The reaction between Na2O·Al2O3 and chromium spinels or chromite in the roasting process was systemically investigated to reveal the effects of Na2O·Al2O3 on chromium oxidation. The results of ...Na2O·Al2O3 roasted with MgO·Cr2O3 illustrate that only about 50% of the chromium in MgO·Cr2O3 can convert into Na2CrO4, while the remaining chromium converts into an Al-bearing spinel Mg(CrAl)O4. Mg(CrAl)O4 is found to be difficult to further react with Na2O·Al2O3. The oxidative roasting of Na2O·Al2O3 with chromite further confirms that Na2O·Al2O3 cannot fully react with some chromium spinels, which reveals the cause of the low chromium oxidation rate in the traditional chromite lime-free roasting process. Based on the experiment results, a strategy for eliminating the impacts of Na2O·Al2O3 in the lime-free roasting process was proposed, in which adding more Na2CO3 for forming Na2O·Al2O3 is considered. The chromium oxidation rate can rise close to 100% with the new strategy. This study contributes to better understanding the chromite lime-free roasting process and provides a new idea for developing the chromate salts manufacturing process more efficiently and cleanly.
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•Physical properties of epitaxial Fe3-xCrxO4 thin films are controlled by stoichiometry.•Epitaxial thin films exhibit different cation site-occupation than their bulk ...counterparts.•Collinear spin structure promotes Curie temperatures above room temperature and higher magnetization.•Electron hopping is facilitated so that the electronic band gap is smaller than expected.
Tuning magnetic and electronic transport properties in spinel oxides requires a faithful description between chemical composition and cation site-occupation. Here this challenge is addressed using Fe3-xCrxO4 thin films grown by oxygen-assisted molecular-beam epitaxy within a wide range of composition (0.0 ≤ x ≤ 1.2). Spectroscopic measurements (e.g., X-ray magnetic circular dichroism), refined by theoretical simulations (e.g., crystal field multiplet), are performed to establish a quantitative link between chromium content, Fe2+/Fe3+ site-occupation and macroscopic physical properties of the layers. It is found that Fe3-xCrxO4 thin films (i) delay the transition from inverse to normal spinel configuration with increasing chromium content and (ii) promote collinear spin structure, at odds with bulk material. As a result, strong antiferromagnetic interactions are preserved between spins in tetrahedral and octahedral spinel sublattices, so that chromium-rich thin films exhibit Curie temperatures above room temperature and higher magnetization. Electron hopping is also favored by this singular cation distribution and electronic band gap is smaller than expected for these thin films. The cation site-occupation is therefore a key feature to consider for applications of Fe3-xCrxO4 thin films in spintronics and photocatalysis, as it enables manipulation of magnetic properties (Curie temperature and magnetization) and band gap engineering.
Black ceramic pigments were prepared from industrial wastes as raw materials, namely, Cr/Ni-rich sludge generated from Cr/Ni plating and Fe-rich galvanizing sludge generated during steel wiredrawing. ...The pigment was based on the chrome–iron–nickel black spinel (Ni,Fe)(Fe,Cr)
2O
4 catalogued as 13-50-9 in the DCMA classification and prepared by the common solid-state reaction method, under optimal formulation and processing conditions. The characterised synthesised pigments were tested in typical ceramic glazes and ceramic bodies. Optimal colour development was achieved when the spinel compound with the three elements Fe, Ni and Cr was the major phase formed. This situation was obtained for mixtures in which the amount of Cr/Ni-rich sludge varied between 50 and 75%. The colouring properties were similar to those imparted by a commercial black pigment.
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•Analyses of rare and informative minerals in Bossoroca and Capané ophiolites.•δ11B of tourmaline is oceanic and #Cr × #Mg of Cr-spinel is mantle-derived.•Firm constraints on early ...Brasiliano Orogen evolution.
Unravelling the complexity of tonian-cryogenian (950–680 Ma) evolution of ophiolites requires the search for rare mineral systems and their quantification with varied techniques. Ophiolites in the Brasiliano Orogen are widely distributed over 2,000 km along the eastern half of South America. We selected two ophiolites from different geotectonic settings of the Sul-Riograndense Shield, southern Brasiliano Orogen, to delimit the evolution of the oceanic phase of the orogen. The southern portion of the Bossoroca ophiolite is inserted in the São Gabriel juvenile terrane and contains rare metasomatic tourmaline in chloritite close to serpentinite and metamorphosed Cr-spinel. The southern Bossoroca ophiolite was intruded by Cerro da Cria and Ramada Granites and the U-Pb-Hf isotopic study of zircon from these rocks constrains the crustal evolution of the São Gabriel juvenile terrane. Capané ophiolite has similar age (793–715 Ma) as the Bossoroca ophiolite and was inserted in the Porongos fold-thrust belt with preserved Cr-spinel of mantellic composition. Integrated use of Cr-spinel mineral chemistry, B isotopes in tourmaline in the Bossoroca ophiolite and zircon U-Pb-Hf isotopes of granites associated with the southern Bossoroca ophiolite revealed several steps in the evolution of the ophiolites in the Dom Feliciano Belt. Capané Cr-spinel cores have mantle-derived compositions (Mg# 0.66 – 0.69; Cr# 0.51 – 0.53), tourmaline from the Bossoroca ophiolite is dravite and has δ11B = 0 to + 3, and granites crystallization ages are 578 ± 3.2 and 612 ± 12 Ma (εHfzrn = −10 to −25). Zircon from other dravite occurrences of the Bossoroca ophiolite were previously dated at 920 Ma. We unraveled the main steps in the evolution of ophiolites from the southern Brasiliano Orogen, with emphasis on the Bossoroca and Capané ophiolites, during their trajectory from mid-ocean ridge (920 Ma), formation of dravite in oceanic crust, preservation of mantellic cores in Cr-spinel, and intrusion of craton-generated granites at 612–578 Ma.
Chromium (Cr) was concentrated as chromium spinel in the vanadium slag produced by pre-oxidizing vanadium-containing hot metal in Pan-steel in China. Due to the chromium-rich Hongge ores used, ...chromium concentration in this slag was much higher than that in common vanadium slags. To propose a process for effective Cr extraction from the slag, mechanisms of Cr oxidation in Pan-steel vanadium slag under sodium roasting conditions were studied experimentally and with thermodynamic simulations. Chromium slags without vanadium were synthesized in the laboratory to mimick mineralogical characteristics of Pan-steel vanadium slag and characterized by XRD, SEM/EDS and TG-DSC techniques. Conditions for Cr extraction from the chromium slag after sodium roasting by leaching with water were studied and optimized. Results showed that Cr spinel was encapsulated in the silicate phase. During sodium roasting, Cr spinel was oxidized and decomposed after oxidation and decomposition of the olivine phase. Sodium carbonate began to react with chromium oxide, which was produced by decomposition of Cr spinel, at 537°C and large amount of sodium chromate appeared after 800°C. However, multi-component liquid phase (mixture of Na2Cr2O4, Na2CrO4 and Na2CO3) was produced after 1000°C, which suppressed the conversion of Cr(III) to Cr(VI) by inhibiting oxygen supply. A conversion about 90% was obtained when the chromium slag was roasted at 1000°C for 2 h with Na2CO3 addition in stoichiometric ratio to total amount of Cr2O3 and SiO2. 96% of Cr in the leaching liquid was recovered as chromium sesquioxide in fine powder form and high purity by reducing and precipitating.
The Mohns Ridge is a very slow‐spreading ridge that, together with the Knipovich Ridge, marks the boundary between the North American and Eurasian plates in the Norwegian‐Greenland Sea. In this ...study, we report the major and trace element composition of spatially associated basalts and peridotites from a gabbro‐peridotite complex ∼20 km west of the Mohns Ridge rift flank. Formation of the ∼4–5 Myr crustal section involved accretion of normal mid‐ocean ridge basalts with Na‐content suggesting derivation from a depleted mantle source. This is consistent with the degree of partial melting estimated for clinopyroxene poor harzburgites using the Cr‐number of spinel (14%–18%) and rare earth element modeling of orthopyroxene (16%–24%) and reconstructed whole‐rock composition (14%–20%). If all the melting took place beneath the paleo‐Mohns Ridge, a crustal thickness of ∼7–8 km is expected, which is nearly double the observed thickness. Orthopyroxene trace elements are not consistent with typical fractional melting expected for mid‐ocean ridges but rather resemble that seen in supra‐subduction zone peridotites. The geochemistry of both the basalts and the peridotites suggests that a water‐rich slab flux in the past has influenced the mantle source. In turn, this caused hydrous melting which increased the depletion of the pyroxene components, leading to a highly depleted mantle that is now underlying much of the Arctic Mid‐Ocean Ridges and represents the source for the spreading related magmatism.
Key Points
We report mantle peridotites and basalts from a 4–5 Myr tectonic window in the Mohns Ridge (Arctic Mid‐Ocean Ridges)
The peridotite and basalt compositions reveal a highly depleted mantle with a subduction signature
The anomalous depleted mantle is likely a consequence of an earlier hydrous melting event
Inclusions in 304 stainless steels are transformed from manganese silicate to manganese chromite (spinel) upon heating in the range of 1000–1350 °C, while this transformation is not observed when ...heating temperature is 1400 °C. Thermodynamic analysis with software FactSage™ is applied in interpreting the transformation. Manganese chromite spinel, with light color under SEM, is deemed to be formed on the surface of MnO–SiO2 inclusions by the reaction between Cr in steel matrix and MnO–SiO2 in inclusions. Heating experiments with different time at 1235 °C (rolling insulation temperature) are carried out to confirm the kinetics of the transformation reaction. Mass transfer coefficients of inclusions’ composition, SiO2, MnO, and Cr2O3 are ranging from 5 × 10−7 to 9 × 10−7 m s−1, depending on diameters and components of inclusions.
Inclusions in 304 stainless steels are transformed form manganese silicate to manganese chromite upon heat treatment at rolling insulation temperature (1235 °C). The transformation is a diffusion‐control process, and shows strong effects of heating time. The mass transfer coefficients of oxides in inclusions are calculated through the change of concentration with time.
The initiation and propagation of microcracks under stress are highly dependent upon the mineralogical and textural characteristics of the various lithotypes. Detailed observation and quantification ...of microcracks before and after uniaxial compression test were conducted. A fresh olivine-rich harzburgite and a serpentinized dunite were analyzed, collected from the Pindos and Vourinos ophiolites (northern Greece) respectively, in order to compare their microcrack patterns. Quantitative analysis indicated that during uniaxial compression the intragranular microcracks, which are the dominating crack-type, are gradually transformed or organized into transgranular cracks. Some of the newly formed transgranular cracks may also be a result of the growth of existing grain-boundary cracks. The new intragranular microcracks in the olivine-rich harzburgite are oriented predominantly parallel to the compressive stress direction, while those in the serpentinized dunite show a scattered orientation presumably due to the mesh texture of this rock-type. The new transgranular cracks of both peridotites tend to be subparallel to the compressive stress direction, however, many of them show a random orientation due to the fact that they have been formed as a result of the propagation of grain-boundary cracks. The occurrence of the soft serpentine along fracture surfaces of olivine, when it is in assemblage with orthopyroxene, tends to absorb the applied stress hampering the development of microcracks in olivine. On the other hand in serpentinized peridotites, the microcracks are enhanced in olivine because it is surrounded by large amounts of the much softer and flexible serpentine. Microcracks are usually formed along the cleavage planes of orthopyroxene porphyroclasts, indicating that such crystallographic preferred orientations act as planes of weakness, controlling the direction of the crack paths. Cr-spinel grains likely comprise locations of initiation of microcracks due to their very hard nature and different mechanical behaviour relative to the surrounding silicate phases. Knowledge of the mineralogical and textural characteristics may assist in the prediction of potential development of failure surfaces of an ultrabasic rock in-service.
► Ιntragranular microcracks transform to transgranular after uniaxial compression stress. ► The soft serpentine significantly controls the development of microcracks. ► Microcracks favour to develop along the weak cleavage planes of orthopyroxene porphyroclasts. ► Petrographic examination may assist in the prediction of the failure of a rock in-service.
The amount of hydrogen dissolved into oxide layers on Fe–10Cr alloy after exposure to steam at 750°C was measured with thermal desorption spectroscopy (TDS). It was found that the dissolved hydrogen ...existed mainly in (Fe, Cr)3O4 spinel layer formed on the alloy surface. This result is related closely to the fact that oxidation of high Cr ferritic steels is accelerated remarkably by the existence of steam at elevated temperatures. Using a diffusion system of (Fe, Cr)3O4/CoO, cation diffusivity was investigated at 750°C in argon atmosphere or in steam. As a result, the diffusivity of Fe ion in (Fe, Cr)3O4 became larger in steam than that in argon atmosphere, indicating that hydrogen (proton) promotes Fe ion outer diffusion from Fe–10Cr alloy. This is direct evidence that high temperature steam accelerates oxidation of Cr containing steels.
The ultramafic body sampled in the Chinese Continental Scientific Drilling (CCSD) Hole PP3 is located in the eastern part of the Dabie–Sulu UHP metamorphic belt near Donghai County. It is about 480 m ...thick, and consists chiefly of garnet peridotite, dunite and serpentinite. The principal minerals include olivine, chromium spinel, diopside, enstatite, garnet with minor secondary augite, phlogopite and amphibole. Both the olivine and orthopyroxene are highly magnesian, and the garnet is pyropic with 5.4–6.4% CaO and 0.3–3.3% Cr
2O
3. Two generations of clinopyroxene are present; an early diopside followed by augite. Chromium spinels are highly variable with Cr#s (100Cr
/
(Cr
+
Al)) between 51 and 89, and their compositions reflect different processes of formation, namely partial melting and eclogite, amphibole and greenschist facies metamorphism. The Mg#s (100 Mg
/
(Mg
+
Fe
2+))of the spinels correlate positively with the Cr#s but negatively with oxygen fugacity. Based on the spinel compositions the ultramafic rocks originated in the shallow mantle, then subducted to depths of more than 100 km and finally exhumed to the surface. They underwent partial melting at shallow depths, mostly in the spinel facies, and were later transformed into garnet peridotites during deep subduction. All of the rocks were weakly metasomatized during exhumation and were subjected to retrograde metamorphism.
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