d-d, charge transfer, and pair d-d absorptions is thus complicated, and criteria for assigning these bands are discussed. Spin-allowed exchange-coupled pair bands should be sought in the spectra of transition metal clusters (trimers and polymers as well as pairs may be involved) whenever geometry favours their origin. It is possible that the bands near 10,000 and 11,500 cm^?1 in blue sapphire, and at about 5,000 cm^?1 in titanian garnets are of this type, but many other examples are likely to occur. Exchange-coupling may involve ions other than Fe³⁺ (e.g., Mn²⁺, also d ⁵), although Fe²⁺-Fe²⁺ coupling is unlikely to be important at laboratory temperatures.     

Experimentelle Sideritbildung aus Calcit + FeCl2

The stability relations of calcite and siderite in the system Ca²⁺-Fe²⁺-CO ₃ ^2– Cl ₂ ^2– -H₂O have been determined between 150 and 400° C in the pressure range from 250 to 2000 bars.It was found that the composition of the fluid phase coexisting with calcite and siderite is very poor in Fe²⁺ and correspondingly very rich in Ca²⁺ (see Tab. 1 and Fig. 3). The mole proportion Ca²⁺/(Ca²⁺+Fe²⁺) exceeds the value 0.98 at temperatures below 250° C when fluid pressure is 1000 bars. The stability field of calcite narrows with decreasing temperature, increasing pressure, and decreasing concentration of dissolved salts (CaCl₂₊+FeCl₂). In our experiments siderite becomes unstable at about 400° C (see p. 158).The experimental data indicate that siderite will be formed by reaction of calcite with FeCl₂-bearing solutions that have very low concentrations of Fe²⁺ as well as high Ca²⁺/(Ca²⁺+Fe²⁺)-ratios. Solutions coexisting with calcite must be very poor in Fe²⁺, otherwise siderite is formed.

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Herrn Prof. Dr. H. G. F. Wikkleb danke ich sehr für sein förderndes Interesse an dieser Arbeit und für eine kritische Durchsicht des Manuskriptes. Der Deutschen Forschungsgemeinschaft gebiihrt Dank für die Arbeitsmöglichkeit mit den Apparaturen, die Herrn Prof. Dr. Winkler zür Verfugung gestellt wurden.     

The Ti4+/Ti3+ ratio of magmatic melts: Application to the problem of the reduction of lunar basalts

Using published experimental data an expression was derived for the Ti⁴⁺/Ti³⁺ ratio as a function of temperature, oxygen fugacity, and melt composition. The equation can be used to estimate Ti³⁺ content in lunar basaltic melts. It was shown that the Ti³⁺ content in melts is probably no higher than the Fe3+ content even under the reduced conditions typical of lunar magmas. Trivalent Ti can lead to some decrease in $f_{O_2 }$ during melt cooling under closed-system conditions, but it cannot reduce Fe²⁺ in melt to metal, because it will be completely consumed by Fe³⁺ reduction to Fe²⁺. The presence of additional reducers, such as Cr²⁺, can be favorable for the formation of metal during melt cooling.