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SiC‐based ceramic matrix composites (CMCs) for turbine engine applications require environmental barrier coatings (EBCs) to protect against water‐vapor induced corrosion. A significant limiting factor of EBC operating lifetime is the formation of a SiO2 thermally grown oxide (TGO) at the EBC–CMC interface at high temperature. Its rapid growth as well as displacive β→α cristobalite phase transformation upon cooling induces strains at this interface, which can result in coating spallation. Recent work has explored upon the use of modifier oxides in EBCs to reduce oxidizing specie transport and possibly stabilize the SiO2 TGO. This work utilizes EBC–glass mixtures to characterize the properties of modifier‐stabilized SiO2 and assess its impact in future EBC systems. [ABSTRACT FROM AUTHOR]

When SiO 2 films undergo oxidation on 4H–SiC, a distinctive crystalline phase, cristobalite, emerges at elevated temperatures. We deeply explored the growth mechanism of the crystallite, focusing on its dependence on the silicon sublimation process (SSO). Activation energy calculations confirmed that the oxidation after SSO above 1350 °C exhibits a lower activation energy. The reduced activation energy suggests the elimination of the solid-phase diffusion process during oxidation. We devised a controllable growth process for the quartz phase to achieve a seamless transition from 0 to 100 % in the area proportion of cristobalite in SiO 2. This strategic approach facilitates the precise preparation of oxide layers and holds potential applications in semiconductor device manufacturing. [ABSTRACT FROM AUTHOR]

In this paper, we discuss the unique challenges posed by Silica f / Silica composite while measuring their thermal conductivity by using Transient Plane Source (TPS) method and specific heat by Differential Scanning Calorimeter (DSC). The measured thermal conductivity values were found reaching a plateau followed by a reduction at temperatures 800 °C and above. Thermal expansion characteristics as measured in Thermomechanical Analyzer (TMA) exhibited strong dependency on static force applied during experiments. Keeping in view the real life application of the developed Silica f /Silica composite, a novel method of sample preparation was implemented which restricted the sample compression at elevated temperatures during TPS experiments. Experiments utilizing DSC under linear and step heating with sample placement in alumina crucible yielded inconsistent values of specific heat (Cp). With further experimentation, the combination of step heating and direct placement of sample in platinum crucible was found most appropriate for accurate measurement of specific heat of Silica f / Silica. [ABSTRACT FROM AUTHOR]

The formation of cristobalite thermally grown oxides (TGO) is a major life limiting factor for environmental barrier coatings (EBCs) with Si bond coat exposed to high temperature steam environments due to its reversible phase transformation and the resultant cracking behaviour during thermal cycling. Therefore, controlling or avoiding cristobalite TGO formation by engineering coating chemistry and architecture will be desirable for a highly durable EBC system. In this work, a multilayer Si/Al-Si-N/Yb 2 Si 2 O 7 EBC was produced using a hybrid vapor deposition process. The oxidation behavior of the EBC was evaluated in flowing 90% H 2 O (g) + 10% air at 1316 °C. After 100 h and 510 h steam test, crack-free mullite and Al-Si-O-N layers formed above Al-Si-N layer and no SiO 2 scale formation of Si bond coat was observed. Detailed TEM analyses show that the Al-Si-N layer successfully prevented the oxidation of underlying Si bond coat and avoided the formation of detrimental cristobalite phase. [ABSTRACT FROM AUTHOR]

At present, developing countries, including Ethiopia, are undergoing a phase of industrial transformation. As a result, there has been a significant rise in the presence of harmful pollutants in groundwater. The release of deleterious heavy metals, such as Cr (VI), into the surrounding ecosystem is a result of various industrial operations. The present investigation utilized activated sodium bentonite clay as an adsorbent agent to facilitate the elimination of Cr (VI). The clay was subjected to acidification and thermal modification techniques to enhance its adsorption capacity. The investigation involved an examination of the particular surface area, capacity for cation exchange, crystalline properties, functional groups on the surface, and structural morphology. The methylene blue test method was employed to ascertain the maximum specific surface area and cation exchange capacity of the adsorbent, which were found to be 512 m2/g and 65.5 meq/100 g, respectively. The clay mineral composition was analyzed using X‐ray diffraction, which indicated that the primary constituent was montmorillonite. The presence of impurities such as quartz, feldspar, muscovite, cristobalite, and hematite was also detected. The study employed response surface methodology, Box–Benkhen design (RSM‐BBD) to investigate the impact of operational parameters, including agitation time, initial Cr (VI) concentration, and adsorbent dosage. Under the optimal conditions, a removal efficiency of 98.05% was achieved for Cr (VI) for a contact duration of 120 min, a dosage of 3.913 g/L, and an initial concentration of 50 mg/L. The sodium bentonite clay that was activated demonstrated a maximum capacity for adsorption of 22.72 mg/g in regards to the adsorption of Cr (VI). The experimental data were best fitted by the Langmuir isotherm, which exhibited a correlation coefficient of 0.9699. The adsorption kinetics were assessed and determined to adhere to a pseudo‐second‐order pattern, exhibiting a coefficient of determination (R2) value of 0.999. Furthermore, the activated sodium bentonite clay demonstrated remarkable potential for repeated utilization in the adsorption of Cr (VI) ions in aqueous solutions. [ABSTRACT FROM AUTHOR]

Knowledge of the phase equilibria in the MgO–CaO–SiO2–Al2O3 slag system is crucial for the nickel laterite smelting process. The phase equilibria of this slag system were experimentally investigated, focusing on the olivine and tridymite/cristobalite primary phase fields, using high-temperature equilibration and quenching methods, followed by Scanning Electron Microscopy–Energy Dispersive X-Ray analysis. The phase equilibria of the MgO–CaO–SiO2 slag system at 1400 °C and 1500 °C were first determined in the absence of ferronickel alloy. The phase equilibria between 1400 °C, 1450 °C, and 1500 °C were then determined under a reducing condition, i.e., at equilibrium with ferronickel alloy and solid carbon. Finally, the effect of Al2O3 addition on the liquidus and solidus compositions in the slag system under the reducing condition was investigated at 1400 °C and 1450 °C. Comparisons between the experimentally constructed diagram, previous data, and FactSage-predicted phase diagrams have been provided and discussed. The present study identified the liquid slag both in the absence and presence of ferronickel alloy and solid carbon, as well as in the presence of Al2O3 impurity, within the formation boundaries of olivine and tridymite/cristobalite solids. Identifying the liquid slag area is essential to ensure that the nickel laterite smelting slag can be tapped from the furnace. [ABSTRACT FROM AUTHOR]

Experimental investigation and thermodynamic modeling of the phase equilibria in the (PbO)-NiO-ZnO-SiO 2 system has been undertaken to characterize Ni behavior in high-Zn slags. Phase equilibria data at 790–1700 °C were obtained through equilibration of synthetic mixtures in sealed silica ampoules or Pt-Ir / Au foils, rapid quenching, and electron probe X-ray microanalysis. Phase equilibria and liquidus isotherms of the NiO-ZnO-SiO 2 system in the tridymite/cristobalite SiO 2 , monoxide (Ni,Zn)O, zincite (Zn,Ni)O, olivine (Ni,Zn) 2 SiO 4 and willemite (Zn,Ni) 2 SiO 4 phase fields were measured and the extent of the 2-liquid immiscibility gap over cristobalite was determined. Additional solid solutions: larsenite Pb(Zn,Ni)SiO 4 and melilite Pb 2 (Zn,Ni)Si 2 O 7 were measured in the PbO-NiO-ZnO-SiO 2 system. It was found that NiO partially substitute ZnO in larsenite and melilite and may form a complete range of solubility in barysilite Pb 8 (Zn,Ni)Si 6 O 21. New data were used for developing a thermodynamic database to describe the Pb/Cu-containing complex system for characterizing Ni behavior in Zn-containing slags. [ABSTRACT FROM AUTHOR]

Authigenic minerals have been studied in Phanerozoic volcanosedimentary deposits in the northern part of the Asian continent–Pacific Ocean transition zone. The following were found: chlorite, mica, corrensite, rectorite, defective chlorite, kaolinite, smectite, calcite, barite, gypsum, epsomite, zeolites, cristobalite, quartz, and goethite. The minerals corrensite and rectorite have significant indicative properties, as do the assemblages corrensite–laumontite, corrensite–epsomite–authigenic calcite and mica–kaolinite–quartz. Such a range of minerals indicates that the thickness of sediments in the studied basins could reach 3–5 km, and their formation temperature could be more than 150°C. The mica–kaolinite assemblage may indicate epicontinental sedimentation conditions associated with coal formation on nearby land, the corrensite–chlorite assemblage may indicate conditions favorable for the evaporation of seawater, and the presence of laumontite in it may indicate periodic calcium supply to the sedimentary basin. The periods of mineral formation, possibly associated with global climatic events, have been identified: 113–120, 110–113, 105–110, 93–95, 72–83, 61–72, 56–61, 33–56 Ma, which can serve as benchmarks to determine sedimentation conditions and the framework for more reliable stratigraphic constructions. [ABSTRACT FROM AUTHOR]

Reactive silica materials like vitreous silica or cristobalite were dissolved at temperatures between 100 and 220 °C for up to 7 days in NaOH lyes. The liquid and solid reaction products were analyzed chemically. Sodium water glasses with silica concentrations of up to 27 wt % and molar SiO2/Na2O ratios of up to 3.7 were obtained. The experimental data were evaluated by an approximation algorithm. The initial size reduction rates ranged between 0.1 and 7 µm h−1, depending on the temperature and the choice of starting materials. Correlations between thermodynamics and kinetics were discussed. [ABSTRACT FROM AUTHOR]

The paper analyzes the effect of nanoadditives of zirconium dioxide, partially stabilized by Y2O3, on the Al2O3–SiO2 matrix during hot pressing by the electroconsolidation method. The microstructure obtained at different compositions and sintering modes is studied. It was established that the introduction of nanopowder ZrO2–3 mol % Y2O3 increases the crack resistance of sintered samples not only due to the transformation of the tetragonal phase to the monoclinic phase, but also due to the formation of a solid solution with aluminum oxide at the interphase boundaries. Starting from a temperature of 1400 °C, complete densification occurs with the formation of nonporous composites while preserving the nanostructure. At this temperature, the formation of mullite with cristobalite phases also begins. The high value of thermocycles makes it possible to use this composite material as a cryogenic one. The fabrication of mullite-corundum composites with additions of ZrO2–3 mol % Y2O3 nanopowders via the method of electrosolidification has enabled the attainment of high fracture toughness, K1C = 14.5 MPa⋅m1/2, and hardness HV10 = 14 GPa. These results evidence excellent mechanical properties, thereby expanding the potential applications of this material. [ABSTRACT FROM AUTHOR]

The microstructural evolution of C sf /SiBCN composites and formation mechanism of the oxide layer after oxidation for 1–10 h at 1500 ℃ were studied. As the oxidation time increases to 10 h, the macrostructure of the porous C sf /MA-SiBCN composite collapses obviously, while the C sf /SiBCN composite densified via the polymer infiltration and pyrolysis (PIP) process maintains structural integrity due to the formation of the oxide layer with a tri-layer structure: the outermost loose layer mainly containing cristobalite, the dense interlayer of amorphous glass and the innermost layer of some unoxidized BN(C) and SiC grains embedded into the amorphous glass, which hinders the diffusion of oxygen. [ABSTRACT FROM AUTHOR]

This study, investigates how adaptable silica made from rice husks could be used in energy storage applications. It is show that rice husk silica (SiO2) can be used in a variety of applications with the right processing. Cold plasma is used to prepare the silica samples in a variety of structural phases which improves their electrochemical characteristics. Using (X-ray diffraction analysis) XRD, Raman spectroscopy, (Fourier-transform infrared spectroscopy) FTIR, (Field emission scanning Electron microscopy) FESEM, and (Energy Dispersive spectroscopy) EDS techniques are used to investigate and evaluate the differences in their structural, morphological, and chemical composition. A three-electrode system is used to assess the material's electrochemical performance. Surprisingly, it is found that after plasma treatment, amorphous silica materials have a large increase in specific capacitance (297 F/g) as a result of an increase in functional groups and better adhesion characteristics. These silica samples also have an outstanding electron reversibility and coulombic efficiency. The effect of the various properties of the silica on super capacitance and catalyst performances in metal air battery is discussed. [ABSTRACT FROM AUTHOR]

Understanding the thermal properties of zeolites is crucial for their industrial applications. This study explores the thermal stability and dehydration process of clinoptilolite using high-temperature X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). Clinoptilolite's thermal decomposition was monitored from 25 °C to 1200 °C, with results analyzed based on its crystal structure. Principal components analysis (PCA) of the DRIFT spectra indicated progressive water removal and dehydration upon heating, leading to the formation of hydrogen bonds. Thermogravimetric analysis (TGA) revealed a two-step endothermic weight loss: initially, physically adsorbed water was lost up to 100 °C, followed by the removal of tightly bound water and hydroxyl groups until 800 °C. Clinoptilolite remained the dominant phase up to 800 °C, after which albite and cristobalite took over. Rietveld refinement showed that the sample initially contained 70% clinoptilolite, 24% albite, and 6% cristobalite. Above 800 °C, clinoptilolite disappeared, leaving 93% albite and 7% cristobalite. FT-IR spectra changes due to water loss were evident: drying of adsorbed water occurred up to 75 °C and minimal changes were observed from 75 °C to 135 °C, followed by further dehydration until 240 °C. Complete dehydration was confirmed by the disappearance of OH stretching vibration bands by 395 °C, consistent with TGA findings. [ABSTRACT FROM AUTHOR]

Silica-rich igneous rims (SIRs) occur commonly as an outer rim layer on porphyritic chondrules in CR (Renazzo-like) chondrites, as well as more rarely in other chondrite groups. Formation conditions for SIRs can provide insight into chemical and physical conditions in the chondrule-forming region of the protoplanetary disk, as well as helping to understand temporal changes to successive thermal events within the CR and other chondrule formation regions. Both accretion and condensation models have been proposed as formation mechanisms of SIRs, but a lack of robust thermal constraints prevents a detailed interpretation. We have carried out an experimental study aimed to define the conditions of SIR formation by reproducing the texture, mineralogy, mineral chemistry, and silica polymorphs present in natural SIRs. Experiments conducted on an SIR bulk composition, at peak temperatures of 1310–1507 °C with linear cooling rates between 30 and 90 °C/hr successfully reproduce natural SIRs that contain cristobalite, low-Ca pyroxene, Ca-rich pyroxene, and glass. Microcrystalline mesostasis was formed in an experiment with lower cooling rates at low temperatures (6 °C/hr from <1200 °C). All silica in the experiments was cristobalite, including in experiments with maximum temperatures as low as 1310 °C. Since the cristobalite stability field in the silica phase diagram is >1470 °C, it is clear that silica polymorphs are not robust temperature indicators and that the presence of cristobalite in natural SIRs does not necessarily indicate high peak temperatures. We suggest that accretion of SIR precursors onto solidified chondrules, followed by melting, is the most likely scenario for their formation. Our constraints on SIR formation are similar to those that are usually discussed for chondrules. Therefore, in the CR chondrule-forming region, a repeatable heating mechanism is required that does not change significantly during the time in which condensing material evolved to highly silica-rich compositions. The need for recurring heating events is a general constraint for modeling chondrule formation. [ABSTRACT FROM AUTHOR]

Kitchen and bathroom countertop is a demanding application, where high aesthetic standards must combine with durability, ease of maintenance, and resistance to heat, stain, scratch and chipping. The hard materials and composites used for high-end countertops usually contain crystalline silica phases that can be inhaled by workers during drilling and cutting operations. The occurrence of silicosis and other respiratory diseases in machining workers makes it important to know exactly how much crystalline silica is present in countertop materials. This paper collects over 300 quantitative determinations of quartz and cristobalite in porcelain stoneware products and compare these contents with other countertop materials. The sum of crystalline silica phases in porcelain stoneware is on average 21 ± 5 % by weight (mostly quartz). This content is lower than granite (∼30 %) and much lower than engineered stone (∼90 %). Possible ways to reduce the amount of crystalline silica phases in ceramic slabs are overviewed and critically discussed.

Towards the end of the 19th century and the early 20th century, France was the world's largest producer of antimony, especially due to the rich deposits in the Brioude‐Massiac area. Even though all the mining and smelting activities are long gone, there are still some remains of those activities. The most original of those is the use of roasting slags as aggregates in former plant walls or industrial building. They are macroscopically characterised by a black or red colour with a vesicular aspect similar to natural pozzolanas. Common petrography and mineralogy tools have been used to characterise those slags: optical mineralogy, scanning electron microscopy coupled with energy dispersive spectroscopy (SEM‐EDS) and X‐ray diffraction (XRD). To gain an insight into their mineral complexity, a QEMSCAN map has been produced. The mineralogy is typical of high‐temperature slags: cristobalite, quartz, mullite, fayalite‐forsterite series and spinel. The antimony content is quite high, up to seven percent, under the form of various antimony oxides and native antimony. LAY DESCRIPTION: In the Brioude‐Massiac area, France, antimony has been heavily mined at the end of the 19th century, giving rise to the generation of large amounts of wastes from the metallurgy of the ores. Those wastes (scoria and slags) have been used locally as aggregates to build walls. They consist of high‐temperature phases (SiO2 polymorphs: cristobalite and tridymite, dendritic spinel, glass) and illustrate an early example of the reuse of waste materials, nowadays termed as 'circular economy'. [ABSTRACT FROM AUTHOR]

Phase equilibria in the CaO–SiO2–CrOx system were experimentally investigated at 1600°C and pO2 of 10−10 to 10−11 atm using the high‐temperature isothermal equilibration/drop quenching/electron probe X‐ray microanalysis technique. The constrained isothermal sections of the CaO–SiO2–CrOx system were constructed at 1600°C and pO2 of 10−10 to 10−11 atm based on the experimental results. The primary phase fields including cristobalite (SiO2), larnite (Ca2SiO4), (Ca,Cr)Cr2O4, and corundum (Cr2O3) were determined. Simulations by thermodynamic software MTDATA, FactSage, and Thermo‐Calc were compared with the present experimental results. [ABSTRACT FROM AUTHOR]

Copyright of Refractories / Naihuo Cailiao is the property of Naihuo Cailia (Refractories) Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Preparation of high modulus solution and high Al/Si residue via leaching SiO2 is a key for large amount utilization of coal gangue based on ferric oxide assisted roasting – alkali leaching silica process. This work focuses on leaching mechanism of SiO2 in activated kaolinite clinker with low modulus sodium silicate solution. The results show that the lattice distorted cristobalite produced in the roasting process is readily soluble in sodium silicate solution, however the leaching of SiO2 was only 75% and the modulus of lixiviant reached 2.5 through leaching in sodium silicate solution with a modulus of ~1.0. The leaching kinetics and dissolution mechanism have also been analyzed by model fitting and microstructural observation. The leaching reaction was controlled by diffusion across the pore. The slit – shaped pores formed by the stacking of hercynite particles prevent the diffusion of the bulk solution, thus inhibiting the leaching of cristobalite inside the particles. [ABSTRACT FROM AUTHOR]

Low-k dielectric materials were prepared by sintering ZSM-5 zeolite powder compacts at temperatures ranged 1200-1300°C, and the microwave dielectric characteristics were investigated together with the structure evolution during sintering. It was failed to obtain ZSM-5 ceramics by high temperature sintering because the densification could never occur below the tolerable temperature and the ordered porous structure completely collapsed above the tolerable temperature. The structure changed from ZSM-5 zeolite to SiO2-Al2O3 glass at 1240°C, and finally to SiO2-based cristobalite ceramics above 1275°C.Alow dielectric constant (2.63-2.78) combined with low dielectric loss (< 0.0005 at 18GHz) and small temperature coefficient of resonant frequency (varied from -18.6 to -8.1 ppm/°C) was achieved in the present materials, and the value of dielectric loss was the lowest one achieved so far in the dielectric materials with a dielectric constant lower than 3. With the structure evolution fromZSM-5 zeolite to SiO2-Al2O3 glass and finally to SiO2-based cristobalite ceramics, the dielectric constant increased from 2.27 to 2.78, while the Qf value indicated a complicated variation tendency, and the higher Qf values were achieved in the amorphous and crystalline states, while significant drop was determined in the partial amorphous state. It should be a great challenge to prepare dense zeolite ceramicswithout collapse of ordered porous structure, and then the ultra-low dielectric constant materials with high Q could be expected. [ABSTRACT FROM AUTHOR]

In the CR (Renazzo‐like) chondrite group, many chondrules have successive igneous rim (IR) layers, with an outer layer that contains a silica mineral and/or silica‐rich glass (silica‐rich igneous rims, SIRs). Models for SIR formation include (1) accretion of Si‐rich dust onto solid chondrule surfaces, followed by heating and cooling and (2) condensation of SiO(gas) onto the surface of partially molten chondrules. We evaluate these models, based on a petrographic study of five Antarctic CR chondrites that have undergone minimal secondary alteration. We obtained electron microprobe analyses of minerals and glass with quantitative wavelength‐dispersive spectroscopy mapping, and identified silica polymorphs with Raman spectroscopy. Common SIRs contain silica, low‐Ca pyroxene, Ca‐rich pyroxene, Fe,Ni metal, ± glass ± plagioclase ± rare olivine. We also describe near‐monomineralic SIRs where a narrow zone of cristobalite occurs at the outer edge of the chondrule. All crystalline silica is cristobalite, except for one SIR that consists of tridymite. Some rims contain silica‐rich glass (>80 wt% SiO2) but no silica mineral. Features such as sharp interfaces and compositional boundaries between chondrules and SIRs indicate that SIRs were formed from solid precursors. Consideration of the stability fields of silica polymorphs and computed liquidus temperatures indicates that SIRs were heated to >1500°C for limited time periods, followed by rapid cooling, similar to conditions for chondrule formation. We infer that in the CR chondrule formation region, the same heating mechanism was repeated multiple times while the chemical composition of the nebular gas evolved to highly fractionated silica‐rich compositions. [ABSTRACT FROM AUTHOR]

High-purity quartz and cristobalite grits were obtained using the dressing technology of quartzite from the Eastern Sayan. Electron paramagnetic resonance (EPR) of structural impurities and defects in the fast-electron-irradiated samples of quartzite grits after various stages of their enrichment has been studied. A computer simulation of complex EPR spectra was performed to clarify the nature of structural impurities and point defects. From EPR studies, it was found that structural germanium impurities are observed as five different paramagnetic centers (PCs) associated with the vacancy of an oxygen atom or with Li and Na atoms in structural c-channels. After calcination of quartz concentrates, the formed cristobalite inherited Ge and Al isomorphic impurities with modified spin Hamiltonian parameters of the EPR spectra. Paramagnetic defects similar to E'γ, E'α1, E'δ centers were observed in pure cristobalite. Additionally, a new Ge E'δ center was detected in quartz-cristobalite concentrate with structural impurities of germanium. [ABSTRACT FROM AUTHOR]

The scarcity of excavated early-stage smelting sites related to copper production presents significant challenges in gaining a comprehensive understanding of the copper production process. However, the archaeological site discovered in 2018 in Daeryang-ri, Jinan-gun, Jeollabuk-do, boasts a substantial number of copper smelting remains and related slags, marking it as the first copper manufacturing production site identified on the Korean Peninsula. Consequently, this study selected 10 slag samples, chosen based on surface color and characteristics indicative of a connection to copper smelting, for scientific analysis to accurately ascertain the site's nature. The primary component analysis of the slags indicated that CuO content ranged from 0.30 to 3.29 wt%, which, although not high, reveals significant quantities of FeO and SiO2. X-ray diffraction analysis revealed the presence of minerals such as cristobalite, along with fayalite and wüstite, commonly found in slags, varying by sample. Furthermore, microstructural observation revealed circular copper particles containing sulfur and iron, indicating the presence of copper particles in a matte state that have not been refined. This analysis suggests that the slags recovered from Jinan Daeryang-ri bear evidence of iron smelting at the site, with the slag being produced as an intermediate by-product during copper production. [ABSTRACT FROM AUTHOR]

Groundmass textures of volcanic rocks provide valuable insights into the processes of magma ascent, crystallization, and eruption. The diktytaxitic texture, characterized by a lath-shaped arrangement of feldspar microlites forming glass-free and angular pores, is commonly observed in silicic dome-forming rocks and Vulcanian ashfall deposits. This texture has the potential to control the explosivity of volcanic eruptions because its micropore network allows pervasive degassing during the final stages of magma ascent and eruption. However, the exact conditions and kinetics of the formation of diktytaxitic textures, which are often accompanied by vapor-phase cristobalite, remain largely unknown. Here, we show that the diktytaxitic texture and vapor-phase minerals, cristobalite and alkali feldspar, can be produced from bulk-andesitic magma with rhyolitic glass under water-saturated, near-solidus conditions (± ~10 MPa and ± ~20 °C within the solidus; 10–20 MPa and 850 °C for our starting pumices). Such crystallization proceeds through the partial evaporation of the supercooled melt, followed by the deposition of cristobalite and alkali feldspar as a result of the system selecting the fastest crystallization pathway with the lowest activation energy. The previously proposed mechanisms of halogen-induced corrosion or melt segregation by gas-driven filter pressing are not particularly necessary, although they may occur concurrently. Diktytaxitic groundmass formation is completed within 4–8 days, irrespective of the presence or composition of the halogen. These findings constrain the outgassing of lava domes and shallow magma intrusions and provide new insights into the final stages of hydrous magma crystallization on Earth. [ABSTRACT FROM AUTHOR]

The effect of soaking time on the properties of porcelain stoneware tiles based on clay mixture and recycled waste glass powder was studied. Mixture was prepared and fired at 1150°C. During firing, raw mixture was subjected to various soaking times: 30, 60, 90 and 120 min. The by-products were characterized by their linear shrinkage, water absorption, bulk density, flexural strength, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results were modelled by using different mathematical expressions. The results demonstrated that physico-mechanical properties of ceramic probe are significantly affected by soaking time. After 30 min, products satisfied the ISO 13006 standard condition for porcelain stoneware wall and floor tiles: water absorption value was lowest (0.07%) and mechanical strength value was 44.4 MPa. Prolonged soaking time was undesirable due to its negative effect (bloating) on the products' physical and mechanical performance. The XRD patterns indicated the presence of mullite, cristobalite, enstatite, anorthite, hematite and quartz as the main crystalline phases. SEM images of the tiles fired at 1150°C for 30 min display a very close texture and dense microstructure. [ABSTRACT FROM AUTHOR]

This paper is a comprehensive review devoted to topical issues in the study of opoka – marine sedimentary siliceous rocks of the Cretaceous-Paleogene age. For almost two hundred years of studying these sedimentary formations, the term “opoka” itself has not received proper recognition globally due to the lack of a detailed mineralogical definition and the general imperfection of the systematics of siliceous rocks. A distinctive feature of the opoka is the presence of opal-CT lepispheres, which form a recognizable framework of siliceous rock or form an obscure lepispheric/ cryptocrystalline mass. The article discusses various aspects of opoka – history of study, nomenclature, mineralogical composition, microtexture, the distribution of Cretaceous- Paleogene opoka, etc. The etymology of the genesis of the opoka and models of its origin at the early and late stages of diagenesis are critically discussed. In this review, attention is focused on recent studies, that provide detailed information about silica polymorphic forms distributed in the opoka, as well as its typical microtexture types. A model of the genesis of opoka and the relationship with global biogeochemical cycle of silica is presented. Differences recorded in opoka of different ages, geological and tectonic positions, indicate both the predetermination of many parameters of the mineral composition from the very genesis, and the disproportionate contribution of individual factors to the evolution of opoka rocks, namely the source of silicic acid, the role of siliceous bios in the mobilization of dissolved silica, the nature and volume of terrigenous influx from the adjacent landmass, secondary changes and mineral formation, etc.

Copyright of Refractories / Naihuo Cailiao is the property of Naihuo Cailia (Refractories) Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The four smectite clays in the CSSJ reference samples have been examined using TEM/STEM imaging and energy dispersive X-ray spectroscopy (EDS) in SEM and STEM, to characterize their chemical compositions, grain sizes, impurity minerals, and other properties. Fine calcite particles were commonly found along with quartz in the Tsukinuno samples (JCSS-3101, 3101b), which serve as the primary source of calcium as reported in previous bulk analyses. Cristobalite, quartz, and feldspar are prevalent impurity minerals in the Mikawa sample (JCSS-3102), and the STEM-EDS analysis revealed that the composition of the smectite clay is definitely montmorillonite with subtle difference from the Tsukinuno montmorillonite. The synthetic saponite (JCSS-3501) displayed a wide range of crystal sizes. Sulfate and sodium ions are concentrated in the aggregates of very fine saponite crystallites compared to the saponite particles with larger sizes. [ABSTRACT FROM AUTHOR]

The possibility of using microsilica in the production of mullite–silica refractories was assessed. The chemical and mineralogical compositions of the raw materials, refractory Arkalyk clay and microsilica, were studied. It has been found that primary mullite and quartz formation occurs due to dehydration of kaolinite with the formation of intermediate metakaolinite. The introduction of alumina and microsilica into the charge composition promotes the formation of secondary mullite due to the interaction of aluminum oxide and highly dispersed chemically active microsilica. Free silica in compositions undergoes polymorphic transformations with the formation of cristobalite and tridymite. Mullite–silica refractories with an open porosity of 21%, a compressive strength of 42 MPa, and a thermal deformation temperature under the load of 0.2 MPa–1350 °C were obtained. [ABSTRACT FROM AUTHOR]

Chromium-doped persistent luminescence nanoparticles (PLNPs) have promising biomedical applications in X-ray charged near-infrared (NIR) luminous imaging. The persistent luminescence (PersL) property was usually adjusted by co-doping with other ions. Impurity phases were thought to be adverse to the PersL. In this study, a composite material, LiInSi 2 O 6 :Cr,Pr·2SiO 2 (LICP-S), was synthesized with a PersL peak at 800 nm after being charged by 1 Gy X-ray while LiInSi 2 O 6 :Cr (LIC) and LiInSi 2 O 6 :Cr,Pr (LICP) possessed no or very weak PersL. Distinctively, excess metastable cristobalite silica was found essential for the much more intense PersL of LICP-S than LICP and LIC. Thermostimulated luminescence spectra studies indicated that cristobalite adjusted the Pr-related traps from deep traps to shallow ones, which facilitated the room temperature PersL. The low dose X-ray charged PersL of LICP-S was applied to demonstrate the luminous labeling of a medical silicone prosthesis that was supposed to be imbedded in body for years via X-ray activated PersL imaging in vitro and in vivo. This work not only provides a kind of PLNPs with 800 nm emission suitable for low dose X-ray activation, but also an unusual strategy for trap engineering in PersL materials by designing multi-phase composites. Metastable cristobalite SiO 2 induces 10-times enhanced NIR persistent luminescence of LiInSi 2 O 6 :Cr,Pr after X-ray activation. [Display omitted] [ABSTRACT FROM AUTHOR]

The study on the origin of quartz and silica in Xuanwei Formation coal in Northwest Yunnan, China, is helpful to understand the relationship between quartz and silica and the high incidence of lung cancer from the root. To address these questions, the mineralogy and microscopic studies of silica in Xuanwei Formation coal were performed. The following results were obtained: (1) silica in the late Permian Xuanwei Formation coal seams originated from detrital input, early diagenesis, and late diagenesis. (2) A more significant contribution comes from early diagenesis, which contains abundant authigenic quartz and amorphous silica. (3) Quartz and silica from inorganic silicon are more symbiotic with kaolinite and from biogenic silicon with chamosite. (4) Three silica polymorphs in coal samples have been identified: opal-A (amorphous silica), opal-CT/-C (cristobalite/tridymite), and α quartz. (5) Opal-A is ubiquitous, while opal-CT/-C and α quartz are rare. (5) Opal-A is an amorphous and nontoxic ordinary silica. (6) Since the toxicity of amorphous silica and its presence in coal is an emerging topic, it should be continuously monitored. [ABSTRACT FROM AUTHOR]

Deynekoite, Ca9□Fe3+(PO4)7 (R 3 c , a = 10.3516(3)Å, c = 37.1599(17)Å, V = 3448.4(3)Å3 and Z = 6), a new mineral of the merrillite group was found in the contact facies of paralava of the Hatrurim Complex in the Daba-Siwaqa pyrometamorphic rock field, Jordan. The paralava, consisting of diopside, tridymite, anorthite, wollastonite and fluorapatite, is enriched in Fe-bearing phosphides and phosphates at the contact with the altered country rock. Cristobalite overgrowing tridymite has a fish-scales texture indicating that temperature of paralava could have reached 1500°C. Deynekoite with empirical formula (Ca8.90Na0.11K0.02)Σ9.03(Fe3+0.62Mg0.30Al0.05)Σ0.97P6.98V5+0.05O27.70(OH)0.30 forms transparent, light-yellow or light-brown grains up to 30–40 μm in size. Microhardness of deynekoite, VHN25 = 319(29) kg/mm2, corresponds to Mohs hardness = 4.5. Its density was calculated as 3.09 g⋅cm–3 on the basis of its empirical composition and structural data. Deynekoite is uniaxial (−), its refractive indices are ω = 1.658(3), ɛ = 1.652(3) (λ = 589 nm), and pleochroism is not observed. The formation of phosphides on the boundary of the paralava and country rock is connected with carbothermal reductive reactions and realised at temperatures above 1300°С. With decreasing temperature and increasing oxygen activity, phosphides are replaced by Fe2+-bearing phosphates. Deynekoite, which contains Fe3+ (substituting for Fe2+-phosphates) and a small amount of water, formed at temperatures of 600–800°C. [ABSTRACT FROM AUTHOR]

Three clay materials (codded MY3, KG and KK) from western Cameroon were selected and investigated for the production of refractory bricks. Samples MY3 and KG are kaolinite-rich materials, having clay mineral contents of 88% and 72%, respectively, whereas KK is a sand-rich material with a total clay content of 44%. Chamottes were prepared using each clay, and the ground chamottes were later used in the refractory formulation, with each raw clay used as a binding phase. After firing, the X-ray diffraction analyses of all of the refractory bricks indicated the same mineral assemblage made of quartz, cristobalite and mullite. The cristobalite is due to high-temperature conversion of quartz, whereas the mullite is due to clay mineral conversion. The evaluation of the linear shrinkage, physical properties (including bulk density and open porosity) and mechanical testing through refractoriness under load and compressive strength indicates that all of these clays could be used as raw materials for standard clay refractory materials. These results stand as a proof for the potential for these locally available materials to be used as raw materials for refractories that could be locally produced to reduce the cost of access to refractory raw materials faced by industries located in Cameroon. [ABSTRACT FROM AUTHOR]

Zeolites are a family of open-framework aluminosilicate minerals used in many diverse fields, including building materials, agriculture, water treatment, and catalysis. In this study, natural zeolites were mechano-chemically treated by co-milling with corundum and cristobalite. The idea behind the study was that co-milling with high-hardness oxides would cause natural zeolite to undergo more structural distortion, potentially increasing its reactivity and sorption capabilities. Corundum has a density of 3.95 g/cm3 and a hardness of 9, while cristobalite has a density of 2.27 g/cm3 and a hardness of 6-7, according to the Mohs hardness scale. In a planetary ball mill, the zeolites and 20 wt.% of various oxides were co-ground for 30 min. The grinding media used were hardened steel balls with a weight ratio of 20:1 between the balls and the minerals. Raw minerals and milled products were evaluated using X-ray diffraction, Fourier-transform infrared spectroscopy and scanning electron microscopy. It revealed that co-milling with different hardness oxides had a minor effect on the structural distortion of raw zeolite. Crystallite size reduction and amorphization were observed in high hardness oxides rather than in zeolite particles. After milling, the amorphization of natural zeolite milled alone was 30.4%, while no significant amorphization was observed when co-milled with corundum and cristobalite. Preliminary results of Cr(VI) adsorption tests on raw and milled zeolites indicate that co-milling with high-hardness oxides is not the preferred method to enhance the activity of natural zeolite.

Alginates are products used as impression materials in dentistry and prosthetics. They consist of polymers, calcium alginates mixed with diatomite and additives. Recently, the occurrence of severe silicosis associated with exposure to respirable dust of such materials has increased the scientific interest in understanding how these materials may pose a toxicological problem to workers. The primary objective of this study is to improve the understanding of both the existence and the characteristics of the toxicant(s) contained in these materials, with the goal of better defining the risk assessment for this occupational setting.Two commercial dental alginates were subjected to a mineralogical, microchemical and spectroscopic investigation. The results indicate the presence of a significant amount of diatomite, clearly identified by micromorphology and formed mainly by cristobalite. The respirable fraction of the dust represents at least 30 % of the total number of particles, and this fraction contains a relevant amount of crystalline silica particles. Conversely, the investigated alginate materials do not exhibit the presence of radical species.The results obtained confirm that the cristobalite detected originates from the high-temperature transformation of amorphous silica during the calcination process of diatomite, prior to mixing with the other components. The same process also produces wollastonite (CaSiO3), which, like cristobalite, is a crystalline phase known for its toxicological effects. The present findings call for a rethinking of dental alginates with regard to the definition of their health risks for technical operators.

The results of investigations of the preparation of electrical porcelain based on available customary raw materials of Uzbekistan are presented. It is shown that in conditions of a shortage of high-grade mineral raw materials for the preparation of electrical porcelain it is advisable to introduce about half of the kaolin in a pre-calcined state into the mixture. In addition the products of firing primary and secondary Angrenskoe kaolins at 1350°C allow us to consider them as a mullite-containing component that subsequently imparts high physicomechanical and dielectric properties to the porcelain material. The phase composition of the resulting porcelain is represented mainly by new crystalline components — mullite and cristobalite, which allows it to be classified as an electric porcelain of the mullite-type. [ABSTRACT FROM AUTHOR]

Although silica‐based ceramic cores have important applications in the precision casting of metallic devices, their high‐temperature stability and removal performances are seriously affected by the liquid phase sintered fused silica. Herein, we develop a manufacturing strategy of high‐collapse silica‐based ceramic core via using cristobalite crystals as the sintering inhibitor, waterglass as the binder, and injection moulding at 100°C and 80 MPa, followed by heat treatment simulating the casting process for sintering at 1200°C and 1500°C. The results demonstrated that the addition of cristobalite crystals could effectively form the core skeleton to ensure high‐temperature performance. Meanwhile, it inhibited the liquid flow during sintering and induced the crytsallization from fused SiO2 glass into cristobalite crystals, and the resulting plenty of micropores and microcracks within the microstructure effectively improve the removal performance. Especially, the porosity was highest up to 35.36% and the flexural strength was only 6.74 MPa when the addition of cristobalite reached 45%, realizing a 100% removing by high‐frequency and fast‐speed specific mechanical vibration. And, the casting is guaranteed to be flat and free of defects. This work provides a simple and flexible strategy to manufacture high‐collapse silica‐based ceramic cores, which can be removed by specific mechanical vibration without immersion in acid or alkali solutions after casting. [ABSTRACT FROM AUTHOR]

Forming high‐temperature α‐Al2O3 phase under soft sparking is an intriguing phenomenon in plasma electrolytic oxidation (PEO) of Al alloys, which contradicts the low energy input of the process. In this study, α‐Al2O3 doped with black CuO is formed beneath an amorphous white outer layer on Al–Cu–Li alloy by PEO in a dilute silicate electrolyte under soft sparking. In comparison, reddish coatings with dominating γ‐Al2O3 are formed under the conventional plasma discharges, although blackish inner layer with α‐Al2O3 can also be exposed by heavily polishing the samples. In order to know the underlying mechanism, temperatures at the coating surface and the underlying substrate have been monitored by a thermocouple under the conventional and soft sparking PEO regimes, respectively. Interestingly, high temperatures are detected in the case of soft sparking rather than PEO with strong discharges. The formation of CuO, quartz, and cristobalite within the soft sparking coating also supports the existence of high temperature. Hence, the formation of α‐Al2O3 under soft sparking can be resolved to the conventional thermal activation mechanism, without the need of seeking other plausible explanations. Thermal condition evaluation for soft sparking PEO suggests that values of the effective thermal conductivity during PEO process for the outer layer and the barrier layer at the coating/substrate interface might be lower than ∼0.05 and ∼0.0017 W m−1 K−1, respectively. It is believed that the amorphous structure of the outer and barrier layers effectively blocks the heat dissipation, facilitating the formation of a highly wear‐resistant inner layer with α‐Al2O3, CuO, and the other high‐temperature species under soft sparking. [ABSTRACT FROM AUTHOR]

Phytoliths (siliceous structures) present in the plants have been employed in the fields of taxonomy and archaeology for many decades. Rostraria cristata is an economically important grass species (Poaceae) which accumulates silica in its different organs in the form of phytoliths. In order to understand the pattern of phytolith production and biochemical architecture of silica in R. cristata, leaf epidermis (blade) using the clearing solution method and different organs using the dry ashing method, X-ray diffraction and Fourier-transform infrared spectroscopy techniques were analyzed. Both abaxial and adaxial leaf epidermis showed the presence of acute bulbosus, rectangular sinuate and stomata phytolith morphotypes. Leaf including sheath and blade had the highest silica content. Characteristic phytolith morphotypes were present in different organs. A total of 34 phytolith morphotypes were present among which nine (9) were articulated and 25 were isolated forms. The most abundant were elongate scrobiculate (48.20%) in root and rectangular sinuate (26.16%) in leaf part. Other common phytolith morphotypes present in different organs of R. cristata were articulated elongate irregular, articulated elongate scrobiculate, acute bulbosus, and polygonal rondel etc. Leaf and synflorescence had the highest similarity based on presence/absence of phytolith morphotypes (Jaccard's similarity index). XRD studies revealed the presence of cristobalite, quartz, tridymite, zeolite etc. forms of silica in different organs. FTIR spectra showed that inplane stretching vibration of Si–C was unique to root, anti-symmetric stretching vibration of C–H was unique to leaf and Al2O3.SiO2 was found in synflorescence only. Our results show the characteristic pattern of phytoliths production in R. cristata. [ABSTRACT FROM AUTHOR]

A novel SiC–Si coating with a high amount of net-like distributed Si was fabricated on the graphite substrate by liquid silicon infiltration (LSI) method to improve the oxidation-resistant performance. The mass gain of this LSI coating after 300 h oxidation at 1873 K was only 3.018%, suggesting an excellent static antioxidant property at 1873 K. The detailed oxidation mechanism of this LSI coating was explored in this work. It was found that the high amount of Si with the net-like distribution in this LSI coating promoted the passivation growth of SiO 2 glass layer and prevented the SiO 2 glass from crystallizing into cristobalite. In addition, the Si in this coating retarded the active oxidation and enhanced the coating stability. In contrast, the infiltration (VSI) SiC–Si coating presented an apparent mass loss, and the pack cementation (PC) SiC coating showed the catastrophic failure. The oxidation mechanisms of these two coatings were also discussed against the backdrop of the LSI coating with high antioxidant performance. [ABSTRACT FROM AUTHOR]

Porous mullite ceramics were prepared from mixtures of Egyptian kaolinite-clay and calcined α-alumina powder. The effects of two methods of compaction as well as two types of binder were investigated. The influence of sintering temperature on the physical properties, crystalline phase, compressive strength and microstructure was studied. Mullite bodies processed by uniaxial pressing utilizing 15wt. % water at 1500°C for 2h exhibited the highest compressive strength of 150 MPa and bulk density of 2.1 g/cm3, as well as, an open porosity of 35%. In the present study, the only stoichiometric mullite was achieved by the crystallization of metakaolin. The mullite crystallizing out from the liquid phase was rich in silica, as determined by EDS, while that resulting from alumina grains was rich in alumina. The microstructure showed that primary mullite appeared at 1200°C, beside the unreacted alumina particles. On the other hand, at 1400°C alumina particles started to react with the glassy phase to form parallel laminates of mullite. [ABSTRACT FROM AUTHOR]

Numerous minerals found in nature contain silica, including quartz, cristobalite, opal, etc. They have the same chemical composition but different crystal structures, and this phenomenon is called "polymorphism" in mineralogy. For these polymorphic and multi-like minerals, in the flotation process, will it directly or indirectly affect the flotation effect. Based on this, this study mainly explores the difference between crystalline SiO2 and amorphous SiO2 in flotation. In this study, two crystal forms of SiO2 were subjected to flotation and adsorption capacity tests. FTIR, other test techniques, the chemical calculation of the flotation solution, and the theoretical calculation of the DLVO can all be used to provide an explanation. Finally, in the flotation experiment, the feedbacks of the two minerals to the change of the pH value of the pulp and the change of the concentration of the reagent are different. Through the comprehensive analysis of the adsorption capacity test and semi-quantitative calculation of the infrared spectrum, the adsorption capacity of crystalline SiO2 to drugs is about 23% higher than amorphous SiO2. Furthermore, during the flotation process, the amorphous SiO2 particles will agglomerate together and entrain into the foam through, resulting in concentrate pollution. So amorphous SiO2 will undoubtedly increase the difficulty of flotation. [ABSTRACT FROM AUTHOR]

The study concerns waste of the fuel and energy complex, ceramic samples, and the phase composition of ceramic samples. The fuel energy complex is one of the main sources of pollution of the natural environment. Usage of waste of the fuel and energy complex in ceramic masses for manufacturing ceramic materials is favorable for the formation of mullite, anorthite, hematite, cristobalite, and wollastonite in the products. [ABSTRACT FROM AUTHOR]

The work describes the production of investment material on the basis of β cristobalite obtained from β quartz at 1100°C in the presence of mineralizers. The obtained mixture is characterized by beginning of solidification of 9 min and compressive strength of 6.1 mPa; it provides a smooth surface and easy extraction from the mold of chromium nickel alloys. [ABSTRACT FROM AUTHOR]

Acid treatment of natural zeolites is considered to be an effective method of “improving” their structure and properties, among which thermal stability occupies a special place, especially for catalytic applications of zeolites. The influence of hydrochloric acid solutions with concentrations up to 2 mol/L and calcination at temperatures up to 1100 °C on the structure and properties of heulandite-containing tuff from the Georgian Dzegvi-Tedzami deposit, selected for the creation of new bactericidal zeolite filter materials for purification and desinfection of water from various sources, was studied by the X-ray energy dispersion spectra, diffraction patterns and thermal analysis, as well as by adsorption of water, benzene and nitrogen methods. It has been found that an acidic environment leads to significant dealumination (Si/Al molar ratio increases from 3.6 to 9.5) and decationization (total charge of metal ions per Al atom decreases from 1 to 0.68) of the sample; solutions of hydrochloric acid do not lead to amorphization of the zeolite microporous crystal structure, but gradually dissolve it. As a result of acid treatment, there is also a sharp increase in the volume of micropores available for large molecules (from ≈ 7 to 80–90 mm³ /g) and surface area (from ≈ 13 to 120–175 m² /g), as well as changes in the mesoporous system, leading to the prevalence of pores with a diameter of up to 4 nm. Heulandite heating leads to stepwise dehydration proceeding up to ≈ 800 °C, amorphization starting at ≈ 250 °C, and structural changes: the transition to methastable heulandite B phase at ≈ 340 °C is not fixed, but at ≈ 500 °C wairakite (Ca(Al2Si4O12)∙2H2O) is formed, at temperatures above ≈ 1000 °C, amorphous aluminosilicate contains crystalline inclusions of cristobalite (polymorph of SiO2), α-quartz, albite (Na(AlSi3O8), hematite (Fe2O3) and magnetite (FeO∙Fe2O3); heating-induced changes in micro- and mesopore systems are insignificant. It is also shown that heat treatment increases the acid resistance of heulandite, which is expressed in a decrease in the degree of dealumination after acid treatment of calcined samples. Thus, acid and heat treatment of heulandite make it possible to obtain materials with different sorption and ion-exchange properties. [ABSTRACT FROM AUTHOR]

Apollo 15 quartz monzodiorites (QMDs) are reported to contain some of the most deuterium-depleted apatite found in lunar samples. In this study, apatite from six Apollo 15 QMDs, including three samples from 15405 not previously investigated, were analyzed for their H and Cl isotopes. Apatite in 15405 are extremely 2H (or D)-poor, with δD values ranging from −658 ± 53 to −378 ± 113‰, comparable to apatite data from related samples 15403 and 15404. In addition to new H isotope data, the first Cl-isotope data for lunar QMDs are presented. Apatite in 15405 and related samples are enriched in 37Cl with respect to Earth, with measured δ37Cl values ranging from +13 to +37‰. These values are within the reported δ37Cl range for KREEP-rich samples. The fact that the Cl isotopic composition of apatite in QMDs are similar to those in other lunar lithologies, but the H isotopic data are distinct and unique, provides possible further evidence for the existence of a D-poor reservoir in the lunar interior. Raman spectroscopy of the silica polymorph in sample 15405 reveals it to be a mixture of quartz and cristobalite. Based on available experimental data on the stability of various silica phases over a range of pressure and temperature regime, a deep-seated origin in the crust for QMDs may be possible which would support an endogenous origin of the H-Cl isotope systematics of the QMDs. The role of impact-induced transformation of silica phases and its contributing towards low D/H ratio in apatite, however, cannot be ruled out. [ABSTRACT FROM AUTHOR]

Hydrogen-based direct reducing iron production (H-DRI) has caused more and more attention in metallurgical industry owing to the near-zero carbon emission. However, the strong reducibility of H 2 at elevated temperature can possibly cause serious corrosion to the lining refractories of furnaces. In this study, four types of Al 2 O 3 –SiO 2 based aggregates (brown corundum, bauxite, mullite and quartz) with various SiO 2 /Al 2 O 3 mass ratios were utilized for the H 2 corrosion test at 1673 K with a soaking time of 8 h. Based on the analysis of weight-loss ratios, phase-composition variation and reduction thermodynamics, the microstructure evolution during H 2 corrosion was investigated in detail. The results indicates that the SiO 2 impurity of brown corundum was partly reduced by H 2 which led to the formation of rod-like or flaky intergranular grains. For bauxite, a porous structure was produced by the accumulation of rod-like or elongated grains owing to the significant reduction of intergranular silicon compounds. Nevertheless, due to the high sintering activity between the Al 2 O 3 products from the reduction of mullite, a high densification structure was realized to effectively prevent the further H 2 penetration, hence, mullite inversely obtained the better H 2 corrosion resistance than bauxite in spite of its higher S/A ratio. The accumulation of SiO and H 2 O gases due to the high reduction rate of SiO 2 led to the formation of a great deal of SiO 2 fiber on the surface of quartz aggregates. The [O] provided by the phase-transformation from α-quartz to cristobalite could be partially captured by the H 2 for the reduction, which caused the deterioration of corrosion. With this study, we aim to provide theoretical assistance for the refractory selection applied in hydrogen metallurgy. [ABSTRACT FROM AUTHOR]