Nonstoichiometry and growth of some Fe carbides

Iron carbides containing from 31 to 17 atomic % carbon, with cohenite XRD structure and optical properties, were grown in experiments in Fe–Ni–S–C, Fe–Ni–C, and in Fe–C at 1, 6, and 7 GPa. X-ray cell volumes increase with C content. Compositions listed above vary considerably outside the nominal (Fe,Ni)₃C stoichiometry of cohenite/cementite. Cohenites coexisting with Fe–C liquid are carbon poor. The Eckstrom-Adcock carbide, nominally Fe₇C₃, was found to show compositions from 29 to 36 atomic % C at 7 GPa in Fe–C. Both these materials are better regarded as solutions than as stoichiometric compounds, and their properties such as volume have compositional dependencies, as do the iron oxides, sulfides, silicides, and hydrides. The fraction of C dissolved in cohenite-saturated alloy is found to become smaller between 1 and 7 GPa. If this trend continues at higher pressures, the deep mantle should be easier to saturate with carbide than the shallow mantle, whether or not carbide is metastable as at ambient pressure. At temperatures below the cohenite-graphite peritectic, cohenite may grow as a compositionally zoned layer between Fe and graphite. The Eckstrom-Adcock carbide joins the assemblage at 7 GPa. Phases appear between Fe and C in an order consistent with metasomatic interface growth between chemically incompatible feed stocks. Diffusion across the carbide layer is not the growth rate limiting step. Carbon transport along the grain boundaries of solid Fe source stock at 1 GPa, to form C-saturated Fe alloy, is observed to be orders of magnitude faster than the cohenite layer growth. Growth stagnates too rapidly to be consistent with diffusion control. Furthermore, lateral variations in carbide layer thickness, convoluted inert marker horizons, and variable compositional profiles within the layers suggest that there are local transport complexities not covered by one-dimensional diffusive metasomatic growth. In contrast to many transport phenomena which slow with pressure, at 7 GPa and 1,162 °C, carbide growth without open grain boundaries is faster than at 1 GPa with fast grain boundary channels, again suggesting C transport is less of a constraint on growth than C supply. C supply at 7 GPa is enhanced by graphite metastability and the absence of fast grain boundary channels to divert C into the Fe instead of growing carbide. At both 1 and 7 GPa, the growth rate of carbide is found to systematically vary depending on which of two stock pieces of graphite are used to form the growth couple, suggesting that some property of each specific graphite, like C release rate, possibly from amorphous binder material, may influence the cohenite growth process. At temperatures near and above the cohenite-graphite peritectic at 1–1.5 GPa, complex intergrowths involving Fe–C liquids and extensive thermal migration transport were encountered, eroding the organized spatial resolution, and the range of cohenite compositions found grown below this peritectic from growth couples of crystalline Fe and graphite. The migration of graphite to a position in the metasomatic sequence between liquid and cohenite demonstrates that the solubility of graphite in liquid increases with temperature above the peritectic, whereas the solubility of graphite in cohenite below the peritectic decreases with temperature. The variable solubility of graphite in cohenite, shown by thermal migration, emphasizes that cohenite does have compositional variations.     

The formation of graphite upon the interaction of subducted carbonates and sulfur with metal-bearing rocks of the lithospheric mantle

Experimental studies of the Fe⁰–(Mg, Ca)CO₃–S system were carried out during 18–20 h at 6.3 GPa, 900–1400°C. It is shown that the major processes resulting in the formation of free carbon include reduction of carbonates upon redox interaction with Fe⁰ (or Fe₃C), extraction of carbon from iron carbide upon interaction with a sulfur melt/fluid, and reduction of the carbonate melt by Fe–S and Fe?S–C melts. Reconstruction of the processes of graphite formation indicates that carbonates and iron carbide may be potential sources of carbon under the conditions of subduction, and participation of the sulfur melt/fluid may result in the formation of mantle sulfides.     

Interaction of Fe and Fe<Subscript>3</Subscript>C with hydrogen and nitrogen at 6–20 GPa: a study by in situ X-ray diffraction

A method of in situ X-ray diffraction at Spring-8 (Japan) was used to analyze simultaneously the hydrogen incorporation into Fe and Fe₃C, as well as to measure the relative stability of carbides, nitrides, sulfides, and hydrides of iron at pressures of 6–20 GPa and temperatures up to 1600 K. The following stability sequence of individual iron compounds was established in the studied pressure and temperature interval: FeS > FeN > FeC > FeH > Fe. A change in the unit-cell volume as compared to the known equations of state was used to estimate the hydrogen contents in carbide Fe₃C and hydride FeH_x. Data on hydride correspond to stoichiometry with x ≈ 1. Unlike iron sulfides and silicides, the solubility of hydrogen in Fe₃C seemed to be negligibly low—within measurement error. Extrapolating obtained data to pressures of the Earth’s core indicates that carbon and hydrogen are mutually incpompatible in the iron–nickel core, while nitrogen easily substitutes carbon and may be an important component of the inner core in the light of the recent models assuming the predominance of iron carbide in its composition.     

Formation of the Fe,Mg-Silicates,Fe<Superscript>0</Superscript>, and Graphite (Diamond) Assemblage as a Result of Cohenite Oxidation under Lithospheric Mantle Conditions

Experimental studies in the Fe₃C–SiO₂–MgO system (P = 6.3 GPa, T = 1100–1500°C, t = 20–40 h) have been carried out. It has been established that carbide-oxide interaction resulted in the formation of Fe-orthopyroxene, graphite, wustite, and cohenite (1100 and 1200°C), as well as a Fe–C–O melt (1300–1500°C). The main processes occurring in the system at 1100 and 1200°C are the oxidation of cohenite, the extraction of carbon from carbide, and the crystallization of metastable graphite, as well as the formation of ferrosilicates. At T ≥ 1300°C, graphite crystallization and diamond growth occur as a result of the redox interaction of a predominantly metallic melt (Fe–C–O) with oxides and silicates. The carbide–oxide interaction studied can be considered as the basis for modeling a number of carbon-producing processes in the lithospheric mantle at fO₂ values near the iron–wustite buffer.     

Conditions of Formation of Iron–Carbon Melt Inclusions in Garnet and Orthopyroxene under <Emphasis Type="Italic">P-T</Emphasis> Conditions of Lithospheric Mantle

Of great importance in the problem of redox evolution of mantle rocks is the reconstruction of scenarios of alteration of Fe⁰- or Fe₃C-bearing rocks by oxidizing mantle metasomatic agents and the evaluation of stability of these phases under the influence of fluids and melts of different compositions. Original results of high-temperature high-pressure experiments (P = 6.3 GPa, T = 1300–1500°С) in the carbide–oxide–carbonate systems (Fe₃C–SiO₂–(Mg,Ca)CO₃ and Fe₃C–SiO₂–Al₂O₃–(Mg,Ca)CO₃) are reported. Conditions of formation of mantle silicates with metallic or metal–carbon melt inclusions are determined and their stability in the presence of CO₂-fluid representing the potential mantle oxidizing metasomatic agent are estimated. It is established that garnet or orthopyroxene and CO₂-fluid are formed in the carbide–oxide–carbonate system through decarbonation, with subsequent redox interaction between CO₂ and iron carbide. This results in the formation of assemblage of Fe-rich silicates and graphite. Garnet and orthopyroxene contain inclusions of a Fe–C melt, as well as graphite, fayalite, and ferrosilite. It is experimentally demonstrated that the presence of CO₂-fluid in interstices does not affect on the preservation of metallic inclusions, as well as graphite inclusions in silicates. Selective capture of Fe–C melt inclusions by mantle silicates is one of the potential scenarios for the conservation of metallic iron in mantle domains altered by mantle oxidizing metasomatic agents.     

Graphite and diamond formation via the interaction of iron carbide and Fe,Ni-sulfide under mantle <Emphasis Type="Italic">P–T</Emphasis> parameters

Experimental research in the Fe₃C–(Fe,Ni)S system was carried out. The objective of the investigation was to model the reactions of carbide–sulfide interaction related to graphite (diamond) formation in reduced lithosphere mantle domains. T ≤ 1200°C is the formation temperature of the Ni-cohenite + graphite assemblage coexisting with two immiscible melts such as sulfide (Fe₆₀–Ni₃–S₃₇)_L and metal–sulfide (Fe₇₁–Ni₇–S₂₁–C₁)_L containing dissolved carbon. Т ≥ 1300°C is the generation temperature of a unified melt such as (Fe₈₀–Ni₆–S₁₀–C₄)_L characterized by graphite crystallization and diamond growth. The extraction of carbide carbon during the interaction with the sulfide melt can be considered as one of the potential mechanisms of graphite and diamond formation in the reduced mantle.     

新型热压WC-Fe基金刚石钻头钻进试验

为满足硬岩钻探的需要,将传统的热压碳化钨(WC)基金刚石钻头胎体中一部分WC用Fe代替,用Excel中的回归分析和非线性规划求解计算出的最优胎体配方,试制了新型热压WC-Fe基金刚石钻头,在可钻性为8级以上的岩石上进行了室内、外钻进试验,取得了较好的效果。      

西藏蛇绿岩中硅铁合金组合及成因探讨

在雅鲁藏布江蛇绿岩带的东端罗布莎蛇绿岩的铬铁矿中发现不寻常的硅铁合金组合。4种硅铁合金的理论分子式分别为Fe0.84Si2.00(Fe3Si7)、Fe7Si3、Fe6Si4以及Fe4Ti3Si2P。这些合金均选自铬铁矿石的人工重砂中，在少数颗粒中见到由3种硅铁合金组成的交生结构，明显表明这些合金的同生和同成因性。推测它们为化学反应成因,来自于核幔边界地带。     

Removal of selected metals from drinking water using modified powdered block carbon

This paper presents the possible alternative removal options for the development of safe drinking water supply in the trace elements affected areas. Arsenic and chromium are two of the most toxic pollutants, introduced into natural waters from a variety of sources and causes various adverse effects on living bodies. Performance of three filter bed method was evaluated in the laboratory. Experiments have been conducted to investigate the sorption of arsenic and chromium on carbon steel and removal of trace elements from drinking water with a household filtration process. The affinity of the arsenic and chromium species for Fe/Fe₃C (iron/iron carbide) sites is the key factor controlling the removal of the elements. The method is based on the use of powdered block carbon (PBC), powder carbon steel and ball ceramic in the ion-sorption columns as a cleaning process. The PBC modified is a satisfactory and practical sorbent for trace elements (arsenite and chromate) dissolved in water.     

Effect of sulfur concentration on diamond crystallization in the Fe–C–S system at 5.3–5.5 GPa and 1300–1370°C

The paper presents results of experiments aimed at diamond synthesis in the Fe–C–S system at 5.3–5.5 GPa and temperatures of 1300–1370°C and detailed data on the microtextures of the experimental samples and the composition of the accompanying phases (Fe₃C and Fe₇C₃ carbides, graphite, and FeS). It is demonstrated that diamond can be synthesized after temperatures at which carbides are formed are overcome and can crystallize within the temperature range of 1300°C (temperature of the peritectic reaction melt + diamond = Fe₇C₃) to 1370°C (of thermodynamically stable graphite) under the appearance experimental pressure. The possible involvement of natural metal- and sulfur-bearing compounds in the origin of natural diamond is discussed.     

Kinetics of the selective extraction of iron oxides in geochemical samples. Association between Fe and Cu in acid brown soils

The use of Tamm's reagent at 60°C and at different reaction times enables the progressive dissolution of iron oxides from the different horizons of acid brown soils. This technique is used to evaluate the association between Cu and Fe in soils from a zone mineralized in Cu. This association is quantified by the coefficient α (moles Cu/moles Fe) which represents the degree of doping of the iron oxides by Cu. For the B horizon of these soils, α values range from 5 to 10 × 10^-3 mole Cu/mole Fe.Calculation of the coefficient α (range = 0.4 to 2.0 × 10^-3) for 150 soil samples of the same type enabled the differentiation of two geochemical populations. Treatment of the total Cu values of these samples did not permit such differentiation.     

Formation and destruction of magnetite in CO3 chondrites and other chondrite groups

Primitive CO3.00–3.1 chondrites contain ∼2-8 vol.% magnetite, minor troilite and accessory carbide and chromite; some CO3.1 chondrites have fayalite-rich veins, chondrule rims and euhedral matrix grains. All CO3.00–3.1 chondrites contain little metallic Fe-Ni (0.4–1.2 vol.%). CO3.2–3.7 chondrites contain 1–5 vol.% metallic Fe-Ni, minor troilite, accessory chromite and 0-0.6 vol.% magnetite. Magnetite is formed in primitive CO3 chondrites from metallic Fe by parent-body aqueous alteration, resulting in decreased metallic Fe-Ni and an increase in the proportion of high-Ni metal grains. The paucity or absence of magnetite in CO chondrites of subtype ≥3.2 suggests that magnetite is destroyed during thermal metamorphism; thermochemical calculations from the literature suggest that magnetite is reduced by H₂ and reacts with SiO₂ to form fayalite and secondary kamacite. Analogous processes of magnetite formation and destruction occur in other chondrite groups: (1) Primitive type-3 OC have opaque assemblages containing magnetite, carbide, Ni-rich metal and Ni-rich sulfide, but OC of subtype >3.4 contain little or no magnetite. (2) Primitive R3 chondrites and clasts (subtype ≲3.5) contain up to 6 vol.% magnetite, but most R chondrites contain no magnetite. The principal exception is magnetite with 9–20 wt.% Cr₂O₃ in a few R4-6 chondrites. Magnetite grains with high Cr₂O₃ behave like chromite and are more stable under reducing conditions. (3) CK chondrites average ∼4 vol.% magnetite with substantial Cr₂O₃ (up to ∼15 wt.%); these magnetite grains also are stable against reduction during metamorphism. (4) The modal abundance of magnetite decreases with metamorphic grade in CV3 chondrites. (5) Chromite occurs instead of magnetite in those rare samples classified CR6, CR7 and CV7.     

Predicting arsenic concentrations in the porewaters of buried uranium mill tailings

The proposed JEB Tailings Management Facility (TMF) to be emplaced below the groundwater table in northern Saskatchewan, Canada, will contain uranium mill tailings from McClean Lake, Midwest and Cigar Lake ore bodies, which are high in arsenic (up to 10%) and nickel (up to 5%). A serious concern is the possibility that high arsenic and nickel concentrations may be released from the buried tailings, contaminating adjacent groundwaters and a nearby lake. Laboratory tests and geochemical modeling were performed to examine ways to reduce the arsenic and nickel concentrations in TMF porewaters so as to minimize such contamination from tailings buried for 50 years and longer. The tests were designed to mimic conditions in the mill neutralization circuit (3 hr tests at 25°C), and in the TMF after burial (5–49 day aging tests). The aging tests were run at, 50, 25 and 4°C (the temperature in the TMF). In order to optimize the removal of arsenic by adsorption and precipitation, ferric sulfate was added to tailings raffinates¹ having Fe/As ratios of less that 3–5. The acid raffinates were then neutralized by addition of slaked lime to nominal pH values of 7, 8, or 9.Analysis and modeling of the test results showed that with slaked lime addition to acid tailings raffinates, relatively amorphous scorodite (ferric arsenate) precipitates near pH 1, and is the dominant form of arsenate in slake limed tailings solids except those high in Ni and As and low in Fe, in which cabrerite-annabergite (Ni, Mg, Fe(II) arsenate) may also precipitate near pH 5–6. In addition to the arsenate precipitates, smaller amounts of arsenate are also adsorbed onto tailings solids.The aging tests showed that after burial of the tailings, arsenic concentrations may increase with time from the breakdown of the arsenate phases (chiefly scorodite). However, the tests indicate that the rate of change decreases and approaches zero after 72 hrs at 25°C, and may equal zero at all times in the TMF at 4°C. Consistent with a kinetic model that describes the rate of breakdown of scorodite to form hydrous ferric oxide, the rate of release of dissolved arsenate to tailings porewaters from slake limed tailings: (1) is proportional to pH above pH 6–7; (2) decreases exponentially as the total molar Fe/As ratio of tailings raffinates is increased from 1/1 to greater than 5/1; and (3) is proportional to temperature with an average Arrhenius activation energy of 13.4 ± 4.2 kcal/mol.Study results suggest that if ferric sulfate and slaked lime are added in the tailings neutralization circuit to give a raffinate Fe/As molar ratio of at least 3–5 and a nominal (initial) pH of 8 (final pH of 7–8), arsenic and nickel concentrations of 2 mg/L or less, are probable in porewaters of individual tailings in the TMF for 50 to 10,000 yrs after tailings disposal. However, the tailings will be mixed in the TMF, which will contain about 35% tailings with Fe/As = 3.0, and 65% tailings with Fe/As = 5.0–7.7. Thus, it seems likely that average arsenic pore water concentrations in the TMF may not exceed 1 mg/L.     

江汉平原沙湖地区浅层含水层第四纪沉积环境演化

地下含水介质是地球关键带的重要研究对象,为了拓展江汉平原地下水环境问题研究维度,综合应用14C测年、δ13C、铁价态以及沉积物溶解态有机质(dissolved organic matter,DOM)的光谱信息等重要环境指标,对沙湖地区近30 ka的浅层含水层第四纪沉积环境演化特征进行了概化分析.沉积物中Zr/Rb、Rb/Sr比值以及δ13C值表明,剖面在5~7 m和16~18 m范围相对其他深度更显暖湿气候特征;Fe3+/Fe2+比值及沉积物颜色说明这两段暖湿期沉积物主要呈现还原环境.基于因子分析和DOM光谱信息可将沉积剖面的沉积环境演化概化为“近地表弱生物碎屑沉积单元”和“暖湿期强沉积单元”等5个单元.本沉积剖面所记录的沉积环境演化信息可为该区的地下水环境等相关问题的深入研究提供重要的背景支撑.      

长江口崇明东滩沉积物地球化学记录及其环境意义

利用2002年在崇明东滩采得的CDS、CDM和CDN三个典型区域沉积物剖面样品,测定了其中的有机碳、活性铁、总磷以及粒度等特征参数,分析了地球化学元素的分布变化特征并对其沉积环境的变化进行了探讨。结果表明崇明东滩沉积物中有机碳的含量较大(0．3％～1．07％),垂向分布上,表层／亚表层含量高且变化复杂,中下层有机碳含量逐渐变小。Fe3 在整个研究区域均是从表层向下逐渐递减的,Fe2 含量逐渐增加。从实验结果判断,所研究区域沉积环境上层以氧化环境为主,呈弱氧化型,中下层以还原环境为主。总磷含量呈现自上而下减少的变化,CDS表现为波浪形变化。分析发现潮滩沉积物各地球化学元素之间不仅相互作用,并且受到沉积物颗粒大小和水动力、物源输入、物理扰动等因素的影响。崇明东滩沉积物氧化还原界面与中、高潮滩划分界大致相当。     

含钙铝铁水解聚合产物的矿物学研究Ⅰ:形态和物相

用X射线粉晶衍射分析技术结合热重分析、红外光谱及电镜观察对合成的含钙聚合氯化铝铁(PAFCCa)及其合成前体含钙聚合氯化铝(PACCa)、含钙聚合氯化铁(PFCCa)的低温干燥样进行了表征.XRD衍射结果表明,Ca(Ⅱ) 分别与Al(Ⅲ)、Fe(Ⅲ)都能形成结晶化合物,在PACCa中结晶化合物主要是Ca3Al2(OH)12,在PFCCa中结晶化合物主要是Ca4Fe14O25,在PAFCCa中形成Al-Ca-Fe三相结晶化合物铁取代的氯钙矾石结构体,其组成是(Ca6(Al,Fe)2[(OH)4Cl2]3·20H2O).热重分析、红外光谱与电镜观察都支持了XRD的衍射结果.     

The oxidation of pyrite at pH 7 in the presence of reducing and nonreducing Fe(III)-chelators

The oxidation rate of pyrite at pH 7, 25°C and at constant partial pressure of oxygen (0.21 and 0.177 atm) was measured in the presence of the Fe(III)-chelators NTA, oxalate, leucine, EDTA, citrate, IDA and the Fe(III)-reductant ascorbic acid. With the exception of leucine and EDTA, non-reducing Fe(III)-chelators increased the oxidation rate relative to the reference state of formation of the Fe(OH)₂⁺ complex at pH 7. The rate increase was proportional to the logarithm of the conditional stability constant of the ligands for the complexation of Fe³⁺. No effect on the oxidation rate was observed in the presence of EDTA, which shifted the redox potential of the redox couple Fe²⁺/Fe³⁺ to a value below that in the absence of any ligand at pH 7. Ascorbic acid decreased the pyrite oxidation rate by a factor of 5 at ascorbic acid concentrations between 10⁻⁴ and 10⁻² mol L⁻¹. Comparison of the rate constants for the oxidation of ascorbic acid by surface bound Fe(III) in the absence and presence of pyrite shows that the pyrite surface accelerates this reaction by a factor of 10. The oxidation of both pyrite and ascorbic acid is of fractional order with respect to ascorbic acid (HAsc): r_py=0.55 c(HAsc)^−0.35 r_HAsc=3.6 c(HAsc)^0.59. Both the results from experiments with Fe(III)-chelating ligands and the Fe(III)-reductant, suggest a very efficient interference in the electron cycling between Fe(II) and Fe(III) at the pyrite surface. The interference seems to be mainly related to the reductive side of the iron cycling. It is therefore concluded that the electron transfer between ferric iron and pyritic sulfur limits the pyrite oxidation rate at pH 7.     

新型热压WC-Fe基金刚石钻头胎体性能研究

为了提高钻头在坚硬致密岩层中的钻进效果并降低钻头成本,将传统的热压碳化钨（WC）基金刚石钻头胎体中一部分WC用Fe代替。用粉末冶金的方法烧制胎体试块并测试其性能。通过对比性能测试,分析了碳化钨和铁的含量变化对胎体性能的影响规律。应用回归分析和规划求解,分别建立了硬度和耐磨性的回归方程,确定了最优的胎体硬度和耐磨性以及对应的胎体配方。      

Mg–Fe carpholites from aluminous schists in the Diahot region and implications for preservation of high-pressure/low-temperature schists, northern New Caledonia

Abstract Mg–Fe carpholite is widespread in the Diahot region of New Caledonia in highly aluminous schists and as veins in what was originally a clay-rich hydrothermal alteration envelope about massive suphide deposits. These carpholites have Fe/(Fe + Mg) ratios of 0.03–0.65 and no significant Mn component. Mg-carpholite + quartz occur in assemblages with chlorite or pyrophyllite, pyrophyllite + kaolinite and pyrophyllite + diaspore. Temperatures of 230–320° C and minimum pressures of 7 kbar are indicated for the Mg–Fe carpholite-bearing rocks. The regional distribution of aragonite and Mg–Fe carpholite parallel to a major zone of dominantly transcurrent movement and oblique to the trend of the subduction complex indicates the high- P /low- T schists owe their rapid uplift and preservation to the vertical component of the transcurrent faulting.     

Absorption of Arsenite on Several Iron （Hydro-）Oxides and Impact from Pre-processing Methods

The absorption reactions of arsenite on Fe （hydro-）oxides are studied. The three absorbent types are Fe（OH）3 gel and two Fe （hydro-）oxides, in which the Fe（OH）3 gel was dried in a microwave oven under vacuum at 80℃. It is found that pH changes from 9.71 to 10.36 in 6 minutes after the Fe （OH）3 gel was mixed with NaAsO2 solution, as the arsenite replaces the OH- in goethite and Fe（OH）3. At the 40th minute after the start of the reaction, pH decreases, which is most probably because that the monodentate surface complex of absorbed arsenite has changed into mononuclear-bidentate complex and released proton. The decline in pH values indicates not the end of the absorption but a change in the reaction type. Temperature and dissolved gas has little effect on these two types of reactions. The total absorption of arsenite increases after the absorbent is irradiated with ultrasound, which also lead to difficulty in separating the solids from solution. The absorption capacity for arsenite of Fe（OH）3 gel dried in a microwave oven under vacuum is 53.18% and 17.22% respectively better than that of Fe （OH）3 gel and gel dried at 80℃. The possible reasons are that the water molecules in the gel vibrates with high frequency under the effect of microwave irradiation, thereby producing higher porosity and improved surface activity.