Silicate Phases Research Articles

Behavior of magnesium impurity during carbochlorination of magnesium-bearing titanium slag in chloride media

Abstract In this study, the phase composition, morphology, and element distribution of Mg impurities in a low-grade titanium slag and in its chlorinated residue after carbochlorination were identified using XRD, SEM-EDS, and MLA. Most Mg impurities existed as anosovite, rutile, titanaugite, and Ti silicate phases before the carbochlorination process. The thermodynamic analysis revealed that the chlorination tendencies of Mg, Ti, Ca, Fe, and Mn oxides in the original titanium slag was much higher than those of Si and Al oxides in the temperature range of 700–850 °C. In addition, the binary phase diagram analysis indicates that the magnesium chloride (MgCl2) byproduct formed Na2MgCl4 eutectic salts in the NaCl-based salt bath. The carbochlorination experiments of the Mg-bearing titanium slag show that the chlorination ratio of magnesium oxide was above 93% under the suitable industrial conditions used for the extraction of titanium from Mg-bearing titanium slag. In addition, the formation of Na2MgCl4 eutectic salts after carbochlorination was confirmed, indicating that the negative effect of MgCl2 on the chlorination process was effectively eliminated. The Mg impurities in the chlorinated residue existed as titanaugite, quartz, chlorite, and rutile phases. Furthermore, we found that the carbochlorination of Mg-bearing titanium slag in chloride media effectively improved the comprehensive utilization efficiency of the Mg-bearing titanium slag. We believe that the proposed carbochlorination mechanism of the Mg-bearing titanium slag will provide thorough understanding and useful guidance on the practical industrial application of Mg-bearing titanium slag.

The role of sulfides in the chalcophile and siderophile element budget of the subducted oceanic crust

Subduction zones are the site of long-term chemical cycling between Earth’s surface and interior. During subduction, dehydration and melting may partition mobile elements into the overlying mantle wedge and arc system. Volcanic arcs also host much of the world’s base (e.g., Cu, Sn, Sb, and Mo) and precious (e.g., Au, Ag) metal deposits. However, slab contributions to the chalcophile and siderophile element (CSE) budget of arc magmas and their associated ore deposits remain controversial. Although previous studies have identified the mobility of some CSE in slab fluids, few studies have examined the contribution from sulfides to the CSE budget of the slab. Here we report in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of Cr, Co, Ni, Cu, Zn, Ga, Ge, As, Mo, Ag, Cd, In, Sn, Sb, Te, Tl, Pb, and Bi in sulfides and silicates in thirteen rocks from six exhumed high-pressure (HP) terranes worldwide. Combined with data from the literature, we demonstrate that sulfides host nearly the entire Cu, As, Ag, Cd, and Te budget of subducted rocks, whereas Co, Ni, Zn, Ga, Ge, Mo, Sn, and Tl are dominantly hosted in silicate and oxide phases. Prograde sulfide-breakdown during subduction is expected to liberate Cu, As, Ag, Cd, and Te into the overlying arc-mantle wedge system, whereas Co, Ni, Zn, Ga, and Pb may be partially lost during lawsonite, epidote, and amphibole dehydration reactions. In contrast, Ge, Tl, Sn, and Mo are largely retained in the eclogitic slab residue. We also demonstrate that CSE zoning in pyrite may be used to track high pressure (HP) metasomatism. Oscillatory Co and Ni zoning in metasomatic pyrite is linked to transitions between fluid and rock buffered regimes, consistent with seismic- and dehydration-induced cyclic fluctuations in fluid pore pressure and flux. Thus, minor and trace CSEs in HP rocks may also be a useful tool to constrain slab fluid migration.

Evidence of formation of an amorphous magnesium silicate (AMS) phase during alkali activation of (Na-Mg) aluminosilicate glasses

There is some ambiguity regarding the fate of Mg during the alkali activation of Mg-rich precursors within the broader field of alkali activated materials (AAMs). The present work addresses this issue by studying the reaction products in AAMs synthesized from (Na-Mg) aluminosilicate glasses. Here, instead of magnesium silicate hydrate (M-S-H) phase, Mg exclusively forms an amorphous magnesium silicate (AMS) phase. Compared to M-S-H, AMS is a more depolymerized phase, which has not previously been well documented. The formation of AMS seems to be driven by the high charge density of the Mg cation which effectively stabilizes the depolymerized silicate species. We also show that the lack of hydrotalcite-group phases is due to aluminum depletion by zeolite formation. This work highlights the need to consider the existence of the AMS phase in Mg-containing AAMs, especially in complex systems, where its identification may be difficult.

Grain boundary diffusion effect on mineral transition of calcium sulpho-aluminate with sodium dopant

The mineral formation-transition mechanism, microstructure morphology evolution, pulverization property and chemical reactivity of calcium sulpho-aluminate with sodium dopant are systematically studied with the molar ratio of CaO to Al2O3 is 0.8, the molar ratio of CaO to SiO2 is 1.5, the mass ratio of Al2O3 to SiO2 is 2.0, and the SO3 accounts for 4%. The results show that sodium dopant could promote 3CaO·3Al2O3·CaSO4, CaO·2Al2O3 and 2CaO·Al2O3·SiO2 transform into 2CaO·SiO2, CaO·Al2O3, 12CaO·7Al2O3 and 2Na2O·3CaO·5Al2O3, and it also can decrease the decomposition temperature of reactive materials and the initial phases formation temperature significantly. Sodium diffusion can destroy the clear grain boundary of calcium aluminate and calcium silicate phases, change the macro-microstructure morphology, improve the crystalline degree of the samples, and then improve the pulverization and alumina leaching property. However, excessive sodium dopant can decrease the melting temperature, inhibit the transformation process of 2CaO·SiO2 from β to γ, and then deteriorate the pulverization property significantly again.

Thermodynamic and experimental approach of the effect of Si on the sintering of Y3NbO7

The effect of silicon on the natural sintering of Y3NbO7 powders is studied. Characterizations shows that Si increases densification rate and grain growth but it also leads to the apparition of a secondary silicate phase. The silicate precipitation results from the segregation of silicon favored by matter transport through a liquid phase. The distribution of the secondary phase have been studied by electronic microscopy and mapped by micro-luminescence. Thermodynamic calculations of Gibbs energy of the different phase as well as binary and ternary phase diagrams have been conducted to support interpretation of the sintering experimental results.

Constraints on the source of reactive phases in sediment from a major Arctic river using neodymium isotopes

Riverine suspended particulate matter (SPM) is essential for the delivery of micronutrients such as iron (Fe) to the oceans. SPM is known to consist of multiple phases with differing reactivity, but their role in the delivery of elements to the oceans is poorly constrained. Here we provide new constraints on the source and composition of reactive phases in SPM from the Mackenzie River, the largest sediment source to the Arctic Ocean. Sequential leaching of SPM shows that river sediments contain labile Fe phases. We estimate the labile Fe flux is substantial (0.21(+0.06,−0.05) Tg/yr) by quantifying Fe concentrations in weak leaches of the SPM. The labile Fe phase hosts a considerable amount of rare earth elements (REE), including neodymium (Nd). We demonstrate that the labile Fe phase and dissolved load have radiogenic Nd isotope ratios that are identical within uncertainty, but up to 8 epsilon units distinct from the silicate phase. We interpret this as evidence for dynamic cycling between Fe-oxide phases in SPM and the river water, demonstrating the high reactivity of the labile Fe phase. Nd isotope and elemental molar ratios suggest that a significant amount of labile Fe- and Nd-bearing phases are derived from Fe-oxides within the sedimentary source rock rather than silicate mineral dissolution. Thus, sedimentary rock erosion and weathering provides an important source of labile Fe, manganese (Mn) and by extension potentially other trace metals. Our results imply that both past and future environmental change in the Arctic, such as permafrost thaw, may trigger changes to the supply of reactive trace metals. These results demonstrate that a re-evaluation of sediment reactivity within rivers is required where uplifted sedimentary rocks are present.

Ferric iron triggers greenalite formation in simulated Archean seawater

AbstractSedimentary rock deposits provide the best records of (bio)geochemical cycles in the ancient ocean. Studies of these sedimentary archives show that greenalite, an Fe(II) silicate with low levels of Fe(III), was an early chemical precipitate from the Archean ocean. To better understand the formation of greenalite, we explored controls on iron silicate precipitation through experiments in simulated Archean seawater under exclusively ferrous conditions or supplemented with low Fe(III). Our results confirm a pH-driven process promoting the precipitation of iron-rich silicate phases, and they also reveal an important mechanism in which minor concentrations of Fe(III) promote the precipitation of well-ordered greenalite among other phases. This discovery of an Fe(III)-triggering iron silicate formation process suggests that Archean greenalite could represent signals of iron oxidation reactions, potentially mediated by life, in circumneutral ancient seawater.

In situ observation of environmental barrier coating against calcium–magnesium–alumina–silicate attack at elevated temperature

AbstractEnvironmental barrier coating (EBC) plays a crucial role in improving the property of ceramic matrix composites at harsh environment. In this work, an in situ observation method was employed to systematically study the resistance of EBC to calcium–magnesium–alumina–silicate (CMAS) phase at elevated temperatures, which were 1200℃, 1300℃, and 1500℃, respectively. The melting and spreading process of the CMAS phase on the surface of the EBC was obtained via this method. In addition, the erosion effect of CMAS phase on EBC at different temperatures was analyzed by combining the scanning electron microscope (SEM) and energy dispersive spectrum (EDS), which provided a guidance for the modification of EBC.

Disorder-induced expansion of silicate minerals arises from the breakage of weak topological constraints

The disordering of the atomic network of minerals typically results in swelling of the structure. However, the origin of such expansion and the extent thereof remain unclear. Here, we conduct a series of molecular dynamics simulations of vitrification- and irradiation-induced disordering to reveal the nature of disorder-induced expansion in silicate phases. We show that pristine crystals are topologically overconstrained—i.e., some interatomic constraints are mutually redundant. Consequently, the weaker atomic constraints yield to the stronger ones— a behavior that manifests itself through some internal elastic stress in the network. Interestingly, we demonstrate that disordering results in a release of this internal stress by breaking the weaker constraints. The release of such internal strain explains the macroscopic expansion exhibited by atomic networks upon disordering.

Release of CO2 in Cold Subduction Zones by Hydrous Carbonatitic Liquids

AbstractThe release of CO2 from subducted slabs is an important process of the global carbon cycle. Previous studies have demonstrated that metamorphic reactions, carbonatitic melts, carbonate dissolutions, and hydrous carbonatitic liquids are effective CO2 recycling mechanisms in hot and warm thermal regimes, whereas the fate of subducted carbonates in cold subduction zones remains poorly understood. Here, we investigate the phase relations of subducted ophicarbonate at pressures beyond sub‐arc depths. The presence of complex quench textures indicates that hydrous carbonatitic liquids formed at temperatures as low as ∼850°C at depths of ∼300–440 km, which corresponds to cold thermal regimes. Water released by the decomposition of dense hydrous magnesium silicate phases can substantially affect the stability of subducted carbonates, which facilitates the formation of hydrous carbonatitic liquids. This provides a mechanism for scavenging CO2 from cold slabs and may promote the formation of metasomatized domains in the deep mantle.

Academic Reminiscences and Thermodynamics-Kinetics of Thermo-Barometry-Chronology

This article has three major components that include, in addition to the technical aspects, reminiscences of my academic upbringing, my move to the USA from India, and my professional career. I have recounted many stories that I hope convey some sense of time, especially in these two countries with vastly different cultures, my personal journey with its ups and downs and how I made the transition to an academic career path in USA even though that was not in my future plan as a young man. The development of the field of thermobarometry and its integration with diffusion and crystal kinetic modelling of compositional zoning (or lack thereof) and cation ordering in minerals have led to important quantitative constraints on the pressure-temperature-time evolution of terrestrial rocks and meteorites. I review the historical developments in these areas and a segment of my own research spanning the period of 1964-2021. The foundational works of the thermometry of metamorphic rocks and palaeothermometry were laid at the University of Chicago around 1950. Subsequently, the synergetic growth of thermodynamics and experimental studies in petrology in the 1960s and 1970s, along with the introduction of electron microprobe as a nondestructive analytical tool with micron scale resolution, gave a major boost to the field of thermobarometry. There were also significant new developments in the field of thermodynamics of solid solutions in the petrology community and demonstration from observational data, countering strong scepticism, that the principles of classical thermodynamics were applicable to “complex natural systems”. The section on thermodynamic basis of thermobarometry concludes with a discussion of the thermodynamics of trace element and single mineral thermometry. I further deal with the experimental protocols, along with selected examples, for phase equilibrium studies that provide the bedrock foundation for the field of thermobarometry based on elemental compositions of coexisting minerals in a rock. It is followed by an account of the controversies and international meetings relating to the aluminum silicate and peridotite phase diagrams that play crucial roles in the thermobarometry of metamorphic rocks and mantle xenoliths, respectively. The construction of quantitative petrogenetic grids to display stability relations of minerals in multicomponent–multiphase systems came into play in the field of metamorphic petrology in the mid-1960s and early 1970s. Augmented by experimental data, these petrogenetic grids led to important discoveries about the P-T-<em>f</em>(O<sub>2</sub>) and bulk compositional controls on the stability of certain “index” minerals that are used to define metamorphic isograds and different types of regional metamorphism; one such grid also opened up a new field that came to be known as ultra-high temperature metamorphism. The construction of petrogenetic grids has now evolved to computer based calculations of complex equilibrium P-T phase diagrams, commonly referred to as “pseudosections”, by minimisation of Gibbs free energy of a system with fixed bulk composition. I discuss these historical developments and modern advancements. Subsequently I highlight some aspects of thermobarometry and diffusion kinetic modelling of selected natural samples along with their broader implications and present a critical discussion of different protocols for thermobarometry of natural assemblages. Following up on the introductory historical perspective of development of palaeothermometry, I discuss the modern advancements using density functional theory (DFT). Examples of DFT based calculations have been shown for hydrogen isotope fractionation in mineral-water/hydrogen systems and “clumped isotope” thermometry. The hydrogen isotope fractionation data led the development of new low temperature palaeothermometers using serpentine-talc/brucite mineral pairs. These results enable simultaneous solutions of both temperature and source of fluid in the serpentinisation process of rocks. The final section is devoted to high temperature thermochronology dealing with the problems of closure temperature of decay systems in minerals and the use of bulk and spatial resetting of mineral age according to a specific decay system to determine cooling rates of the host rocks. Complications arise in the interpretation of mineral ages determined by such decay systems as <sup>176</sup>Lu-<sup>176</sup>Hf or the short- lived system <sup>53</sup>Mn-<sup>53</sup>Cr in which the parent nuclide has a much lower closure temperature than the corresponding daughter product. Numerical simulations help explain the discrepancy between the <sup>176</sup>Lu-<sup>176</sup>Hf and <sup>147</sup>Sm-<sup>143</sup>Nd ages of garnets in metamorphic rocks and enable construction of the entire T-t cycle from the discrepant ages and some additional constraints.

Dissolution Rates of Allophane With Variable Fe Contents: Implications for Aqueous Alteration and the Preservation of X-Ray Amorphous Materials on Mars

AbstractRecent measurements from Mars document X-ray amorphous/nano-crystalline materials in multiple locations across the planet. Despite their prevalence, however, little is known about these materials or what their presence implies for the history of Mars. The X-ray amorphous component of the martian soil in Gale crater has an X-ray diffraction pattern that can be fit partially with allophane (approximately Al2O3⋅(SiO2)1.3–2⋅(H2O)2.5–3), and the low-temperature water-release data are consistent with allophane. The chemical data from Gale crater suggest that other silicate materials similar to allophane, such as Fe-substituted allophane (approximately (Fe2O3)0.01–0.5(Al2O3)0.5–0.99⋅(SiO2)2⋅3H2O), may also be present. In order to investigate the properties of these potential poorly crystalline components of the martian soil, Fe-free allophane (Fe:Al = 0), Fe-poor allophane (Fe:Al = 1:99), and Fe-rich allophane (Fe:Al = 1:1) were synthesized and then characterized using electron microscopy and Mars-relevant techniques, including infrared spectroscopy, X-ray diffraction, and evolved gas analysis. Dissolution experiments were performed under acidic (initial pH values pH0 = 3.01, pH0 = 5.04), near-neutral (pH0 = 6.99), and alkaline (pH0 = 10.4) conditions in order to determine dissolution kinetics and alteration phases for these poorly crystalline materials. Dissolution rates (rdiss), based on the rate of Si release into solution, show that these poorly crystalline materials dissolve approximately an order of magnitude faster than crystalline phases with similar compositions at all pH conditions. For Fe-free allophane, logrdiss = –10.65–0.15 × pH; for Fe-poor allophane, logrdiss = –10.35–0.22 × pH; and for Fe-rich allophane, logrdiss = –11.46–0.042 × pH at 25°C, where rdiss has the units of mol m–2 s–1. The formation of incipient phyllosilicate-like phases was detected in Fe-free and Fe-rich allophane reacted in aqueous solutions with pH0 = 10.4 (steady-state pH ≈ 8). Mars-analog instrument analyses demonstrate that Fe-free allophane, Fe-poor allophane, and Fe-rich allophane are appropriate analogs for silicate phases in the martian amorphous soil component. Therefore, similar materials on Mars must have had limited interaction with liquid water since their formation. Combined with chemical changes expected from weathering, such as phyllosilicate formation, the rapid alteration of these poorly crystalline materials may be a useful tool for evaluating the extent of aqueous alteration in returned samples of martian soils.

Early performances of cement paste in the presence of triethanolamine: Rheology, setting and microstructural development

AbstractThe effect of triethanolamine (TEA) at various dosages on the early performance of cement paste was systematically evaluated through the techniques of rheological measurements, penetration tests, and ultrasonic pulse velocity. The correlation of early performance to the chemical hydration process was analyzed by calorimetry, zeta potential, in situ XRD, and pore solution analysis. It is found that the effect of TEA on the early performance of cement paste is strongly dependent on its dosage. With the TEA dosage below 0.1 wt%, the setting and microstructural development of cement paste are retarded. Meanwhile, the yield stress of fresh paste is decreased due to the increasing zeta potential of cement grains. The promoted formation of ettringite (AFt) and monosulfate (AFm) caused by TEA decreases the rheological retention ability. At dosages ≥0.2 wt%, the reaction of aluminate‐containing phases is greatly accelerated and a flash setting is observed. Besides, the importance of ferric phase on the reaction of cement with TEA is highlighted. At a low dosage, TEA prefers to accelerate the dissolution of tetracalcium aluminoferrite (C4AF) first and increases the [Fe] in the pore solution of cement paste. In cement without C4AF, the retardation of TEA on silicate phase hydration is significantly alleviated.

Uniform oxygen fugacity of shergottite mantle sources and an oxidized martian lithosphere

Martian meteorites are the only available samples that can be directly measured to constrain the geological evolution of Mars. It has been suggested that the oxygen fugacity (fO2) of martian shergottite meteorites, which have low (∼7 wt.%) to high-MgO (∼30 wt.%) compositions, correlates with incompatible trace element enrichment (i.e., La/Yb), and 87Sr/86Sr, 143Nd/144Nd, 187Os/188Os and 176Hf/177Hf at the time of crystallization. These relationships have been interpreted to result from early magmatic processes segregating enriched and more oxidized from depleted and more reduced reservoirs in Mars. Here we use the V-in-olivine oxybarometer to constrain the fO2 of shergottites and the dunitic chassignites. These data, utilizing early crystallizing silicate phases, constrain the shergottite fO2 range to between −3.72 ± 0.07 and −0.21 ± 0.55 ΔFMQ (log units relative to the fayalite-magnetite-quartz buffer), with no correlation with trace element enrichment or Nd isotope systematics. Previously employed oxybarometers that use later-formed or multiple mineral phases, and that show such correlations, likely differ from the V-in-olivine oxybarometer in that they record effects from late-stage magmatic processes. In contrast to shergottites, chassignites are relatively oxidized, at +2.1 ± 0.4 to +2.2 ± 0.5 ΔFMQ. The chassignites, along with the nakhlites, have been proposed to be sourced from metasomatized lithospheric mantle, and their high fO2 strengthens this model. The new data implies that the martian mantle sources of shergottites have fO2 of −2.1 ± 1.8 ΔFMQ. This estimate indicates that the mantle and core of Mars are not in redox equilibrium and therefore that oxidation of the martian mantle following core formation is required.

Cement-based materials: Pozzolanic activities of mineral additions are compromised by the presence of reactive oxides

Presence of reactive oxides, such as sodium, potassium, and magnesium oxides, compromise the pozzolanic activities of mineral additions to cement-based materials. That is because these oxides react with water to form hydroxides. Subsequently, these hydroxides may react with the otherwise non-reactive oxides, such as silicon and aluminum oxides, to form various silicate and aluminate phases in the cement-based material, that finally compete with the desired CSH phase. Further, the strong pozzolanic activity of the silica fume is seemingly due to its large ability to form the calcium silicate hydrated systems when compared with both metakaolin and fly ashes. All these results have been established by X-ray fluorescence, microstructural, and chemometric analyses that were applied to investigate the pozzolanic activities, thermodynamic, structural, and mechanical properties of a set of mineral systems widely applied to cement-based materials (silica fume, metakaolin, and fly ashes). The chemometric analyses revealed that the materials prepared with silica fume exhibited the largest mechanical properties, whereas the reference the lowest. The cement-based materials with metakaolin and fly ashes present intermediary resistances and similar behaviors. In conclusion, strict control of the purity of the material always needs to be performed to obtain mineral systems with powerful pozzolanic activities.

Electrosprayed calcium silicate nanoparticle-coated titanium implant with improved antibacterial activity and osteogenesis

To ensure clinical success, the implant and the surrounding bone tissue must not only be integrated, but also must not be suspected of infection. In this work, an antibacterial and bioactive nanostructured calcium silicate (CaSi) layer on titanium substrate by an electrospray deposition method was prepared, followed by annealing at 700, 750 and 800 °C to improve the bonding strength of the CaSi coating. The phase composition, microstructure and bonding strength of the CaSi coatings were examined. Human mesenchymal stem cells (hMSCs), Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) species were used to analyze the osteogenic and antibacterial activity of the coatings, respectively. Experimental results showed that the as-prepared CaSi coating was mainly composted of β-dicalcium silicate phase with a particle size of about 300 nm. After annealing, the thickness of the oxidation reaction layer increased obviously from 0.3 μm to 1 μm with increase in temperature, which was confirmed by the cross-sectional morphology and element depth profile. The bonding strength of the coating annealed at 750 °C (19.0 MPa) was significantly higher (p < 0.05) than that of the as-prepared coating (4.4 MPa) and the ISO 13,779 standard (15 MPa). The results of antibacterial efficacy and stem cell osteogenesis consistently elaborated that the 750 °C-annealed coating had higher activity than the as-prepared coating and the Ti control. It is concluded that after annealing at 750 °C, the CaSi nanoparticle-coated Ti implant had good bond strength, osteogenic and antibacterial activity.

Biosilicate® Glass-Ceramic Foams From Refined Alkali Activation and Gel Casting

Biosilicate® glass-ceramics are among the most valid alternatives to 45S5 Bioglass. They combine a similar bioactivity and bioresorbability as the 45S5 with superior mechanical strength, owing to the crystallization of a Na–Ca silicate phase. This crystallization may be experienced upon viscous flow sintering of fine glass powders, thus configuring a sinter-crystallization process. As crystallization is seldom complete, sintering can also be applied to semicrystalline powders. The sintering/crystallization combination may be exploited for shaping highly porous bodies, to be used as scaffolds for bone tissue engineering, in the form of foams. The present study aims at exploring a gel-casting process, based on the room temperature foaming of powders suspended in a “weakly alkaline” (1 M NaOH) aqueous solution, followed by sintering at 1,000°C. The gelation of suspensions is attributed to the formation of hydrated compounds, later decomposed upon firing. Amorphous powders provided more intense gelation than semicrystalline ones, promoted a more homogeneous foaming, and stimulated a substantial crystallization upon firing. The homogeneity of foamed samples was assessed using micro-tomography and was further improved by casting foamed suspensions (“foam casting”) before setting.

Characterizations on phase reconstruction, microstructure evolution and separation of magnetic ferrite ceramics from low-grade manganese ores by novel uphill reaction diffusion and magnetic separation

Traditional utilization of ferruginous manganese ore has mainly concentrated on the separation of Mn and Fe. However, an alternative process of novel uphill reaction diffusion followed by magnetic separation was developed for the synchronous recovery of manganese and iron as the form of ferrite ceramics from the ferruginous manganese ore in this work. The phase reconstruction, interfacial uphill reaction diffusion, ferrite size quantification and separation behavior were characterized via various means of XRD, XPS, TEM, VSM, SEM-EDS, optical microscopy, microhardness and thermodynamic analyses. In the phase reconstruction course, manganese and iron oxides reacted to form ferrites, whereas the impurities were enriched in liquid silicate phases due to the uphill reaction diffusion. Meanwhile, the ferrite particles grew and the liquid silicates aggregated owing to the intensified reaction diffusion by CaO addition. During the water quenching process, microcracks at the cleavage plane between the silicate and ferrites are preferentially generated due to fast cooling shrinkage of liquid silicates. Huge differences in hardness, magnetism, fusibility and cooling shrinkage of ferrites and silicates provided excellent mineralogical conditions for the successful grinding and magnetic separation of ferrites. Eventually, a soft magnetic manganese ferrite precursor with recoveries of 89.2 wt% iron and 85.6 wt% manganese was obtained. The interfacial reaction mechanisms for the formation of ferrites and silicates were also discussed in this work.

Preparation and properties of carbonated steel slag used in cement cementitious materials

Attributable to the advantages of low cost, easy availability and high density, et al., steel slag has become the research focus of the cement concrete industry. In this paper, the indirect carbonation process of steel slag is studied, the cementitious materials are prepared by replacing part of cement with carbonated steel slag, the mechanical properties and volume stability of the cementitious materials were studied. The reasons for the influence of carbonated steel slag on the properties of cementitious materials are explored from the hydration properties. The results show that carbonation reaction can reduce the content of free calcium oxide in steel slag. The carbonation products are mainly calcite and are in the form of non-uniform particles and carbonation layer morphology. The volume stability of cementitious materials containing carbonated steel slag is improved effectively due to the consumption of the expansion phase. Although the silicate phase in steel slag will be carbonated, the calcium carbonate produced by carbonation fills the microcracks or pores of the cementitious materials, or becomes the core of the hydration product, which significantly improves the 28-day strength of the cementitious materials.

Lithium and potassium isotope fractionation during silicate rock dissolution: An experimental approach

This study experimentally investigates the isotopic behaviors of Li and K during the dissolution of silicate rocks (i.e., basalt and granite). Proton-driven dissolution (in 0.8 M HNO3) and ligand-driven dissolution (in 5 mM critic acid or oxalic acid) experiments were performed in batch-closed systems over 15 days. We provide a time-series interpretation of Li and K isotope fractionation during silicate dissolution in ultra-acidic (unidirectional) and near-natural (biologically affected) environments. As the reaction progressed, we measured large isotope fractionation between the liquid (l) phase and the pristine silicate (s) phase, ranging from −10.3 to 0.1‰ (Δ7Lil-s) and from −1.01 to −0.11‰ (Δ41Kl-s) through the early stage of dissolution (<24 h). The enrichment of lighter Li and K isotopes in the solutions rapidly diminished as rock dissolution continued and gradually approached equilibrium to the end of experiments. In contrast, resorption of pre-leached isotopically lighter Li on silicate residuals during ligand-driven dissolution produced lighter isotope enrichment in the solutions compared to the initial rock by up to 2.8‰. Despite the preferential dissolution of specific minerals, the isotope fractionation patterns of Li and K do not vary with lithology, indicating limited inter-mineral isotopic differences. During the experiments, the Li and K isotopic pattern could be divided into two-to-three stages. The initial enrichment of light isotopes in the liquids can be ascribed to the kinetic isotope effect, confounded by diffusion and ion solvation. A later transition towards no isotope fractionation of Li and K may be explained by (i) the masking effect from dissolution, and (ii) an imprint from the destruction of 7Li/41K-enriched surface layers. Lateral resorption of solute Li after ~100 h reaction could be facilitated by the electrostatic attraction from increasing surface negative charges and active hydroxyls with increasing pH during ligand-driven dissolution (pH ~ 4) relative to proton-driven dissolution (pH ~ 0.2). Therefore, the presence of organic ligands impacts dissolution stoichiometry, and potentially modifies Li isotope fractionation in natural weathering environments. In comparison, K isotope fractionation driven by rock dissolution stops immediately (within days) after starting the experiments. This research helps to understand the mechanisms of Li and K isotope fractionation during chemical weathering and trace long-term climate change using geological records.