SiO2 Precipitation Research Articles

Exploring the influences of Li2O/SiO2 ratio on Li2O – Al2O3 – SiO2 – B2O3 – BaO glass-ceramic bonds for vitrified cBN abrasives

Abstract Li2O–Al2O3–SiO2–B2O3–BaO glass-ceramic bonds for vitrified cBN abrasives with various Li2O/SiO2 ratio were prepared by the traditional melt-quenching method. The structure of parent glass was identified by X-ray diffraction (XRD) and Fourier Transform infrared spectroscopy (FTIR). The reduction in the Li2O/SiO2 ratio increased the connectivity of the glassy network, resulting in a decrease of the crystallization ability for the parent glasses. Besides, a comprehensive understanding of measured and calculated coefficient of thermal expansion (CTE) for glass-ceramic bonds demonstrated that the simultaneous presence of a higher content of LiAlSi2O6 and LiAlSi3O8 along with the absence of SiO2 precipitation were believed to contribute largely to the lower CTE of glass-ceramic bonds. Lastly, the highest bending strength of cBN composites (100.53 MPa) relied on the sufficient wetting of B3 glass-ceramic bonds (Li2O/SiO2 = 0.11) on cBN and the close CTE between them when composites were sintered at 860 °C for 2 h, indicating B3 is a kind of suitable and promising glass-ceramic bonds for cBN abrasives.

High‐purity amorphous silica from rice husk: Preparation and characterization

AbstractAmorphous silica has a wide range of applications in chemical, biological and environmental sectors. In this work, we studied the preparation of amorphous silica from rice husk, an abundant silica‐rich agricultural waste in Vietnam. We investigated and compared two methods of preparation: (1) via selective removal of alkaline and alkaline‐earth oxides from rice husk ash to yield amorphous silica; and (2) via selective precipitation of silica from aqueous solutions which had been prepared by dissolving rice husk ask in NaOH solutions. The obtained silica was characterized by using XRD, EDX, SEM and BET techniques. The results show that the best obtained silica is a white amorphous and porous powder with high purity of SiO2, in the form of aggregates of 50 nm‐sized primary particles, and a specific surface area of 186 m2/g. The SiO2 recovery is around 8.3 (g) per 100 (g) rice husk. Moreover, in the context of the present work, method (1) can be a more straightforward route for the preparation of amorphous silica in terms of silica's purity, porousity and amorphousness. In contrast, method (2) is shown for a lower grade of silica purity which could be due to strong adsorption of ions from the solution onto the amorphous SiO2 precipitate during precipitation process. The obtained silica has promising applications as adsorbents, fillers and fertilizers.

Investigation of micron-sized fish-scale surface structures on tool steel surfaces using laser galvanometer scanning

Micronized structured surface exhibits excellent surface functions for surface science applications. However, the cost is high for fabricating, especially specialized structure surfaces. The work prepared the micron-sized fish-scale surface structures on rough DC 53 tool steel surfaces. CW (continuous wave) laser beam irradiation method was used from a scanning galvanometer in the filling scan mode. The surface morphology and structure before and after laser irradiation were analyzed by optical microscopy, laser scanning confocal microscopy, and scanning electron microscopy. The elemental distribution of the structure was measured by energy dispersive spectroscopy (EDS) and Auger electron spectroscopy (AES), with the chemical analyses detected by the X-ray photoelectron spectroscopy (XPS). The combination of the AES with XPS analysis was also developed by the ion sputtering with the depth etching. It is found that the fish-scale surface structures can be prepared using a crossover laser scanning method, and their formation is based on superficial surface melting achieved by laser irradiation. The XPS results showed that the oxide layer of the fish-scale surface was composed of FeO, Fe2O3 and SiO2. The AES and EDS results showed the high content of Si and O in the fish-scale surface structure boundary. The combination of AES and XPS results showed that the boundary of the fish scale was composed of SiO2. Laser fluence and scanning mode were the key factors in the preparation of micron-sized fish-scale surface structure. A laser fluence of 33.16 J/mm2 induced a well-defined fish-scale corrugated structure. At the scanning speed of 140 mm/s, the flow of material on the surface of the tool steel was gradually controlled, and the shape of the fish-scale structure tended to be obvious. The scanning method has a significant effect on the spacing of the fish scales on the surface. By adjusting the appropriate scanning pitch, a regular distribution of the fish scale surface structure can be obtained. It is indicated that the precipitation of SiO2 (inclusions) has a great potential for the structuring of the fish-scale surface on DC 53 tool steel.

Prospects for an ancient dynamo and modern crustal remanent magnetism on Venus

Venus lacks an internally generated magnetic field today. Whether one existed in the past is unknown, but critical to atmospheric evolution and potential habitability. Canonical models assume the core of Venus is cooling too slowly for convection and thus a magnetic dynamo to occur today. Core/mantle heat flow is suppressed in these models after a putative transition in mantle dynamics associated with widespread, volcanic resurfacing. However, recent studies of impact craters and other surface features support more steady heat loss over geologic time. Precipitation of MgO and/or SiO2 from the core can also drive compositional convection even with slow cooling. Here we reevaluate the likelihood that Venus has an “Earth-like” (at least partially liquid and chemically homogeneous) core using numerical simulations of the coupled atmosphere–surface–mantle–core evolution. An Earth-like core is only compatible with the modern lack of a dynamo if the thermal conductivity of core material is towards the higher end of modern estimates (i.e., >100 W m−1 K−1). If lower estimates like ∼40–50 W m−1 K−1 are actually correct, then we favor recent proposals that the core has completely solidified or preserved primordial stratification. Any simulation initialized with a homogeneous, liquid core predicts a global magnetic field with Earth-like surface strength for >2–3 billion years after accretion—consistent with all available observations—and also sporadic activity within the surface age while temperatures remain below the Curie point of magnetite. Therefore, future spacecraft missions should prioritize the first-ever magnetometer measurements below the ionosphere to search for crustal remanent magnetism.

Theoretical analysis of the kinetics of precipitation of lizardite and magnesite from olivine alteration

The present theoretical study analyzes the kinetic characteristics of precipitation of magnesite and lizardite as a result of hydrothermal alteration of olivine under high carbonate alkalinity. It makes use of a simulation strategy in which, both the chemical evolution of the aqueous solution and the solid phases are considered. The simulation is performed with the help of the NANOKIN code. It includes a full treatment of speciation processes in the aqueous solution, a rate equation for the dissolution of olivine, and a full account of nucleation and growth processes during the formation of secondary particles. The comparison between experimental (Lafay et al., 2014) and simulation results puts strong constraints on the simulation parameters, in particular those related to the nucleation and growth of the particles. Modeling points to an incongruent olivine dissolution and reveals a transitory precipitation of SiO2, not detected experimentally, showing that the fate of Si is more complex than a mere incorporation in lizardite. It highlights how lizardite, magnesite and SiO2 compete for the incorporation of silicon and magnesium released by the olivine dissolution.

Clean and efficient process for the extraction of rubidium from granitic rubidium ore

Abstract The extraction of rubidium from natural resources has attracted much attention due to its growing application in many fields. This paper presents a novel clean process for extracting rubidium from granitic rubidium ore. The process consists of alkaline leaching, desilication, and solvent extraction. The experiment results show that the leaching ratio of Rb can be greater than 95% at a leaching temperature of 230 °C, NaOH concentration of 200 g/L, particle size of 100 mesh, liquid/solid ratio of 10:1, leaching time of 1 h, and stirring rate of 500 rpm. The optimum conditions for the desilication were determined to be the temperature of 95 °C, mass ratio of CaO to SiO2 of 1.2, and reaction time of 1 h. The SiO2 precipitation under optimum conditions was up to 96.2%. Approximately 98% of Rb in desilication liquor was extracted using 1 mol/L 4-tert-butyl-2-(α-methylbenzyl) phenol (in xylene) in three stages at a phase ratio (O/A) of 3:1 for 1.5 min and more than 99% of Rb in loaded organic phase was stripped using 1 mol/L HCl in two stages at a phase ratio of 10:1. The raffinate of Rb can be sent to the leaching step again after the extraction of potassium to close the loop. The results of mineralogy research suggest that the rubidium-bearing micas and feldspar were altered to zeolite and cancrinite during the hydrothermal alkaline leaching, which contributed to the release of rubidium from ore. Compared with the existing processes, this process has the advantages of lower energy consumption and no wastewater/emission gas disposals. In addition, the solid wastes can be utilized as the value-added products.

Leaching of iron concentrate separated from kiln slag in zinc hydrometallurgy with hydrochloric acid and its mechanism

It is taken as a novel prospective process to treat iron concentrate from hydrometallurgical zinc kiln slag for comprehensive utilization of valuable metals by a hydrochloric acid leaching-spray pyrolysis method. The leaching mechanism of different valuable metals was studied. The results revealed that the leaching rates of Ag, Pb, Cu, Fe, As and Zn were 99.91%, 99.25%, 95.12%, 90.15%, 87.58% and 58.15%, respectively with 6 mol/L HCl and L/S ratio of 10:1 at 60 °C for 120 min. The action of SiO2 in leaching solution was also studied. The results showed that the precipitation and settlement of SiO2(amorphous) adsorbed part of metal ions in solution, which greatly inhibited the leaching of Cu, Fe, As and Zn, so it is crucial to control the precipitation of amorphous SiO2.

Micro-Ikebana by Biomimetic Crystallization of Alkaline Earth Carbonates

Biomineralization is the process of forming hard exo- or endoskeltons by biological organisms. The physical properties and the morphology of the composite material that is formed depends on the mode of crystal-growth, which depends on the diffusion of the constituents (metal cations, their anions, and additives). We chose a model system, strontium carbonate / silica, because the precipitation of SrCO3 and SiO2 is pH dependent. A lower pH accelerates SiO2 formation while it slows down SrCO3 crystallization, and vice versa. Thus, the precipitation of the composite can occur in an alternating fashion, regulating the morphology of the carbonate precipitate. The solution of SrCl2 and water glass is poured into a shallow container and a glass slide is floated on top, eliminating the effects of bulk precipitation and convection. The pH is adjusted to a starting value of 12, and a small amount of dimethyl carbonate is added. This compound slowly hydrolyses and gives off carbon dioxide, the carbonate source for the crystal growth. The crystal morphology is clearly dendritic and shows several distinct periods of growth. First, a few crystals grow from a common seed, and after a while, the blocking of surface sites on the carbonate crystals by water glass, leads to the start of a more needle-like growth with a significant increase of branching density. Finally, after seven days, coral-like structures are formed. [DOI: 10.1380/ejssnt.2017.65]

Source and sink of fluid in pelagic siliceous sediments along a cold subduction plate boundary

Subduction zones where old oceanic plate underthrusting occurs are characterized by thick pelagic sediments originating from planktonic ooze as well as cold thermal conditions. For a better understanding of dehydration from pelagic sediments and fluid behavior, which would play a key role in controlling the dynamics in the shallow portion of the subduction zone, as observed in the 2011 Tohoku earthquake and tsunami, we investigate cherts in a Jurassic accretionary complex in Japan. The microstructure and microchemistry of these cherts indicate dissolution of SiO2 from a pressure solution seam and precipitation of SiO2 to the “white chert layer,” which would act as a fluid conduit. The amount of water necessary to precipitate SiO2 in the white chert is ~102 times larger than that produced by compaction and silica/clay diagenesis. Other fluid sources, such as hydrated oceanic crust or oceanic mantle, are necessary to account for this discrepancy in the fluid budget. A large amount of external fluid likely contributed to rising pore pressure along cold plate boundaries.

Comparison of adsorbent materials for herbicide diuron removal from water

Diuron herbicide is a persistent and frequently detected compound in surface and groundwater. In this work, a comparative study of the performance of four types of adsorbent (two precipitated silica, zeolite Y and carbon molecular sieves (CMS)) for removing diuron from water was carried out. Precipitated silica samples were obtained by controlled precipitation of SiO2 with sulphuric acid from water glass; zeolite Y was synthesized by a microwave-assisted hydrothermal method; CMS were synthesized by the method of controlled pyrolysis of wheat straw and chemical vapour deposition of organic matter. It was found that adsorption isotherms for precipitated silica and zeolite type Y are of linear shape, whereas the one for CMS exhibits at low concentrations a concave shape followed by an inflection point which suggests cooperative adsorption and the formation of adsorbate multilayers. CMS adsorbent showed the highest adsorption capacity and this was likely due to its high hydrophobicity. Temperature has a negligible effect on precipitated silica and zeolite Y while it strongly affects adsorption properties of CMS. Adsorption is not thermodynamically favoured at high temperature. An unexpected decrease in the rate of adsorption was observed with increase in temperature. This is possibly related to the different adsorption behaviour of diuron conformers and aggregates.

Formation Mechanism of SiO2-Type Inclusions in Si-Mn-Killed Steel Wires Containing Limited Aluminum Content

The origin, formation mechanism, and evolution of SiO2-type inclusions in Si-Mn-killed steel wires were studied by pilot trials with systematical samplings at the refining ladle, casting tundish, as-cast bloom, reheated bloom, and hot-rolled rods. It was found that the inclusions in tundish were well controlled in the low melting point region. By contrast, MnO-SiO2-Al2O3 inclusions in the as-cast bloom were with compositions located in the primary region of SiO2, and most CaO-SiO2-Al2O3-MnO inclusions lied in primary phase region of anorthite. Therefore, precipitation of SiO2 particles in MnO-SiO2-Al2O3 inclusions can be easier than in CaO-SiO2-Al2O3-MnO inclusions to form dual-phase inclusions in the as-cast bloom. Thermodynamic calculation by the software FactSage 6.4 (CRCT-ThermFact Inc., Montreal, Canada) showed that mass transfer between liquid steel and inclusions resulted in the rise of SiO2 content in inclusions from tundish to as-cast bloom and accelerated the precipitation of pure SiO2 phase in the formed MnO-SiO2-Al2O3 inclusions. As a result, the inclusions characterized by dual-phase structure of pure SiO2 in MnO-SiO2-Al2O3 matrix were observed in both as-cast and reheated blooms. Moreover, the ratio of such dual-phase SiO2-type inclusions witnessed an obvious increase from 0 to 25.4 pct before and after casting, whereas it changed little during the reheating and rolling. Therefore, it can be reasonably concluded that they were mainly formed during casting. Comparing the evolution of the inclusions composition and morphology in as-cast bloom and rolled products, a formation mechanism of the SiO2-type inclusions in wire rods was proposed, which included (1) precipitation of SiO2 in the formed MnO-SiO2-Al2O3 inclusion during casting and (2) solid-phase separation of the undeformed SiO2 precipitation from its softer MnO-SiO2-Al2O3 matrix during multipass rolling.

Origin of John’s Stone: A quartzitic boulder from the site of the 1908 Tunguska (Siberia) explosion

An exotic meter-size quartzitic boulder known as John’s Stone was found by John Anfinogenov in 1972 buried in permafrost close to the epicenter of the 1908 Tunguska blast in a region of Siberia dominated by Permian–Triassic Siberian Trap basalts. The boulder is made almost entirely of well-cemented quartz grains, mostly around 100μm in size; it contains zones with coarser or finer grain sizes. Rare zircon and rutile crystals are scattered within the quartz matrix. Quartz is often dissected by strain lamellae. The rock contains abundant scattered internal vugs rimmed by euhedral quartz crystals. We cannot exclude that John’s Stone is a fragment of a Permian granite-derived sandstone unit. However, based on structure, mineralogy and chemistry the quartzitic boulder may have originated due to silica deposition from hydrothermal solutions that had reacted with basaltic rocks. Anfinogenov et␣al. (Anfinogenov, J. et␣al. [2014]. Icarus 243, 139–147) interpreted features observed in the permafrost at the base of the boulder as indicating it impacted from above, suggesting the boulder may be a meteorite, possibly of martian origin, given the reported presence on Mars of silica-rich deposits. Triple oxygen isotope ratios determined on two samples of the quartzite reveal a terrestrial rather than a martian meteorites composition. Oxygen isotope data suggest also that the precipitation of SiO2 could have occurred in equilibrium with hydrothermal water (δ18Ow≈−19.5‰) at the temperature of about 50°C. The thermal event that generated the quartzite may be related either to the century-old Tunguska Event, or, more probably, to Permian–Triassic Siberian Traps magmatism, although an extraterrestrial origin cannot be completely ruled out.

Numerical simulation of porosity and permeability evolution of Mount Simon sandstone under geological carbon sequestration conditions

A numerical model was developed with the use of reactive transport code CrunchFlow to estimate porosity, permeability and mineral composition changes of Mount Simon sandstone under typical geological carbon sequestration conditions (P=23.8MPa and T=85°C). The model predicted a permeability decrease from 1.60mD to 1.02mD for the Mount Simon sandstone sample in a static batch reactor after 180days of exposure to CO2-saturated brine, which is consistent with measured permeability results. Model-predicted solution chemistry results were also consistent with laboratory-measured solution chemistry data. SiO2 (am) was the primary mineral that causes permeability decrease, followed by kaolinite. Both SiO2 (am) formation and kaolinite formation were attributed to the dissolution of quartz and feldspar. This study shows that the formation of SiO2 (am) and kaolinite in the pore space of host rock is possible under typical CO2 sequestration conditions. SiO2 (am) and kaolinite precipitation at the CO2 plume extent could reduce the permeability of host rock and improve lateral containment of free-phase CO2, contributing to overall security of CO2 storage.

ZnO-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub>系ガラスの核生成に及ぼすHfO<sub>2</sub>の影響

The effect of HfO2 addition on the crystallization behavior of ZnO-Al2O3-SiO2 glasses, which shows volume crystallization, was studied. The crystallization peak temperature (TC) obtained by DTA for as cast glasses decreased with the addition of HfO2. The crystallization peak temperature also decreased by nucleation heat treatments (NHT), and the difference of TC among samples depending on HfO2 content became less with the increase of NHT time. It was found that HfO2 works as a nucleation agent because the addition of HfO2 suppressed the precipitation of SiO2 by promoting the precipitation of Gahnite(ZnAl2O4). The effect of HfO2 on nucleation is less than TiO2 or ZrO2 which are known well as nucleation agents. The low diffusion rate of Hf due to its heavier atomic weight than Ti and Zr would be a reason for the low nucleation ability of HfO2.

Surface treatment optimization of pigmentary TiO2 from an industrial aspect

Industrial processes are most often limited in the means of production conditions, e.g., chemical agents’ selection and consumption, and time and energy conditions. In the present work, we succeeded in synthesizing a homogeneous silica layer on titanium dioxide (TiO2) core using moderate time and temperature conditions. The structure of the surface coated pigmentary TiO2 was characterized by scanning electron microscopy, transmission electron microscopy, and EDS. In our research isoelectric point (IEP) of TiO2 was determined, which occurred when the solution pH was ~4. In the meantime, the precipitation of SiO2 on TiO2 surface resulted in the shift of IEP. In addition, Fourier transform infrared spectroscopy and Brunauer–Emmett–Teller analysis were applied in order to determine differences in surface microstructure. Coating parameters were optimized to attain desired morphology of precipitated Si-hydroxide, which in turn provides improvements in one or more important performance properties such as hiding power efficiency, which was determined by the means of UV–Vis light transmittance.

Precambrian banded iron formations in the North China Craton: Silicon and oxygen isotopes and genetic implications

Banded iron formations (BIFs) are Precambrian chemical marine sedimentary formations that record major transitions of chemical composition, and oxidation–reduction state of oceans at the time of their deposition. In this paper, we report silicon and oxygen isotope compositions of a variety of BIFs from the North China Craton (NCC) in order to deduce the mechanism of their formation. Quartz in the various types of BIFs from the NCC are generally depleted in 30Si, where δ30SiNBS-28 values range from −2.0‰ to −0.3‰ (average, −0.8‰), similar to δ30SiNBS-28 values measured from modern submarine black chimneys and sinters. The δ18OV-SMOW values of quartz in the BIFs are relatively high (8.1‰–21.5‰; average, 13.1‰), similar to those of siliceous rock formed by hydrothermal activities. The δ30SiNBS-28 values of quartz in magnetite bands are commonly lower than those of quartz in adjacent siliceous bands within the same sample, whereas δ18OV-SMOW values are higher in the magnetite bands. A negative correlation is observed between δ30SiNBS-28 and δ18OV-SMOW values of quartz from siliceous and magnetite bands in BIF from Fuping, Hebei Province. The isotopic compositions of silicon and oxygen of quartz in BIFs provide insights for the formation mechanisms of silicon–iron cyclothems in BIFs. After the silicon- and iron-rich hydrothermal solution was injected onto the seabed, the abrupt temperature drop caused oversaturation of silicic acid, resulting in rapid precipitation of SiO2 and deposition of siliceous layers. Ferric hydroxide was precipitated later than SiO2 because of low free-oxygen concentration in the ocean bottom. Progressive mixing of hydrothermal solution with seawater caused a continuous drop in temperature and an increase in Eh values, resulting in gradual oxidation of hydrothermal Fe2+ and deposition of iron-rich layers. In summary, each silicon–iron cyclothem marks a large-scale submarine hydrothermal exhalation. The periodic nature of these exhalations resulted in the formation of regular silicon–iron cyclothems. The widespread distribution of BIFs indicates that volcanism and submarine hydrothermal exhalation were extensive; the low δ30SiNBS-28 and high δ18O V-SMOW values of the BIFs indicate that the temperature of seawater was relatively high at the time of BIF formation, and that concentrations of Fe2+ and H4SiO4 in seawater were saturated.

Pyridine-containing block copolymer/silica core–shell nanoparticles for one-step preparation of superhydrophobic surfaces

Two poly(4-vinylpyridine)-b-polystyrene diblock copolymer/silica core-shell nanoparticles (P4VP-b-PS/SiO2 NPs) are developed in this work. Confirmed by DLS analysis and TEM observation, one comprises a SiO2 core surrounded by a P4VP-b-PS shell and the other comprises a P4VP-b-PS core surrounded by a SiO2 shell, which is facilely prepared by the in situ hydrolysis of tetraethyl orthosilicate (TEOS) using cationic P4VP-b-PS micelles obtained in a THF-H2C2O4 (aq, 0.1 mol L(-1)) mixture and a DMF-H2C2O4 (aq, 0.01 mol L(-1)) mixture as template, respectively. The SCA, CAH, SA and SEM measurements reveal that one-step deposition of P4VP-b-PS/SiO2 NPs with SiO2 cores formed at a high level of TEOS creates a superhydrophobic surface with an SCA of 160°, a CAH of 2° and an SA of around 4° originating from the formation of a typical micro-nanoscale binary structure (MNBS). For the NPs with SiO2 cores formed at a low level of TEOS, the superhydrophobicity with a SCA of 151°, CAH of 3° and SA of around 5° can be induced by the transition of the surface microstructure from an uneven and discontinuous MNBS, created by a one-step deposition process, to the coexistence of MNBS and a nanoscale structure (NS) after annealing with toluene for 30 min. In contrast, one-step deposition of P4VP-b-PS/SiO2 NPs with P4VP-b-PS cores and SiO2 shells usually results in the inhomogeneous precipitation of SiO2 from bulk P4VP-b-PS along with the production of micro-cracks, with which is impossible to achieve surface superhydrophobicity.

Precipitation in silicon wafers after high temperature preanneal studied by X-ray diffraction methods

We have investigated oxygen precipitation in Czochralski silicon wafers focusing on influence of nucleation temperature and high temperature pre-anneal during common three step treatment. Thick Si wafers were studied mainly by x-ray diffraction in Laue transmission geometry using Mo x-ray tube, but were also compared to reciprocal space maps obtained in Bragg reflection geometry. The analysis of measured diffraction scans in Laue geometry was performed by means of Takagi equations and statistical dynamical theory of diffraction. From the simulated Laue diffraction curves we find the size of the individual defect area and the fraction of strain area volume in the wafer. The results obtained from x-ray diffraction were compared to loss of interstitial oxygen according to infrared absorption spectroscopy and the size of SiO2 precipitate core was estimated. These techniques are in agreement with transmission electron microscopy images.

Decreasing CO2partial pressure triggered Mg-Fe-Ca carbonate formation in ancient Martian crust preserved in the ALH84001 Meteorite

We retrace hydrogeochemical processes leading to the formation of Mg–Fe–Ca carbonate concretions (first distinct carbonate population, FDCP) in Martian meteorite ALH84001 by generic hydrogeochemical equilibrium and mass transfer modeling. Our simple conceptual models assume isochemical equilibration of orthopyroxenite minerals with pure water at varying water-to-rock ratios, temperatures and CO2 partial pressures. Modeled scenarios include CO2 partial pressures ranging from 10.1325 to 0.0001 MPa at water-to-rock ratios between 4380 and 43.8 mol mol−1 and different temperatures (278, 303 and 348 K) and enable the precipitation of Mg–Fe–Ca solid solution carbonate. Modeled range and trend of carbonate compositional variation from magnesio-siderite (core) to magnesite (rim), and the precipitation of amorphous SiO2 and magnetite coupled to magnesite-rich carbonate are similar to measured compositional variation. The results of this study suggest that the early Martian subsurface had been exposed to a dynamic gas pressure regime with decreasing CO2 partial pressure at low temperatures (approximately 1.0133 to 0.0001 MPa at 278 K or 6 to 0.0001 MPa at 303 K). Moderate water-to-rock ratios of ca. 438 mol mol−1 and isochemical weathering of orthopyroxenite are additional key prerequisites for the formation of secondary phase assemblages similar to ALH84001’s ‘FDCP’. Outbursts of water and CO2(g) from confined ground water in fractured orthopyroxenite rocks below an unstable CO2 hydrate-containing cryosphere provide adequate environments on the early Martian surface.

Nucleation and growth mechanism of apatite on a bioactive and degradable ceramic/polymer composite with a thick polymer layer

Nucleation and growth mechanism of apatite on a bioactive and degradable PLLA/SiO2–CaO composite with a thick PLLA surface layer were investigated compared to that on a bioactive but non-degradable polyurethane (PU)/SiO2–CaO composite with a thick PU surface layer. The bioactive SiO2–CaO particles were made by a sol–gel method from tetraethyl orthosilicate and calcium nitrate tetrahydrate under acidic condition followed by heat treatment at 600 °C for 2 h. The PLLA/SiO2–CaO and PU/SiO2–CaO composites were then prepared by a solvent casting method which resulted in thick PLLA and PU surface layers, respectively, due to precipitation of SiO2–CaO particles during the casting process. Two composites were exposed to SBF for 1 week and this exposure led to form uniform and complete apatite coating layer on the PLLA/SiO2–CaO composite but not on the PU/SiO2-CaO composite. These results were interpreted in terms of the degradability of the polymers. A practical implication of the results is that a post-surface grinding or cutting processes to expose bioactive ceramics to the surface of a composite with a thick biodegradable polymer layer is not required for providing apatite forming ability, which has been considered as one of the pragmatic obstacles for the application as a bone grafting material.