Formation Of Amorphous Silica Research Articles

Water-Rock Interaction and Freeze-Thaw Cycles as Drivers of Acid Rock Drainage Generation by a Rock Glacier in the European Alps.

Little is known to date about the processes governing natural acid rock drainage (NARD) generated by rock glaciers. We used paragneiss samples from a catchment with NARD generated by a rock glacier in the Italian Alps for long-term leaching experiments under conditions that are possible within rock glaciers. The findings clearly suggest that at a low acid neutralization capacity of the rock, the dissolution of sulfide minerals, even if they are present in trace amounts, may be the most important process that controls the groundwater acidity at 1 °C, a typical temperature of groundwater discharge from rock glaciers. The acidic conditions increase the solubility and mobility of aquifer lithology-specific trace elements, and concentrations of some heavy metals of geogenic origin (e.g., Mn and Ni) may greatly exceed health standards after a six month interaction of water with paragneisses. Diurnal freeze-thaw cycles were found to be 6-7 times more effective in transformation of coarse rock fragments to fine-grained debris with fresh, reactive mineral surfaces, compared with temperatures above freezing. Cyclic freezing favors an enhanced formation of amorphous silica, a highly effective adsorbent for metal ions, and its redissolution within unfrozen layers of rock glaciers may represent an additional source of trace elements.

Variations of silicon species, dissolution and crystallinity within sichars prepared under different heating rate

Silicon within Si-rich biochars (sichar) plays a crucial role in immobilizing heavy metals and providing slow-releasing bioavailable silicon for silicophilic plants. However, the impact of heating rate on the silicon properties and carbon‑silicon interactions in sichars remains unclear. In this study, rice husk was used as a silicon-rich biomass to prepare sichars at different heating rates (10, 30 and 60 °C per minute, and ultra-fast-pyrolysis), then experiments such as silicon concentration measurement, Raman and XRD characterization were conducted. The results showed that a faster heating rate reduced the carbon content during pyrolysis while promoted the formation of amorphous silica, resulting in a threefold increase in dissolved silicon in sichars prepared at 400 °C. Additionally, we observed the formation of a meta-stable SiO2 polymorph (tridymite) in rice husk-derived biochars under fast heating, differing from the previously observed quartz generated at slow heating rates. Regarding the CSi relationship, a faster heating rate facilitated the removal of the surface carbon layer, exposing the underlying silicon layer. This led to more soluble silicon species and less encapsulated silicon, resulting in a continuous release and cumulative silicon dissolution amount 1.2 times and 1.6–1.9 times higher, respectively, than those in slow heating rate-derived sichars. Consequently, this enhanced silicon uptake in rice seedlings. Our findings indicate that beyond pyrolysis temperature, the heating rate significantly affects the silicon species, silicon dissolution behavior, and carbon‑silicon relationships of biochar, ultimately determines the properties and applications of sichars.

Дослідження впливу параметрів процесу хонінгування силікатними брусками на якість обробленої поверхні

Numerous studies have shown that the kinematics of the honing process, which determines the trajectory of the cutting grain during machining, has a major impact on the parameters of macro- and microrelief. The complex working movement during honing of holes creates favourable conditions for the fuller use of the cutting capacity of the bars and uniform wear of the tool, which positively affects the machining performance and accuracy of the geometric shape of the surface. It is shown that, in this regard, there is a need to investigate the influence of the parameters of the honing process with silicate bars on the quality of the machined surface. The paper presents the results of a study of the effect of the contact pressure of the bars and the axial and circumferential speeds on the quality of the treated surface. The study was performed on a vertical single-spindle honing machine. The samples used were cylinder liners of car engines made of grey alloy cast iron. The evaluation was based on the roughness of the machined surface and the mass removal of metal during machining. The best performance was achieved by those bars that resulted in the lowest roughness of the samples. It was found that with an increase in the circumferential speed, the surface quality improves, and with an increase in the axial speed, it deteriorates after processing with diamond, abrasive and silicate stones. With an increase in the contact pressure of the bars, the surface quality deteriorates after machining with diamond bars and improves after machining with abrasive and silicate bars. It has been proven that the high surface quality after treatment with silicate blocks is due to the formation of amorphous silica during honing, which, under the influence of abrasive grains, falls into the cavities of the treated surface and adheres to the surface, thus saturating the treated surface with silicon oxide. It has been established that with an increase in the axial speed of the bars, metal removal increases, and with an increase in the circumferential speed, it decreases after machining with diamond, abrasive and silicate bars. With an increase in the contact pressure of the stones, metal removal increases after machining with diamond stones and gradually decreases after honing with abrasive and silicate stones. It was found that the metal removal after machining with silicate stones is 2 times less than after honing with diamond stones. It is proposed to set the allowance for finishing honing with silicate stones for cylinder liners made of alloy cast iron (HB 240) within 0.01-0.02 mm.

Sustainable eco‐friendly sub‐micron NaCl crystal powder‐assisted method to synthesize SiOx/C as anode materials originated from rice husk for lithium‐ion batteries

AbstractIn the present study, we produced SiOx/C from rice husk by the NaCl sub‐micron crystal‐assisted synthesis method which can mitigate environmental degradation and hazards, simplify preparation, and improve electrochemical performance. During synthesis, NaCl induces catalytic graphitization, carbon activation, and amorphous silica formation. Moreover, it is only partially consumed and can be recrystallized and reused indefinitely. Our NaCl sub‐micron crystal powder‐assisted method created lithium‐ion batteries (LIBs) with rice husk‐derived SiOx/C anodes that exhibited a high initial charge/discharge capacity of 422.05/915.93 mAh∙g−1 at 0.05 A∙g−1 current density and stable cycling performance. In addition, the SiOx/C electrode produced by the NaCl micro‐crystal method had 333.96 mAh∙g−1 capacity at 0.05 A∙g−1 current density. By contrast, bare rice husk electrode exhibited a lower capacity of 333.96 mAh∙g−1at the same condition.image

Geochemical processes and mechanisms for cesium enrichment in a hot-spring system

Abstract Geothermal systems in Tibet, a crucial geothermal region in China, belong to the Mediterranean-Himalayan geothermal belt and are characterized by a broad distribution of cesium (Cs) bearing geyserite deposits. Targejia, one of the largest Cs-bearing geyserites in southern-western Tibet, contains 1.446 × 104 tons of Cs. The highest ore grade reaches 2.89 wt%, and the ore-forming process can be subdivided into mineralization stages I to V. Cs is heterogeneously distributed in geyserites. Herein, two Cs-bearing ores are investigated, with distinct characteristics of (1) low-Cs-bearing ore (amorphous silica opal-A and opal-CT type) with low Cs (average of ~0.2 wt%), Na, K, Al, and Ca contents, and (2) high-Cs-bearing ore (clay type) with high-Cs (average of ~1.40 wt%), -Na, -K, -Al, and -Ca contents. It is reported for the first time that Cs primarily exists in clay rather than in amorphous silica opal. The Cs-enrichment mechanisms are different for the above two Cs-bearing geyserite types: (1) The deprotonated –OH, surrounded by water molecules, controls the amount of Cs absorbed on the geyserite surface (Si–OH) in the low-Cs-bearing ore. (2) The variable Cs content depends on the Al content because Al substitutes Si, yielding more negative charges to absorb Cs in the high-Cs-bearing ore. Geothermal fluid loading-mass elements, such as Cs and SiO2, precipitate as amorphous silica (opal) with clay minerals. Mineral saturation index modeling was used to predict the most applicable physical parameters for ore formation. The results confirm that the ore forms at ~85 °C and a pH of ~8.5 in the Na-Cl system at stage V. The degree of Cs enrichment reduces from the latest stage V (0–4 ka) to the early stage IV (4–17 ka), and is controlled by clay dissolution, which might further relate to the climate change in Tibet’s Holocene. Fluid-rock interaction modeling shows that dissolution–reprecipitation induces a higher order of amorphous silica formation and clay dissolution at >40 °C and pH of 5–9 at stages V and III, excluding Cs from the ore.

Chemical and Isotopic Composition of Sulfide Minerals from the Noho Hydrothermal Field in the Okinawa Trough

Studies of the element contents and isotopic characteristics of sulfide minerals from seafloor hydrothermal sulfide deposits are a significant method of investigating seawater-fluid mixing and fluid-rock and/or sediment interactions in hydrothermal systems. The seafloor hydrothermal sulfide ores from the Noho hydrothermal field (NHF) in the Okinawa Trough (OT) consist of pyrrhotite, isocubanite, sphalerite, galena, and amorphous silica. The Rh, Ag, Sb, and Tl contents mostly increase in galena as the fluid temperature decreases in the late ore-forming stage. In the sulfide minerals, the rare earth elements are mainly derived from the hydrothermal fluids, while the volcanic rocks and/or sediments are the sources of the sulfur and lead in the sulfide minerals. After the precipitation of galena, the redox state becomes oxidizing, and the pH value of the fluid increases, which is accompanied by the formation of amorphous silica. Finally, neither pyrite nor marcasite has been observed in association with pyrrhotite in the NHF sulfides, likely indicating that the amount of sulfur was limited in this hydrothermal system, and most of the residual Fe was incorporated into the sphalerite. This suggests that the later pyrite and/or marcasite precipitation in the seafloor hydrothermal sulfide deposit is controlled by the sulfur content of the fluid. Furthermore, it is possible to use hydrothermal sulfides and their inclusions to trace subseafloor fluid circulation processes.

Acid treated halloysite nanotubes reinforced PP/ABS blends and its composites: Influence on mechanical and thermal properties

Halloysite nanotube (HNTs) is treated with sulphuric acid (H2SO4) which results in the formation of amorphous silica over the HNTs surface. The acid-treated HNTs and untreated HNTs are reinforced in a ratio of 80:20 (wt/wt) of polypropylene (PP) to acrylonitrile butadiene styrene (ABS) blends using melt extrusion. The dual compatibilizers of grafted maleic anhydride monomer over polypropylene and styrene ethylene butadiene styrene are used as compatibilizers for compatibilizing HNTs filled blend and its composites. The cryofractured and etched HTNs filled blends and its composites reveal the development of droplet embedded in the matrix. A significant decrease in average droplet diameter is observed with the incorporation of acid-treated HNTs and untreated HNTs filled blends and its composites. Acid-treated HNTs and pure HNTs filled blends and its composites influence the crystallinity of the PP phase as compared to pure blends. In addition, it has influenced thermal stability. The refined morphology and compatibilization improves the tensile and impact properties of untreated HNTs and acid-treated HNTs filled blends and its composites in comparison to pure blend. Tensile fractured scanning electron microscope (SEM) image shows the formation of fibrils. Whereas the impact fractured SEM image shows the resistance to dislodge the ABS droplets. Acid-treated HNTs filled blends and its composites show the highest tensile and impact properties with ∼37% increase in tensile modulus and ∼49% increase in impact strength in comparison to pure blends.

Synthesis and characterization of SO42-/KCC-1 catalyst as an alternative candidate for simultaneous esterification and transesterification reactions

Sulfate-impregnated acid catalysts can be used to replace conventional homogeneous base catalysts which are very dangerous, corrosive and environmentally unfriendly. Sulfate impregnation on porous support material will increase the surface area of the catalyst. The use of KCC-1 as a supporting material can produce catalysts with excellent properties. The method used in this research is the hydrothermal method for the synthesis of KCC-1 and the direct impregnation method for the synthesis of SO42-/KCC-1. Characterization of the resulting material using X-ray diffraction (XRD), FTIR spectrometer, and low temperature emission scanning electron microscope (SEM). The characterization results showed that the synthesis of KCC-1 and SO42-/KCC-1 had been successfully carried out. This is evidenced by the formation of amorphous silica, the presence of peaks of silica and sulfate groups, and scanning electron microscope images showing nano-sized materials.©2021 JNSMR UIN Walisongo. All rights reserved.

Direct Observation of Carbonate Formation in Partly Hydrated Tricalcium Silicate by Dynamic Nuclear Polarization Enhanced NMR Spectroscopy

Solid-state NMR combined with dynamic nuclear polarization (DNP NMR) is used to study hydration processes in tricalcium silicate (Ca3SiO5, abbreviated as C3S) samples. The studied C3S samples have experienced early stage hydration (1–30 h) and slow aging (9 years) processes. The appearance of Q3 and Q4 lines in the 29Si MAS and 1H → 29Si CP MAS NMR spectra obtained for partly hydrated C3S samples indicated the formation of amorphous silica which corresponds to their carbonation, which was corroborated by complementary FTIR data. Significant DNP signal enhancements obtained for the studied samples allowed to further investigate the C3S carbonation process in detail employing the 1H → 29Si CP MAS FSLG HETCOR technique. Finally, DNP enhanced 1H → 13C CP MAS and 1H → 13C CP MAS FSLG HETCOR techniques enabled to directly observe the formation of carbonate moieties in partly hydrated C3S samples.

Amorphous silica coating on flank deposits of the 1783 CE eruption at Asama volcano

Silica-rich coatings of up to 660 μm thickness were found on the juvenile volcanic lapilli and blocks of the 1783 CE pyroclastic flow deposits 1.9–3.6 km northeast of the Maekake crater of Asama volcano in Central Japan. These coatings are composed of fine-grained volcanic ash particles with silica-rich cement. Raman spectroscopic measurements revealed that the interstitial silica-rich phase is hydrous amorphous silica and is considered to be a reaction product of ash particles with acidic volcanic rain and fog like volcanic smog (vog) as it has been recorded in Hawaii. The formation of amorphous silica without jarosite and the preferential alteration of plagioclase compared with pyroxene indicate that the reaction occurred with a sulfuric acid solution of pH < ~3. The porous structure and large surface area of the ash layer would have been favorable for the dissolution of sintered volcanic ash and the precipitation of amorphous silica. The thickness of the coating and degree of alteration in the plagioclase phenocrysts adjacent to the coating tend to decrease with increasing distance from the volcanic crater, which is consistent with the absence of fumarolic areas in the flank. The Asama-Maekake volcano has repeated Vulcanian and phreatic eruptions since 1783 CE, especially in the first half of the 20th century. The amorphous silica coating recognized in this study is assumed to have formed mainly during this period, which can thus be an indicator of the acidity and distribution of past acidic volcanic rain and fog.

Experimental and theoretical study of deactivated HDT catalysts by Si species deposited on their surfaces: Models proposition, structural and thermodynamic analysis

It is known that silicon species originate from the thermal degradation of the polydimethylsiloxane, adsorb on the catalysts surface used for hydrotreatment in the oil refining industry. This severely affects the performance of these catalysts and considerably reduce their lifetime. Combining 29Si and 13C CP/MAS NMR experiments and DFT calculations, we investigated the poisoning process of these catalysts by silicon species in order to obtain a better understanding of this process. Two structural models (mono and bilayer) for amorphous silica deposited on the γ-Al2O3 surface and some intermediates that can be formed during this process were proposed. From the calculated 29Si chemical shifts, it was possible to identify chemical shifts variations due to different structural elements for each species, thereby, it was possible to make a more appropriate assignment of the resonances found in 29Si CP/MAS NMR spectra in the literature. Besides that, through of the good agreements obtained between the experimental and simulated 29Si CP/MAS NMR spectra, we identified that probably the D4, T1, T3, T4, Q1 and Q2 species are present in the initial stage of the contamination and that the amorphous silica bilayer model deposited on the γ-Al2O3 surface best characterizes the final stage of the contamination. Moreover, the thermodynamic analysis revealed that the chemisorption reaction of the Hexamethylcyclotrisiloxane (D3) compound is much more stable than the Octamethylcyclotetrasiloxane (D4) compound and of a fragment (corresponding to 13 or 14 of the D3 and D4compounds, respectively), and the chemisorption reaction of the fragment presented the highest values of ΔG°. This leads us to deduce that the D3 and D4 compounds chemisorb preferentially without breaking. Besides, it also showed that the amorphous silica formation process on the γ-Al2O3 surface is spontaneous and the bilayer formation reaction is more stable than the monolayer.

Silicon isotopic re-equilibration during amorphous silica precipitation and implications for isotopic signatures in geochemical proxies

Abstract Water-rock interactions influence the isotopic signatures archived during secondary mineral formation and thus the fidelity with which these geochemical proxies retain pristine geochemical records over geologic timescales. The extent to which these isotopic signatures are “re-equilibrated” or “reset” has been suggested to be dependent on surface area and the depth into mineral surface that the surrounding fluid can access, but this relationship has yet to be incorporated in a generalized modeling framework. In this study, we evaluate the timing and extent of silicon isotopic re-equilibration during amorphous silica formation, a common secondary mineral in weathering environments, through a combination of both laboratory experiments and multi-component numerical modeling. A series of amorphous silica precipitation experiments were conducted over a range of surface areas and solid to fluid ratios at 20 °C and near neutral pH for a period of 30 days. These experiments show evidence that surface area exerts a first order control on the extent of isotopic re-equilibration following secondary mineral precipitation. New mineral growth onto higher surface areas seeds resulted in rapid isotopic re-equilibration (over a period of ∼2 weeks) whereas precipitation onto lower surface area grains retained a kinetic signature due to overall slower kinetics. Experimental results were further supported by a modified numerical reaction network model that has the novel capacity to explicitly treat the depth into the mineral that the fluid can chemically and isotopically exchange with through time. These numerical simulations demonstrate that the fluid-mineral surface interaction depth is larger for low surface area seed crystals where both a larger mass of amorphous silica was formed, and slower precipitations rates kept the reaction far from equilibrium, thereby impeding the influence of the back reaction and corresponding isotopic re-equilibration. These results highlight the significant role that water-rock interaction and, specifically, the mineral surface plays in the preservation of isotopic signatures within secondary silicate phases. Further, we suggest that a transient model approach is necessary to improve quantitative interpretations of observed isotope behavior in natural systems.

Efficient silica synthesis from tetra(glycerol)orthosilicate with cathepsin- and silicatein-like proteins

Silicateins play a key role in biosynthesis of spicules in marine sponges; they are also capable to catalyze formation of amorphous silica in vitro. Silicateins are highly homologous to cathepsins L – a family of cysteine proteases. Molecular mechanisms of silicatein activity remain controversial. Here site-directed mutagenesis was used to clarify significance of selected residues in silica polymerization. A number of mutations were introduced into two sponge proteins – silicatein A1 and cathepsin L from Latrunculia oparinae, as well as into human cathepsin L. First direction was alanine scanning of the proposed catalytic residues. Also, reciprocal mutations were introduced at selected positions that differ between cathepsins L and silicateins. Surprisingly, all the wild type and mutant proteins were capable to catalyze amorphous silica formation with a water-soluble silica precursor tetra(glycerol)orthosilicate. Some mutants possessed several-fold enhanced silica-forming activity and can potentially be useful for nanomaterial synthesis applications. Our findings contradict to the previously suggested mechanisms of silicatein action via a catalytic triad analogous to that in cathepsins L. Instead, a surface-templated biosilification by silicateins and related proteins can be proposed.

Effect of silicone resin as precursor and binder on the properties of alumina-based ceramic cores using ball-shaped powders

Al2O3-based ceramic cores with uniform microstructure were fabricated by sintering ball-shaped Al2O3 powders as the matrix and silicone resin as the precursor and binder in air atmosphere. Effects of silicone resin content and sintering temperature on phase composition, microstructure and properties of the Al2O3-based ceramic cores were investigated. Results showed that a low sintering rate was beneficial for preparing high-quality Al2O3-based ceramic cores with no defects and cracks. Silicone resin was uniformly adhered on the surface of the ball-shaped Al2O3 particles, playing a well role in connecting Al2O3 particles. Sintering process made the organic-inorganic transformation process of silicone resin happen, leading to the formation of amorphous silica at 1300 °C, followed by cristobalite appeared by the crystallization of amorphous silica as the temperature was higher than 1400 °C. The sintering necks between ball-shaped Al2O3 grains were formed by the pyrolyzed products of silicone resin. As the temperature was lower than 1500 °C, the phase composition consisted of Al2O3 and silica. However, at 1600 °C, new mullite phases were in situ formed on the surface of ball-shaped Al2O3 particles. The increasing silicone resin content promoted the formation of more mullite phase. Al2O3-based ceramic cores prepared had well dimensional stability during sintering, and the linear shrinkage rate reached the minimum value of 0.23% in the ceramic cores with silicone resin content of 20 wt% at 1400 °C. The open porosities of Al2O3-based ceramic cores prepared were 24.2~30.0% and relative densities were 63.9~70.2%. The bending strength increased with the increase in the sintering temperature, and the maximum value was obtained in the ceramic cores with silicone resin content of 15 wt% at the same sintering temperature. In our research, the highest bending strength reached to 77.06 MPa in the ceramic cores with silicone resin content of 15 wt% at 1500 °C.

Influence of the Structure and Experimental Surfaces Modifications of 2:1 Clay Minerals on the Adsorption Properties of Methylene Blue

In this work the adsorption capacities of methylene blue on Mg-smectite and sepiolite and its derived material obtained after acid treatment and/or the addition of Fe at its surfaces are studied. Natural smectite with high Mg-content in its octahedral sheet (Mg-smectite) displays higher adsorption capacity than the sepiolite due to the ability of Mg-smectite to expand the basal spacing to accommodate methylene blue cations between adjacent layers and the inability of sepiolite to adsorb these cations within the structural channels. The acid-activation of Mg-smectite causes a clear decrease in the adsorption capacity attributed to the partial loss of the interlayer positions by the loss of the octahedral sheet and subsequent formation of amorphous silica. Moreover, the adsorption of the Mg-smectite decreases even more when iron oxohydroxides species are incorporated in its interlayer spacing making inaccessible the interlayer active sites for the adsorption of methylene blue cations. On the contrary, the microwave-assisted acid treatment of sepiolite causes a slight increase in the adsorption capacity related with the dispersion of fibers. Nevertheless, contrary to that observed with Mg-smectite, higher increasing of adsorption capacities are obtained after the formation of new adsorption centers due to the incorporation of iron oxohydroxides species at the external surfaces of sepiolite.

Effect of bismuth oxide on structural and thermal diffusivity of waste rice ash zinc silicate (willemite) glass ceramics

This work demonstrates the formation of amorphous silica and zinc oxide nanoparticles acquired from waste rice ash by the solid-state method. The XRD of zinc silicate pure and doped with bismuth oxides reveals the crystalline phase. The FTIR results show less significant variations with the increment of dopant concentrations. The FESEM does not show any feature of bismuth oxides in all the samples; this signifies all bismuth oxides absorbed by the samples. The thermal diffusivity value of samples fluctuates; the highest value is for the Bi 3 wt% with 0.2329–0.1592 mm2/s. Moreover, the least values are for the Bi 1 wt% with 0.2008 to 0.1392 mm2/s all sintered at 1000 °C. All the thermal diffusivity measurements carried out at room temperature 30 to 500 °C, respectively.

Relations Between Oxidation Induced Microstructure and Mechanical Durability of Oxide Scales

Most industrial heat-resistant stainless steels contain silicon as a minor constituent. At high temperature, the internal formation of amorphous silica reduces oxidation rates but decreases the metal/oxide interface toughness. Tensile testing experiments performed on AISI 304L previously oxidized in synthetic air for 50 h at 900 or 1000 °C showed a relation between the silica morphology and location and the crack patterns. A micromechanical modeling using cohesive zone models to describe interfaces fracture behavior is proposed to investigate relevant parameters controlling the silica/alloy interface debonding. Calculations carried out using the finite elements method have shown that location of silica inclusions and silica/metal interface toughness are key parameters determining the cracks pattern morphology and the critical strain at failure.

Modified macromolecules in the prevention of silica scale

Abstract Silicic acid polycondensation leads to the formation of amorphous silica. This process is of great importance to the survival of certain living organisms, such as diatoms and sponges, but presents a significant problem in various production facilities that use water for heating or cooling. In the latter, amorphous silica can be a recalcitrant deposit that can hamper proper system operation. Hence, inhibition of silicic acid polycondensation by chemical inhibitors is an intensely sought strategy by water system operators. In this manuscript, we report the inhibitory effect of zwitterionic phosphonated analogs (PPEI’s) of the cationic polymeric chemical additive polyethyleneimine (PEI) in mildly supersaturated silica solutions (500 ppm/8.3 mM “Si”) at pH=7. The inhibition efficiency of PPEI’s depends on a variety of parameters, such as concentration and degree of phosphonomethylation of the parent PEI polymer.

Molecular dynamics simulations of the structure and mechanical properties of silica glass using ReaxFF

Assessment of the empirical reactive force field ReaxFF to predict the formation of amorphous silica from its crystalline structure and the determination of mechanical properties under tension using molecular dynamics simulations is presented. Detailed procedures for preparing amorphous silica from crystalline silica are presented and the atomic structure is in good agreement with experimental results. Tensile properties of silica are predicted over a wide range of strain rates (2.3 × 108 s−1–1.0 × 1015 s−1) allowing comparison with results reported in the literature for other force fields. Quasi-static modulus obtained from power-law fitting of the low-stain rate modulus predicted by ReaxFF is in good agreement with experimental results. A transition strain rate of approximately $$ 2.5 \times 10^{11} {\text{s}}^{ - 1} $$ is identified where modulus increases rapidly to a plateau level. Tensile strength also increases significantly in this range of strain rate and plateaus at the theoretical upper bound for silica. A detailed study is presented to understand the mechanisms associated with strain rate effects on the overall stress–strain response of silica. Bond breakage which evolves into void growth leading to failure is predicted to occur at approximately 27 % strain for all strain rates. Stress relaxation simulations indicates that the transition strain rate occurs when the characteristic time for high-strain rate loading and stress relaxation times are the same order. The effects of cooling rate and temperature on the structure and the stress–strain response of the silica glass are also investigated. Low-cooling rate and low-cooling temperature enhance the properties of silica.

Chemical models for martian weathering profiles: Insights into formation of layered phyllosilicate and sulfate deposits

Numerical chemical models for water–basalt interaction have been used to constrain the formation of stratified mineralogical sequences of Noachian clay-bearing rocks exposed in the Mawrth Vallis region and in other places on cratered martian highlands. The numerical approaches are based on calculations of water-rock type chemical equilibria and models which include rates of mineral dissolution. Results show that the observed clay-bearing sequences could have formed through downward percolation and neutralization of acidic H2SO4–HCl solutions. A formation of weathering profiles by slightly acidic fluids equilibrated with current atmospheric CO2 requires large volumes of water and is inconsistent with observations. Weathering by solutions equilibrated with putative dense CO2 atmospheres leads to consumption of CO2 to abundant carbonates which are not observed in clay stratigraphies. Weathering by H2SO4–HCl solutions leads to formation of amorphous silica, Al-rich clays, ferric oxides/oxyhydroxides, and minor titanium oxide and alunite at the top of weathering profiles. Mg-Fe phyllosilicates, Ca sulfates, zeolites, and minor carbonates precipitate from neutral and alkaline solutions at depth. Acidic weathering causes leaching of Na, Mg, and Ca from upper layers and accumulation of Mg-Na-Ca sulfate-chloride solutions at depth. Neutral MgSO4 type solutions dominate in middle parts of weathering profiles and could occur in deeper layers owing to incomplete alteration of Ca minerals and a limited trapping of Ca to sulfates. Although salts are not abundant in the Noachian geological formations, the results suggest the formation of Noachian salty solutions and their accumulation at depth. A partial freezing and migration of alteration solutions could have separated sulfate-rich compositions from low-temperature chloride brines and contributed to the observed diversity of salt deposits. A Hesperian remobilization and release of subsurface MgSO4 type solutions into newly-formed depressions could account for formation of some massive layered sulfate deposits through freezing or evaporation. This scenario explains the observed deficiency of salts in Noachian formations, a paucity of Hesperian phyllosilicates, and the occurrence of sulfate deposits in Valles Marineris troughs, chaotic terrains, and some craters of the Hesperian age.