Spinel Compositions Research Articles

Preparation and characterization of high infrared emissivity Mn-doped NCO spinel composites

NiCr2O4 (NCO) spinel composites with different Mn/Ni atomic ratios (Mn/Ni = 0.05, 0.10, 0.15, and 0.20) were synthesized via solid state reaction method. Phase compositions and microstructure of samples were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The TG-DSC curves showed that the appropriate baking temperature for Mn-doped NCO spinel preparation was approximately 1 320 °C. X-ray diffraction patterns exhibited the formation of NCO spinel with Fd-3m space group. Valence state of the Mn ions was determined from 2p and 3s X-ray photoelectron spectra. Manganese ions were mostly in divalent and trivalent states, and the ratio of Mn2+/Mn3+ was 0.78-0.98. Fourier transform infrared spectroscopy (FTIR) was used to analyze the spectral emissivity of Mn doped NCO spinel. It was revealed that the infrared emissivity of Mn-doped NCO spinel in 1.8-5 μm could be significantly enhanced with increasing content of Mn2+, reaching as high as 0.9398. Mn-doped NCO spinel showed excellent radiation performance and good prospect in high emissivity applications in the temperature range of 800-1 200 °C.

Copper ferrite supported gold nanoparticles as efficient and recyclable catalyst for liquid-phase ethanol oxidation

Copper-containing spinel-supported gold nanoparticle catalysts are optimized toward high activity and stability in liquid-phase aerobic oxidation of aqueous bioethanol to acetic acid under mild hydrothermal conditions (3 MPa O2, 140 °C). In this work, we investigate the structure-performance relationships of supported gold catalysts by changing the composition and calcination temperature of spinel supports to gain insight into the gold-support interaction. The support effect specially acts on the rate-controlling ethanol activation step, with ferrite-spinel outperforming aluminate and chromite-spinels, and the activity increasing with the copper content in the spinels. Moreover, support calcination at 700 °C results in the optimal Au/CuFe2O4 catalyst, which can be magnetically separated and reused at least five times. The improved reusability is likely attributed to the alloying between AuNPs and Cu0 on the partially reduced CuFe2O4. The presence of negatively charged Auδ− and redox active Fe2+ species in the spent Au/CuFe2O4 catalyst points to a specific Auδ−-Fe2+ synergy for the enhanced activation of O2 and ethanol. With the help of negatively charged AuNPs, heterogeneous Fenton-like reaction is suggested to provide hydroxyl radicals for the CH bond activation of ethanol.

The origin of the ultramafic rocks of the Tulu Dimtu Belt, western Ethiopia – do they represent remnants of the Mozambique Ocean?

AbstractThe East African Orogen contains a series of high-strain zones that formed as Gondwana amalgamated. The Tulu Dimtu shear belt is one of these N–S structures within the Barka–Tulu Dimtu zone in western Ethiopia, and contains ultramafic bodies of equivocal origin. Identifying the petrogenetic origin of these enigmatic rocks provides evidence for the geodynamic significance of these shear zones. Owing to their altered state, these ultramafic rocks’ well-preserved chrome spinels provide the only reliable evidence for their source and tectonic affiliation. Chrome spinels have high Cr2O3 (30.04–68.76 wt %), while recalculated Fe2O3 (< 2 %) and TiO2 (0.01–0.51 %) values are low. The Cr# (molar Cr3+/Cr3+ + Al2+) and Mg# (Mg2+/Mg2+ + Fe2+) have averages of 0.88 and 0.22, respectively. Based on olivine–spinel equilibria, the calculated fO2 values (FMQ +3.03) for the dunites reveal a highly oxidized environment. This spinel chemistry (high Cr# > 0.6 and low Ti) supports a supra-subduction origin, with an oxidized mantle source more refractory than depleted MORB mantle (DMM). These spinel compositions indicate that some ultramafic bodies in western Ethiopia, including those from Daleti, Tulu and Dimtu, are serpentinized peridotites emplaced as obducted ophiolite complexes. By contrast, the ultramafic rocks from the Yubdo locality have a different spinel chemistry, with strong affiliation with igneous spinels formed in Alaskan-style mafic intrusions. These collective results suggest that regardless of their origin as supra-subduction ophiolites or as Alaskan-type intrusions, these spinels were formed on a convergent-subduction margin.

Oxidation Kinetics and Oxide Scale Characterization of Nickel-Based Superalloy IN738LC at 900 °C

The high-temperature isothermal oxidation behavior of the polycrystalline nickel-based superalloy IN738LC was investigated at 900 °C in air for up to 1000 h. The results from the tests suggest that the alloy showed single-stage parabolic oxidation behavior during isothermal oxidation. The oxidized samples were characterized using SEM and SEM/EDS, and the results show that the alloy is comprised of an outer dense chromia scale with titania proving Type II oxidation behavior. In addition, the formation of a spinel composition adjacent to the external layer and a discontinuous needle-shaped alumina scale in the alloy subsurface zone were also observed. The depletion of gamma prime (γ′) phase leads to a precipitate-free zone formation in the subscale zone. A JMatPro thermodynamic analysis showed that an increase in titanium content from 1 to 3.44 wt.% increased the chromium activity by 50%. Therefore, the results suggest that the presence of high amounts of titanium (~3.44 wt.%) in IN738LC increased the oxidation kinetics by increasing the chromium scale growth rate and resulting in an oxidation rate constant of 2.79 × 10−6 mg2 cm−4 s−1.

New Occurrence of Pyroxenites in the Veria-Naousa Ophiolite (North Greece): Implications on Their Origin and Petrogenetic Evolution

The Veria-Naousa ophiolite represents a dismembered unit in north Greece, which includes variably serpentinised lherzolite and harzburgite, locally intruded by a sparse network of dykes or thin layers of websterite and olivine-orthopyroxenite composition. The websterite and the olivine-orthopyroxenite show abundant petrographic and geochemical evidence (relic olivines with mantle affinities, Cr-rich spinels, low Al2O3, depletions in incompatible elements, and concave upwards rare earth element patterns) that they comprise replacive bodies from refractory subarc mantle precursors. The occurrence of these pyroxenites in dykes implies that channelled percolation of melts account for their replacive character. High CaO/Al2O3, low Zr and crystallisation of diopside suggest that a melt of ankaramitic/carbonatitic composition percolated in lherzolite replacing porphyroclastic olivine and forming the pyroxenes in the websterite. At a shallower level, harburgites were impregnated by boninitic melts (inferred by U-shape rare earth element patterns and very rich in Cr spinels) triggering the replacement of porphyroclastic olivine by orthopyroxene for the formation of olivine-orthopyroxenite. These peritectic replacements of olivine commonly occur in a mantle wedge regime. The peculiar characteristics of the Veria-Naousa pyroxenites with LREE and compatible elements enrichments resemble the subarc pyroxenites of Cabo Ortegal implying a similar environment of formation. Whole-rock and mineralogical (spinel and clinopyroxene) compositions are also in favour of a backarc to arc environment. It is recommended that the evolution of the Veria-Naousa pyroxenites record the evolution of the subarc region and the opening of a backarc basin in a broad SSZ setting in the Axios Zone of eastern Greece.

Peridotite-chromitite complexes in the Eastern Desert of Egypt: Insight into Neoproterozoic sub-arc mantle processes

The Neoproterozoic peridotite-chromitite complexes in the Central Eastern Desert of Egypt, being a part of the Arabian-Nubian Shield, are outcropped along the E–W trend from Wadi Sayfayn, Wadi Bardah, and Jabal Al-Faliq to Wadi Al-Barramiyah, from east to west. Their peridotites are completely serpentinized, and the abundance of bastite after orthopyroxene suggests harzburgite protoliths with subordinate dunites, confirmed by low contents of Al2O3, CaO and clinopyroxene (<3vol%) in bulk peridotites. The primary olivine is Fo89.3–Fo92.6, and the residual clinopyroxene (Cpx) in serpentinites contains, on average, 1.1wt% Al2O3, 0.7wt% Cr2O3, and 0.2wt% Na2O, similar in chemistry to that in Izu-Bonin-Marian forearc peridotites. The wide range of spinel Cr-number [Cr/(Cr+Al)], 0.41–0.80, with low TiO2 (0.03wt%), MnO (0. 3wt%) and YFe [(Fe3+/(Cr+Al+Fe3+)=0.03 on average)] for the investigated harzburgites-dunites is similar to spinel compositions for arc-related peridotites. The partial melting degrees of Bardah and Sayfayn harzburgites range mainly from 20 to 25% and 25 to 30% melting, respectively; this is confirmed by whole-rock chemistry and Cpx HREE modelling (~20% melting). The Barramiyah peridotite protoliths are refractory residues after a wide range of partial melting, 25–40%, where more hydrous fluids are available from the subducting slab. The Neoproterozoic mantle heterogeneity is possibly ascribed mainly to the wide variations of partial melting degrees in small-scale areas, slab-derived inputs and primordial mantle compositions. The Sayfayn chromitites were possibly crystallized from island-arc basaltic melts, followed by crystallization of Barramiyah chromitites from boninitic melt in the late stage of subduction. The residual Cpx with a spoon-shape REE pattern is rich in both LREE and fluid-mobile elements (e.g., Pb, B, Li, Ba, Sr), but poor in HFSE (e.g., Ta, Nb, Zr, Th), similar to Cpx in supra-subduction zone (SSZ) settings, where slab-fluid metasomatism is a prevalent agent. The studied chromitites and their host peridotites represent a fragment of sub-arc mantle, and originated in an arc-related setting. The systematic increase in the volume of chromitite pods with the increasing of their host-peridotite thickness from Northern to Southern Eastern Desert suggests that the thickness of wall rocks is one factor controlling the chromitite size. The factors controlling the size of Neoproterozoic chromitite pods are the thickness, beside the composition, of the host refractory peridotites, compositions and volumes of the supplied magmas, the amount of slab-derived fluids, and possibly the partial melting degree of the host peridotites.

Whole rock and spinel compositions of serpentinized peridotites from the Divriği-Sivas region, eastern Turkey: Implications for their tectonic setting

In this study we investigate the spinel and whole rock chemistry of ultramafic rocks in the Divriği-Sivas region hosting one of the largest ophiolite suites in Turkey, which were emplaced following the convergence between the African and Eurasian plates in the Late Cretaceous. The ophiolitic mélange (Güneş Ophiolite) in Divriği is composed of ultramafic rocks, amphibolites, sub-ophiolitic metamorphic rocks and calc-silicates. The ultramafic rock section is made up of serpentinized wehrlites overlain by irregular segregations of pyroxenite levels.Whole rock chemistry of Divriği wehrlites resembles serpentinized peridotites in Himalayas. They have high MgO (average 31.27 wt%) and low Al2O3 (average 0.56 wt%) concentrations and Mg/Si and Al/Si ratios imply that peridotites formed in a supra-subduction zone environment. The U-shaped REE patterns of serpentinized wehrlites are formed by partial melting, coupled with interaction between peridotite and hydrous melts in the mantle wedge. The spinels collected from serpentinized wehrlites show compositions ranging from chromite in the core to ferritchromite and Cr-magnetite at the rims and have high Cr2O3 (46.5–56.2 wt%) and very low TiO2 contents (<0.3 wt%). Cr# values (0.71–0.84) of studied spinels are consistent with those of selected spinels from Anatolian peridotites but their Mg# values (0.26–0.46) are significantly lower. Cr# values of Divriği spinels correspond to SSZ peridotites and indicate high-degree partial melting (>%35). Parental melt compositions computed (8.69–11.71 wt% for Al2O3 and 0.1 to 0.37 wt% for TiO2) from chromite-melt equilibrium conditions yield a boninitic affinity. Our data suggest that spinels from serpentinized wehrlites in the Divriği area are similar to peridotite xenoliths from the Kamchatka arc and West Bismarck.

O-136 房絶半島南部嶺岡山地第三系の砂岩組成と砕屑性クロムスピネル(14. 砕屑物組成・組織と続成作用,口頭発表,一般発表)

O-136 房絶半島南部嶺岡山地第三系の砂岩組成と砕屑性クロムスピネル(14. 砕屑物組成・組織と続成作用,口頭発表,一般発表)

Petrography, chemical composition, and Raman spectra of chrome spinel: Constraints on the diamond potential of the No. 30 pipe kimberlite in Wafangdian, North China Craton

Conventional diamond exploration seldom searches directly for diamonds in rock and soil samples. Instead, it focuses on the search for indicator minerals like chrome spinel, which can be used to evaluate diamond potential. Chrome spinels are preserved as pristine minerals in the early Paleozoic (∼465Ma), hydrothermally altered, Group I No. 30 pipe kimberlite that intruded the Neoproterozoic Qingbaikou strata in Wafangdian, North China Craton (NCC). The characteristics of the chrome spinels were investigated by petrographic observation (BSE imaging), quantitative chemical analysis (EPMA), and Raman spectral analysis. The results show that the chrome spinels are mostly sub-rounded with extremely few grains being subhedral, and these spinels are macrocrystic, more than 500µm in size. The chrome spinels also have compositional zones: the cores are classified as magnesiochromite as they have distinctly chromium-rich (Cr2O3 up to 66.56wt%) and titanium-poor (TiO2<1wt%) compositions; and the rims are classified as magnetite as they have chromium-poor and iron-rich composition. In the cores of chrome spinels, compositional variations are controlled by Al3+-Cr3+ isomorphism, which results in a strong Raman spectra peak (A1g mode) varying from 690cm−1 to 702.9cm−1. In the rims of chrome spinel, compositional variations result in the A1g peak varying from 660cm−1 to 672cm−1. The morphology and chemical compositions indicate that the chrome spinels are mantle xenocrysts. The cores of the spinel are remnants of primary mantle xenocrysts that have been resorbed, and the rims were formed during kimberlite magmatism. The compositions of the cores are used to evaluate the diamond potential of this kimberlite through comparison with the compositions of chrome spinels from the Changmazhuang and No. 50 pipe kimberlites in the NCC. In MgO, Al2O3 and TiO2 versus Cr2O3 plots, the chrome spinels from the Changmazhuang and No. 50 pipe kimberlites are mostly located in the diamond stability field. However, only a small proportion of chrome spinels from No. 30 pipe kimberlite have same behavior, which indicates that the diamond potential of the former two kimberlites is greater than that of the No. 30 pipe kimberlite. This is also supported by compositional zones in the spinel grains: there is with an increase in Fe3+ in the rims, which suggests that the chrome spinels experienced highly oxidizing conditions. Oxidizing conditions may have been imparted by fluids/melts that have a great influence on diamond destruction. Here, we suggest that chrome spinel compositions can be a useful tool for identifying the target for diamond potential in the North China Craton.

Microwave-assisted synthesis and dielectric properties of Al2O3–MgAl2O4 spinel composites from ironsand

In this study, we performed the synthesis and sintering of Al2O3 and MgAl2O4 spinel composites using microwave radiation. Al2O3 was used as the ceramic matrix and ironsand with concentrations of 25, 50, and 75 vol% was added to alumina. Specimens were microwave sintered for 30, 60, and 90 min. Heat treatment in an electric furnace was also performed for comparison with the method proposed in this work. The results revealed that the material developed crystalline phases of Al2O3 and MgAl2O4 spinel-type structure with anorthite and iron oxide as minor phases. The composites exhibited high dielectric constants at low frequency. High density values were found for materials composed of 25% ironsand and 75% alumina. The treatment by microwave radiation was presented as an alternative to conventional heat treatments, with reduced synthesis and sintering times at high processing temperatures. The results showed that the developed material can be useful for microelectronic applications.

Topotactic Synthesis of Porous Cobalt Ferrite Platelets from a Layered Double Hydroxide Precursor and Their Application in Oxidation Catalysis.

Monocrystalline, yet porous mosaic platelets of cobalt ferrite, CoFe2 O4 , can be synthesized from a layered double hydroxide (LDH) precursor by thermal decomposition. Using an equimolar mixture of Fe2+ , Co2+ , and Fe3+ during co-precipitation, a mixture of LDH, (FeII CoII )2/3 FeIII1/3 (OH)2 (CO3 )1/6 ⋅m H2 O, and the target spinel CoFe2 O4 can be obtained in the precursor. During calcination, the remaining FeII fraction of the LDH is oxidized to FeIII leading to an overall Co2+ :Fe3+ ratio of 1:2 as required for spinel crystallization. This pre-adjustment of the spinel composition in the LDH precursor suggests a topotactic crystallization of cobalt ferrite and yields phase pure spinel in unusual anisotropic platelet morphology. The preferred topotactic relationship in most particles is [111]Spinel ∥[001]LDH . Due to the anion decomposition, holes are formed throughout the quasi monocrystalline platelets. This synthesis approach can be used for different ferrites and the unique microstructure leads to unusual chemical properties as shown by the application of the ex-LDH cobalt ferrite as catalyst in the selective oxidation of 2-propanol. Compared to commercial cobalt ferrite, which mainly catalyzes the oxidative dehydrogenation to acetone, the main reaction over the novel ex-LDH cobalt is dehydration to propene. Moreover, the oxygen evolution reaction (OER) activity of the ex-LDH catalyst was markedly higher compared to the commercial material.

Water, lithium and trace element compositions of olivine from Lanzo South replacive mantle dunites (Western Alps): New constraints into melt migration processes at cold thermal regimes

Replacive mantle dunites are considered to be shallow pathways for extraction of mantle melts from their source region. Dunites offer a unique possibility to unravel the compositional variability of the melts produced in the upper mantle, before mixing and crystal fractionation modify their original signature. This study includes a quantification of H2O, Li and trace elements (Ni, Mn, Co, Sc, V, Ti, Zr, Y and HREE) in olivine from large replacive dunite bodies (>20m) within a mantle section exposed in the Western Italian Alps (Lanzo South ophiolite). On the basis of olivine, clinopyroxene and spinel compositions, these dunites were previously interpreted to be formed by melts with a MORB signature. Variations in Ni, Mn, Co and Ca contents in olivine from different dunite bodies suggested formation by different melt batches. The variable H2O and Li contents of these olivines agree with this idea. Compared to olivine from residual peridotites and olivine phenocrysts in MORB (both having H2O <5ppm; Li >1ppm), the Lanzo South dunite olivine has high H2O (18–40ppm) and low Li (0.35–0.83ppm) contents. Geochemical modelling suggests that the dunite-forming melts were produced by low melting degrees of a mixed garnet-pyroxenite-peridotite mantle source, with a contribution of a garnet pyroxenite component variable from 20 to 80%. The Lanzo dunites experienced migration of melts geochemically enriched and mainly produced in the lowermost part of the melting region. Extraction of enriched melts through dunite channels are probably characteristic of cold thermal regimes, where low temperatures and a thick mantle lithosphere inhibit mixing with melts produced at shallower depths.

Inversion in Mg1-xNixAl2O4 Spinel: New Insight into Local Structure.

A wide variety of compositions adopt the isometric spinel structure (AB2O4), in which the atomic-scale ordering is conventionally described according to only three structural degrees of freedom. One, the inversion parameter, is traditionally defined as the degree of cation exchange between the A- and B-sites. This exchange, a measure of intrinsic disorder, is fundamental to understanding the variation in the physical properties of different spinel compositions. Based on neutron total scattering experiments, we have determined that the local structure of Mg1-xNixAl2O4 spinel cannot be understood as simply being due to cation disorder. Rather, cation inversion creates a local tetragonal symmetry that extends over sub-nanometer domains. Consequently, the simple spinel structure is more complicated than previously thought, as more than three parameters are needed to fully describe the structure. This new insight provides a framework by which the behavior of spinel can be more accurately modeled under the extreme environments important for many geophysics and energy-related applications, including prediction of deep seismic activity and immobilization of nuclear waste in oxides.

Pink and Red Spinels In Marble: Trace Elements, Oxygen Isotopes, and Sources

The oxygen isotopic composition of pink to red and other colored spinels hosted by marbles worldwide have been combined with trace elements investigated by electron microprobe analysis (EMPA). For the first time, this database enables the characterization of the geographic origin of the main productive and historical sources of gem spinel. Three consistent sets of δ18O values were found for primary and placer deposits: (1) between 5.6 and 8.6‰ for the historical Kuh-i-Lal spinels from Tajikistan; (2) from 12.1 to 18.5‰ for spinels from An Phu and Cong Troi in Vietnam, and Namya from Myanmar; and (3) between 19.0 and 24.2‰ for spinels from Paigutan in Nepal, Kiswila and Ipanko in Tanzania, Pamreso in Kenya, Mogok in Myanmar, and An Phu in Vietnam. The variation in the δ18O values for most of the deposits can be related to the buffering of the metamorphic fluids by the oxygen isotope composition of the impure marbles. The δ18O values of Kuh-i-Lal spinels result from a metasomatic skarn process between granitic intrusives and marble. Trace elements, such as Fe-Zn-Cr-V-Ti, presented in various new chemical discrimination diagrams allow for the differentiation of all the pink to red spinels. The slight overlaps observed for some chemical domains can be resolved by combination with the O-isotope composition of spinel. The present study shows that it is possible to discriminate gem spinels hosted by marbles from different geographic regions of the world and provides new insights into the traceability of historical spinels such as those from Kuh-i-Lal.

Fabrication of MgO-NiO-Fe2O3 materials and their corrosion in Na3AlF6-AlF3-K3AlF6 bath

Ledge-free sidewall is preferred because it probably reduces the energy requirement of aluminum production by about 30%. However, such approach poses great material challenge since the sidewall materials would be in direct contact with corrosive electrolytes. In the present study, MgO-NiO-Fe2O3 materials were fabricated using fused magnesia, nickel oxides and ferric oxide powders as starting materials. All the specimens were corroded in a Na3AlF6-AlF3-K3AlF6 bath to estimate the corrosion resistance. The results revealed that reaction sintering occurs in the MgO-NiO-Fe2O3 system during firing process. After firing at 1400 to 1600○C, all the specimens prepared are composed of single-phase NixMg1-xFe2O4 composite spinel, whose lattice parameter decreases and chemical stability improves with increasing Ni2+ ion concentration. And densification of the MgO-NiO-Fe2O3 specimens can be enhanced by increasing the NiO content. The corrosion results show that the increase of NiO content has an obvious positive effect on corrosion resistance of the specimens due to the improved chemical stability of the NixMg1-xFe2O4 phase. However, the incorporation of high MgO content into the specimens is favorable for formation of a dense and stable ceramic layer on their surface during the corrosion process, which effectively promotes the electrolyte penetration resistance of the specimens as it functions as the blocking layer between the specimens and electrolyte.

Spinel-embedded lithium-rich oxide composites for Li-ion batteries

Spinel-embedded lithium-rich oxides are synthesized and their structural phases are analyzed. The type of spinel LiM0.5Mn1.5O4 (M = Ni, Co, Mn) embedded is varied by controlling the spinel composition and content. Of the various composites fabricated with different spinel phases, the LiCo0.5Mn1.5O4-embedded over-lithiated layered oxide (OLO) shows the best electrochemical performance as a cathode because of the absence of a parasitic phase and its high structural stability. The formation energy of the LiCo0.5Mn1.5O4-embedded oxide is determined through first-principles calculations and is found to be lower than that of the pristine oxide as well as other spinel-phase-embedded oxides. It is also observed that use of OLO with optimal embedded spinel LiCo0.5Mn1.5O4 in a cylindrical 18650-type cell results in improvement in the full-cell electrochemical performance.

Geochemical, mineralogical and Re-Os isotopic constraints on the origin of Tethyan oceanic mantle and crustal rocks from the Central Pontides, northern Turkey

Chromite, ultramafic and mafic rocks from Eldivan, Yaprakli, Ayli Dag, Kure, Elekdag and Kizilirmak in northern Turkey have been studied to determine their mineral and whole-rock geochemical, and Re-Os isotope geochemical characteristics. Most of the studied peridotites display depleted but commonly V-shaped chondrite-normalized rare-earth element (REE) patterns while some peridotites as well as pyroxenites from all areas exhibit light REE depleted patterns. Olivine (forsterite 82 to 92 mol%) and spinel (chromium number 13 to 63) in the studied peridotites exhibit a wide range of compositions. Compositions of spinels suggest that peridotites from Eldivan, Ayli Dag and Kure experienced relatively large degrees of partial melting (~ 15 and 19 wt%), whereas those of the Kizilirmak area most likely reflect lower melting degrees (~ 4–6 wt%). Whole-rock and mineral chemical data indicate that the ultramafic rocks are similar to abyssal and supra-subduction zone peridotites. The ultramafic rocks of the investigated areas exhibit a wide range of 187Re/188Os (0.12 to 6.6) and measured 187Os/188Os (0.122–1.14), while the basaltic rocks from Kure, Eldivan and Kizilirmak areas have high 187Re/188Os (128–562) and measured 187Os/188Os (0.724–1.943). On the other hand, chromite from Eldivan, Elekdag and Kizilirmak show high Os contents (21.81–44.04 ppb) and low 187Re/188Os (0.015–0.818) and 187Os/188Os (0.122–0.133). Re-Os model ages (TChur) for all analyzed samples yielded scattered ages ranging from Jurassic to Proterozoic. Overall, geochemical data are interpreted to reflect different degrees of partial melting, melt – rock interactions and metasomatic effects that produced a heterogeneous mantle in a supra-subduction setting.

Characterisation of chromites, chromite hosted inclusions of silicates and metal alloys in chromitites from the Indo-Myanmar ophiolite belt of Northeastern India

Chromite is commonly used to decipher the petrogenesis and geodynamic setting of ultramafic rocks. Podiform chromitites associated with the mantle sequence of the Indo-Myanmar ophiolite belt are studied here to characterise the major and trace element concentrations of the chromites. The molar ratios Cr# (100×Cr/(Cr+Al)) and Mg# (100×Mg/(Mg+Fe2+)) range from 67 to 82 and 66 to 71, respectively, in the chromites. Calculated equilibrium conditions for chromite crystallisation are 19–25kbar pressure, 1080–1240°C temperature and oxygen fugacity ranging between 0.30 and 0.58 log units above the FMQ buffer, showing that the samples formed in an upper mantle environment. Minor and trace element concentrations in the chromites (Ga 23–33ppm, Ni 1203–1843ppm, V 585–780ppm, Sc 2–5ppm) are comparable to other podiform chromitites hosted in the mantle sections of ophiolite complexes. Using chrome spinel compositions, the parental magma for the chromitite is identified as boninite. Tectonic discrimination diagrams using the primary composition of chromites indicate a supra-subduction zone setting for the Manipur ophiolite chromitites. The chromites host inclusions of silicates, base metal sulphides and base metal alloys. The diversity of these inclusions reflects the important role played by hydrothermal fluids. Inclusions (base metal sulphide and alloys) of linear trails with enriched Cr2O3 (63wt.%, Cr# ∼80) in the chromite grains (Cr2O3, Cr# 70) indicate mobility of this component in chromites.

High alumina castables: a comparison among various sol-gel bonding systems

A comparison among various sol-gel bonding systems has been done for high alumina castable refractory. Four different sol systems, namely, alumina, boehmite, mullite, and spinel, have been synthesized and used individually as sole binder in high alumina castable system with particle size distribution coefficient of 0.23 as per Dinger and Funk model. The castable compositions were processed conventionally and properties were evaluated after heat treatment at different temperatures. These developed sol-containing compositions were also compared against commercially available silica sol-containing composition prepared under exactly similar conditions. Mullite sol-bonded composition showed higher cold strength, whereas spinel sol resulted in better hot properties. Improved thermal shock resistance against commercial sol binder was observed in spinel and mullite sol-containing compositions, which may be associated with the microcracking effect produced from the thermal expansion mismatch. The slag corrosion resistance was also observed better for the spinel sol-containing composition. The commercial silica sol-bonded compositions showed higher strength due to its higher solid content but resulted in inferior hot and corrosion properties.

Petrology and geochemistry of peridotites and podiform chromitite in the Xigaze ophiolite, Tibet: Implications for a suprasubduction zone origin

The Xigaze ophiolite, which lies in the middle section of the Yarlung Zangbo suture zone (YZSZ) of southern Tibet, is composed from the base upward of tectonized harzburgites with minor dunites, and a magmatic sequence of wehrlites, pyroxenites, troctolites and gabbros. Both abundant high-Cr, metallurgical and sparse, dunites occur within the harzburgites. The peridotites show abundant evidence of mantle melt extraction at various scales as indicated by the Fo values of olivine (92.4–93.5) in the harzburgites and 92.8–94.0 in the dunites. Whole-rock MgO contents are 41–50wt%, similar in composition to forearc peridotites. Most of the chromitites have high Cr# numbers (61–81), although there are systematic changes in olivine and chromian spinel compositions from harzburgites, to dunites to chromitites, reflecting variable partial melting and melt-rock reaction. The primitive mantle U-shaped rare earth element (REE) patterns of the harzburgites indicate refertilization of depleted peridotites by suprasubduction melts. The PGE abundances in the Xigaze rocks provide additional constraints on their geochemical evolution and show multiple episodes of melt extraction from the mantle source, giving it a subchondritic character. Compositional zoning of minerals likewise attest to modification by migrating melts, formed in a suprasubduction zone mantle wedge.