Fe–Mg Interdiffusion in (Mg,Fe)O

We have performed a series of interdiffusion experiments on magnesiowüstite samples at room pressure, temperatures from 1,320° to 1,400°C, and oxygen fugacities from 10^?1.0 Pa to 10^?4.3 Pa, using mixed CO/CO₂ or H₂/CO₂ gases. The interdiffusion couples were composed of a single-crystal of MgO lightly pressed against a single-crystal of (Mg_1-x Fe _x )_1-δO with 0.07<x<0.27. The interdiffusion coefficient was calculated using the Boltzmann–Matano analysis as a function of iron content, oxygen fugacity, temperature, and water fugacity. For the entire range of conditions tested and for compositions with 0.01<x<0.27, the interdiffusion coefficient varies as $$\tilde D\, =\,2.9\times10^{ - 6}\,f_{{\text{O}}_2 }^{0.19}\,x^{0.73}\,{\text{e}}^{ - (209,000\, -\,96,000\,x)/RT}\,\,{\text{m}}^{\text{2}} {\text{s}}^{ -1} $$ These dependencies on oxygen fugacity and composition are reasonably consistent with interdiffusion mediated by unassociated cation vacancies. For the limited range of water activity that could be investigated using mixed gases at room pressure, no effect of water on interdiffusion could be observed. The dependence of the interdiffusion coefficient on iron content decreased with increasing iron concentration at constant oxygen fugacity and temperature. There is a close agreement between our activation energy for interdiffusion extrapolated to zero iron content (x=0) and that of previous researchers who used electrical conductivity experiments to determine vacancy diffusivities in lightly doped MgO.     

Pressure dependence of Néel transition in (Mg,Fe)O

Pressure dependence of Néel temperature (T _N) in (Mg_0.20Fe_0.80)O, (Mg_0.25Fe_0.75)O, and (Mg_0.30Fe_0.70)O was newly measured up to 1.14 GPa, using superconducting quantum interference device magnetometer and piston–cylinder-type pressure cell under hydrostatic condition. The dT _N/dP values of (Mg_0.20Fe_0.80)O, (Mg_0.25Fe_0.75)O, and (Mg_0.30Fe_0.70)O were determined as 4.0 ± 0.3, 2.7 ± 0.3, and 4.4 ± 0.4 K/GPa, respectively, in linear approximation; however, the T _N deviated from the linearity under nonhydrostatic conditions. The compositional dependence of dT _N/dP in (Mg_1?X Fe _X )O showed a rapid decay with increasing Mg components at X ≥ 0.75 and the trend ended at X = 0.70. The estimated Néel transition pressure at room temperature by extrapolating these linearities are very similar to the rhombohedral distortion determined by previous X-ray diffraction studies for X ≥ 0.75, which suggests that the rhombohedral phase of (Mg_1?X Fe _X )O (X ≥ 0.75) at room temperature is antiferromagnetic under hydrostatic conditions.     

Analysis of nanomolar concentrations of phosphate in seawater with Liquid Waveguide after Mg (OH)_2 coprecipitation

Phosphorous is an essential nutrient used by plants and animals for growth and energy transport. Phosphate cycling studies are important in understanding biological processes, and phosphate analyses are a standard part of the biogeochemists’ toolbox. How…     

Influence of hydrogen on Fe–Mg interdiffusion in (Mg,Fe)O and implications for Earth’s lower mantle

Interdiffusion of Fe and Mg in (Mg,Fe)O has been investigated experimentally under hydrous conditions. Single crystals of MgO in contact with (Mg_0.73Fe_0.27)O were annealed hydrothermally at 300 MPa between 1,000 and 1,250°C and using a Ni–NiO buffer. After electron microprobe analyses, the dependence of the interdiffusivity on Fe concentration was determined using a Boltzmann–Matano analysis. For a water fugacity of ∼300 MPa, the Fe–Mg interdiffusion coefficient in Fe _x Mg_1−x O with 0.01 ≤ x ≤ 0.25 can be described by with and C = −80 ± 10 kJ mol⁻¹. For x = 0.1 and at 1,000°C, Fe–Mg interdiffusion is a factor of ∼4 faster under hydrous than under anhydrous conditions. This enhanced rate of interdiffusion is attributed to an increased concentration of metal vacancies resulting from the incorporation of hydrogen. Such water-induced enhancement of kinetics may have important implications for the rheological properties of the lower mantle.

硼掺入Mg(OH)2形式及机理

采用pH=9.5～13.0的无镁合成海水进行了Mg(OH)₂沉积时B的掺入实验,证实了B(OH)₃优先掺入Mg(OH)₂的硼同位素分馏特征。在所研究的pH范围内,Mg(OH)₂沉积的δ11B均高于无镁合成海水的δ11B,它们之间的硼同位素分馏系数α沉积/海水为1.017 7~1.056 9,平均为1.032 9±0.009 32(SD)。硼同位素的这种分馏特征与无机碳酸盐沉积时的硼同位素分馏存在明显差异,表明B掺入Mg(OH)₂沉积具有不同的机理。B在Mg(OH)₂沉积上的吸附以及B(OH)₃与Mg(OH)₂的沉积反应同时存在并相互制约是其主要特征,造成了B(OH)₃优先掺入的总结果,这并不意味B(OH)₃在掺入的分数上占有优势,相反在所研究的pH范围内,Mg(OH)₂沉积的B(OH)₃/B(OH)-₄大都小于1,因此吸附作用决定了Mg(OH)₂沉积中B浓度的变化特征。采用这种模型能很好地解释沉积中B浓度、B在沉积和海水间的分配系数Kd以及沉积与海水间的分馏系数α随海水pH的变化特征。石珊瑚中Mg(OH)₂的普遍存在和Mg(OH)₂中B(OH)₃的优先掺入也许会影响珊瑚的硼同位素组成与海水pH的定量对应关系,给δ11B作为古海水pH的代用指标带来一定的不确定性。     

Equilibrium experiments in the system MgO–SiO2–H2O (MSH): stability fields of clinohumite-OH [Mg9Si4O16(OH)2], chondrodite-OH [Mg5Si2O8(OH)2] and phase A (Mg7Si2O8(OH)6)

The water-pressure and temperature stability fields of clinohumite-OH, chondrodite-OH and phase A were determined in reversed equilibrium experiments up to 100 kbar within the system MgO–SiO₂–H₂O. Their PT-fields differ from results from former synthesis experiments. Bracketing experiments on the reaction phase A + low P-clinoenstatite ⇆ forsterite + water resulted in a slightly steeper dP/dT-slope compared to earlier experiments for this equilibrium. Clinohumite-OH and chondrodite-OH both have large stability fields which extend over pressure ranges of more than 80 kbar. However, they are hardly relevant as hydrous minerals within the subducted oceanic lithosphere. Both are too Mg-rich for a typical mantle bulk composition. In addition, the dehydration of subducted oceanic lithosphere – due to (forsterite + water)-forming reactions – will occur before the two humite-group phases even become stable. Restricted to the cool region of cold subducting slabs, phase A, however, might be formed via the reactions phase A + low P-/high P-clinoenstatite ⇆ forsterite + water or antigorite + brucite ⇆ phase A + water, before dehydration of the oceanic lithosphere occurs. Received: 22 July 1997 / Accepted: 12 March 1998     

Hydrostatic compression of γ-(Mg0.6, Fe0.4)2SiO4 to 50.0 GPa

The bulk modulus, K ₀, and its pressure derivative K₀, of -(Mg_0.6, Fe_0.4)₂SiO₄ have been accurately determined to 50.0 GPa under hydrostatic conditions at room temperature in a diamond cell using synchrotron radiation. Our results agree with Brillouin and ultrasonic measurements on -Mg₂SiO₄ at low pressure, indicating normal elastic behaviour in the metastable pressure range of this high pressure mineral. Our values of K ₀ and k₀ are 183.0 GPa and 5.4, respectively.     

Thermochemical study of Mg–Fe chlorites

The thermochemical study of two natural trioctahedral Mg–Fe chlorites—clinochlores was carried out using high-temperature melt solution calorimetry with a Tian–Calvet microcalorimeter. The enthalpies of formation of clinochlores of compositions (Mg_4.9Fe _0.3 ²⁺ Al_0.8)[Si_3.2Al_0.8O₁₀](OH)₈ (–8811 ± 12 kJ/mol) and (Mg_4.3Fe _0.7 ²⁺ Al_1.0)[Si_3.0Al_1.0O₁₀](OH)₈ (–8696 ± 13 kJ/mol) from elements were determined. The values of the standard entropies and the Gibbs energies of formation of the studied natural minerals as well as thermodynamic properties of Mg–Fe chlorites of theoretical composition were estimated.     

Thermal infrared (vibrational) spectroscopy of Mg–Fe olivines: A review and applications to determining the composition of planetary surfaces

The reststrahlen features in thermal infrared, or vibrational, spectra of Mg-Fe olivines ((Mg,Fe)₂SiO₄) exhibit trends in position, strength, and number of features that are diagnostic of the relative proportions of the Mg and Fe cations in the minerals. Although band positions move to lower wavenumbers (longer wavelengths) across the forsterite–fayalite compositional binary in a generally linear manner, specific feature shifts in transmittance data are described best by two linear fits with a break in slope near Fo₇₀. The break in slope may be accompanied by an offset as well; both traits are attributed to structural changes in olivine brought about by distortion of the crystal lattice by Fe. Reflectance and emissivity spectra exhibit similar trends in band position with composition, and all three types of data demonstrate that some olivine band strengths change across the Mg–Fe solid solution series and also are diagnostic of composition. Olivines have been identified in a wide array of thermal infrared spectra of planetary materials and have been interpreted as being present on the surfaces of Mercury, the Moon, Mars, and a number of asteroids based on the analysis of thermal infrared spectra. New linear least squares models of the emissivity spectra of olivine-bearing Martian meteorites enable a preliminary estimation of the accuracy with which quantitative estimates of olivine abundance and solid solution composition can be derived from the spectra of mixtures.     

Formation of helictite in the cave Dragon Belly (Sardinia,Italy)—Microstructure and incorporation of Mg,Sr, and Ba

In the Dragon Belly cave helictites, a special type of irregular speleothem, are found, which grew on stalactites in all vertical and horizontal directions without any affinity to gravity. Microstructural and mineralogical analyses of this stalactite–helictite system indicate that its evolution is initiated by clogging of the central stalactite channel at its tip, probably when the cave was flooded by muddy water. Clogging caused the formation of secondary channels (≈0.2 mm in diameter) for water passage through the outer surface of the stalactite, where helictites start to grow. The secondary channel passes into the central channel of the helictite.The helictites consist of stacked idiomorphic calcite crystals with uniform orientation. Growth of calcite is essentially controlled by water transfer through the central channel and via canalicules (narrow channels of ≈0.05 mm in diameter) following the crystal boundaries of the calcite mesocrystal induced by capillary hydrostatic forces. At straight parts of the helictites calcite crystals are almost uniform in size, but at bended parts crystals are significantly smaller inside (≤0.1 mm in length) than outside of the bend (≤0.5 mm). It is proposed that the difference in calcite volume (larger crystals) vs. the inside of the bend leads to a helix form, which explains the origin of the term helictite.The Sr and Ba concentrations measured by laser ablation along helictites can be explained by cation incorporation during calcite precipitation close to equilibrium. Dilution effects caused by seasonality control the elemental distribution in the helictite, which result in a positive correlation between Sr and Ba. Variability of Mg is unrelated with Sr and Ba, and is probably due to the incongruent dissolution of Mg–calcite from the host rock.     

Paired Measurements of Foraminiferal δ~ (18) O and Mg/Ca Ratios of Indian Monsoons Reconstructed from Holocene to Last Glacial Record

The effect of seasonally reversing monsoons in the northern Indian Ocean is to impart significant changes in surface salinity(SS).Here,we report SS changes during the last 32 kyr in the Lakshadweep Sea(southeastern Arabian Sea)estimated from paired measurements of δ~(18)O and sea surface temperature(SST)using Globigerinoides sacculifer,an upper mixed layer dwelling foraminifera.The heaviest δ~(18)O_(G.sacculifer)(-0.07±0.08‰)is recorded between 23 and 15 ka,which could be defined as the last glacial maxi...     

Effects of Mg and Si ions on the symmetry of δ-AlOOH

We conducted powder neutron diffraction for δ-AlOOH samples with and without Mg and Si ions under ambient conditions in order to investigate the long-standing problem of the symmetry of this phase. The observed reflection conditions clearly show that the space group of pure δ-AlOOH is P2₁ nm with ordered hydrogen bonds, whereas that of δ-(Al_0.86Mg_0.07Si_0.07)OOH is Pnnm or Pnn2 with disordered hydrogen bonds. It is more likely that the space group of δ-(Al_0.86Mg_0.07Si_0.07)OOH is Pnnm, because cation or hydrogen ordering that breaks the mirror plane perpendicular to c axis in the Pnnm structure would not occur. The previously reported inconsistency for the space group of this phase was caused by the substitution of Mg and Si ions to Al site, i.e., the disordered cations with different valences may fluctuate hydrogen positions, and the disordered hydrogen causes the symmetry change.     

Occurrence of wagnerite in Mg–Al granulites of Sonapahar,Meghalaya

We report for the first time the occurrence of rare phosphate wagnerite as a stable phase from the Mg–Al granulites of Sonapahar. The wagnerite bearing assemblages consist of the spinel, phlogopite, brucite and corundum. The wagnerite appears in the Mg–Al granulites due to the break-down of spinel and fluorapatite. The mineral chemistry of the phases has been discussed from the EPMA data, which reveals that the fluorine content of the wagnerite is relatively low due to the exchange of F to coexisting phases. The major oxide analysis of the rocks show the low content of Ca, which is the requisite for the occurrences of wagnerite.     

A Raman spectroscopic study of Fe–Mg olivines

End-member synthetic fayalite and forsterite and a natural solid-solution crystal of composition (Mg_1.80,Fe_0.20)SiO₄ were investigated using Raman spectroscopy. Polarized single-crystal spectra were measured as a function of temperature. In addition, polycrystalline forsterite and fayalite, isotopically enriched in ²⁶Mg and ⁵⁷Fe, respectively, were synthesized and their powder spectra measured. The high-wavenumber modes in olivine consist of internal SiO₄ vibrations that show little variation upon isotopic substitution. This confirms conclusions from previous spectroscopic studies that showed that the internal SiO₄ vibrations have minimal coupling with the lower-wavenumber lattice modes. The lowest wavenumber modes in both forsterite and fayalite shift in energy following isotopic substitution, but with energies less than that which would be associated with pure Mg and Fe translations. The low-wavenumber Raman modes in olivine are best described as lattice modes consisting to a large degree of mixed vibrations of M(2) cation translations and external vibrations of the SiO₄ tetrahedra. The single-crystal spectra of forsterite and Fo₉₀Fa₁₀ were recorded at a number of temperatures from room temperature to about 1200 °C. From these data the microscopic Grüneisen parameters for three different A_g modes for both compositions were calculated, and also the structural state of the solid solution crystal was investigated. Small discontinuities observed in the wavenumber behavior of a low-energy mixed Mg/T(SiO₄) mode between 700 and 1000 °C may be related to minor variations in the Fe–Mg intracrystalline partitioning state in the Fo₉₀Fa₁₀ crystal, but further spectroscopic work is needed to clarify and quantify this issue. The mode wavenumber and intensity behavior of internal SiO₄ vibrations as a function of temperature are discussed in terms of crystal field and dynamic splitting and also ₁ and ₃ coupling. Crystal-field splitting increases only very slightly with temperature, whereas dynamical-field splitting is temperature dependent. The degree of ₁–₃ coupling decreases with increasing temperature.     

改性Mg(OH)2对多种重金属污染土壤的修复效果

为了研究一种高效的多种重金属污染土壤修复剂,本文采用了一种具有OH^-缓释功能的改性Mg（OH）₂,通过重金属污染土壤稳定化修复实验,探讨了改性Mg（OH）₂对污染土壤中多种重金属（Pb、Cd、Cu、Zn）的稳定效率及对多种重金属形态分布的影响。结果表明,投加改性Mg（OH）₂对土壤中多种重金属均有稳定作用,对Pb、Cd、Cu、Zn的稳定效率分别为72.42%、34.53%、87.64%和97.65%,且改性Mg（OH）₂的投加使重金属交换态质量明显减少、残渣态质量增加,进一步提高了重金属的稳定性,降低了重金属生物有效性;另外,改性Mg（OH）₂具有OH^-缓释性,可使土壤长期保持一定的碱性,是一种经济有效的土壤修复剂。     

Heat capacity anomalies at incommensurate-normal transition of åkermanite solid solution (Ca,Sr)2(Mg,Co, Zn,Fe)Si2O7

The heat capacity of åkermanite solid solutions was measured by a small scale adiabatic calorimeter near the incommensurate-normal (I-N) transition. The heat capacity anomalies caused by the I-N transition show the type characteristic behavior implying the presence of dynamical fluctuations. The heat capacity anomalies were observed over the whole range of the åkermanite solid solutions Ca₂Mg_1-xCo_xSi₂O₇ and Ca₂Mg_1-x-Zn_xSi₂O₂. With increase of Co or Zn atoms, the transition temperature, T_i, rises linearly from ca. 83° C to 220° C and to 130° C, respectively. In the system Ca₂CoSi₂O₇-Ca₂FeSi₂O₇ and Ca₂MgSi₂O₇-Ca₂-FeSi₂O₇ electronic microscopy revealed that the temperature of the heat capacity anomaly decreases with increasing Fe content, whereas the T_i rises. This unusual behavior is ascribed to the microdomains observed in high resolution lattice images.     

Surface complexation of U(VI) on goethite (α-FeOOH)

Sorption of U(VI) to goethite is a fundamental control on the mobility of uranium in soil and groundwater. Here, we investigated the sorption of U on goethite using EXAFS spectroscopy, batch sorption experiments and DFT calculations of the energetics and structures of possible surface complexes. Based on EXAFS spectra, it has previously been proposed that U(VI), as the uranyl cation , sorbs to Fe oxide hydroxide phases by forming a bidentate edge-sharing (E2) surface complex, >Fe(OH)₂UO₂(H₂O)_n. Here, we argue that this complex alone cannot account for the sorption capacity of goethite (α-FeOOH). Moreover, we show that all of the EXAFS signal attributed to the E2 complex can be accounted for by multiple scattering. We propose that the dominant surface complex in CO₂-free systems is a bidentate corner-sharing (C2) complex, (>FeOH)₂UO₂(H₂O)₃ which can form on the dominant {101} surface. However, in the presence of CO₂, we find an enhancement of UO₂ sorption at low pH and attribute this to a (>FeO)CO₂UO₂ ternary complex. With increasing pH, U(VI) desorbs by the formation of aqueous carbonate and hydroxyl complexes. However, this desorption is preceded by the formation of a second ternary surface complex (>FeOH)₂UO₂CO₃. The three proposed surface complexes, (>FeOH)₂UO₂(H₂O)₃, >FeOCO₂UO₂, and (>FeOH)₂UO₂CO₃ are consistent with EXAFS spectra. Using these complexes, we developed a surface complexation model for U on goethite with a 1-pK model for surface protonation, an extended Stern model for surface electrostatics and inclusion of all known UO₂-OH-CO₃ aqueous complexes in the current thermodynamic database. The model gives an excellent fit to our sorption experiments done in both ambient and reduced CO₂ environments at surface loadings of 0.02-2.0 wt% U.     

Seasonal changes of fulvic acid,Ca and Mg concentrations of water samples collected above and in the Béke Cave of the Aggtelek karst system (Hungary)

Magnesium and Ca concentration ratios, fulvic acid content, total dissolved inorganic carbon (DIC) and pH were determined in seepage water and drip water samples collected during one seasonal cycle between June 2000 and May 2001 above and in the Béke Cave of Aggtelek (Hungary). Seepage water samples were collected at 0.5 and 7 m below ground level from an observation point situated above the cave. Drip water was collected 40 m underground from a group of stalactites. The fulvic acid concentrations were determined by fluorescence spectrometry after pre-concentration on a XAD-8 chromatographic column. Calcium and Mg concentrations were measured by inductively coupled plasma atomic-emission spectrometry. DIC was determined with a CO₂ – selective electrode. DIC values increased and the fulvic acid concentrations and Mg and Ca concentration ratios, generally, decreased with depth. The highest flux of fulvic acid was observed in spring. The fulvic acid flux increased by a factor of 2.6–3.6 and 1.4 for groundwater and drip water, respectively, compared with those registered in the winter samples. The variations in the Ca, Mg and fulvic acid concentrations of the seepage and drip water samples relate to the variable drip rate. The results revealed that there is a strong correlation between the daily average surface temperature, daily amount of precipitation and drip water rate registered in the cave.     

Structural Evolvement of Heating Treatment of Mg/AI-LDH and Preparation of Mineral Mesoporous Materials

Although hydrotalcite, or layered double hydroxides （LDHs）, is not a common mineral, it is an important material that can be easily synthesized in laboratory. In this study, structural evolvement and BET surface area changes of heat treated Mg/AI-LDH is evaluated by XRD, TEM and N2-BET analyses. The results indicate that the magnesium-aluminum LDH with carbonate as interlayer anion, periclase-like oxides was formed at temperatures of 400-800℃. Meanwhile, 2-3 nanometer mesoporous were formed during decomposition of LDH. However, the heat treated samples still preserve the morphology of the original LDH plates. Periclase-like formed from LDH heat treatment may re-hydrolyze and recover the structure of LDH. However, crystallinity of the recovered LDH is lower than that of the original LDH. This heat treatment will result in formation of （Mg, Al）-oxide nano-crystals and nanopores among the nano-crystals. When heating temperature exceeds 1000, the periclase-like （Mg, Al）-oxide is transformed into a composite with periclase （MgO） and spinel phases. The periclase can be re-hydrolyzed and dissolved in HCl solution. After acid treatment, the sample with a high surface area is composed of spinel nano-crystals and nanopores among them. Our results will provide a new and economic way to synthesize mesoporous materials through pathways of phase transformation of precursor materials with different composition.