Substitution Of Al For Si Research Articles

Incorporation of Al into diatom frustule as a proxy of seawater Al/Si ratios in the Antarctic Zone of the Southern ocean: Implications for surface silicic acid concentrations during the last glacial period

Knowledge of silicic acid concentrations at the surface Southern Ocean helps better understand the role of the biological pump in regulating atmospheric CO2 concentrations in the past. While the silicon isotope compositions of diatom provide a measure of Si utilization efficiency, proxies on surface silicic acid concentration are not yet available. Here we determined the diatom frustule Al/Si ratio and dust flux to constrain the surface silicic acid concentrations in the Atlantic sector of the Southern Ocean. Our results based on multiple methods, including inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscope (SEM) and energy-dispersive spectroscopy (EDS), and nuclear magnetic resonance spectroscopy (NMR), suggest the isomorphic substitution of Al for Si by frustule biosynthesis. This enables us to transfer the diatom Al/Si ratio into seawater Al/Si ratio using the partition coefficient of Al between diatom silica and seawater. By further constraining dust flux using the sediment 230Th-normalized 232Th flux, we found that the markedly higher frustule Al/Si ratios cannot be solely explained by increased Al dissolution from dust and call for much lower surface silicic acid concentrations in the Antarctic Zone during the Last Glacial Period. The lower silicic acid concentration coupled with lower Si utilization efficiency inferred previously from silicon isotopes in the Antarctic Zone is likely due to the combined effects of reduced upwelling and iron-fertilization-regulated uptake stoichiometry, contributing to a lower atmospheric CO2 during the Last Glacial Period.

Microstructural and high-temperature performance evolution of Al-doped lamellar porous Ti3SiC2 using freeze casting

To enhance high-temperature oxidation resistance, Al was introduced into lamellar porous Ti3SiC2, reducing wall porosity from 16% to 7%, signifying wall densification. DFT calculations revealed that Al substitution for Si in the silicon layer caused lattice distortion, raising lattice parameter (d) from 0.2278 to 0.2289 nm, and disrupting energy, weakening bonds and reducing fracture energy absorption. Consequently, diffusion activation energy dropped from 370 kJ/mol to 230 kJ/mol, significantly impacting high-temperature stability. Yet, Al doping improved wall and oxide film densification, lowering the oxidation rate from 0.0097 to 0.0073 mg2/cm4·h, leading to remarkable high-temperature electromagnetic shielding with SSE of 71.1 dB.

The effects of calcium content of fly ash on hydration and microstructure of ultra-high performance concrete (UHPC)

In this study, the effects of low-calcium fly ash (LFA) and high-calcium fly ash (HFA) on C–S–H structure, hydration products, hydration heat, and compressive strength of UHPC paste/concrete were investigated and compared via XPS, MIP, FT-IR, 29Si NMR, XRD, SEM, and the thermal analysis method. The results showed that fly ash reduced the rate of exothermic hydration and early strength, but it increased the compressive strength and reduced the porosity in the middle and late ages, and denied the microstructure of UHPC. By contrast, HFA exhibited the higher band fluctuation of Si–O and Al–O and lower Ca2p-Si2p energy separation values, which indicated that more amorphous silicon oxide and alumina participated in the pozzolanic reaction. This also demonstrated that HFA played an important role in enhancing the polymerization degree and increasing a higher level of Al-substitution for Si in C–S–H than LFA. HFA enhanced the pore volume with diameters less than 10 nm from 28 to 90 days due to pozzolanic reaction and filling effect, whereas the LFA mainly played a filling role and a lower pozzolanic reaction during this period. Durability results showed that fly ash reduced the drying shrinkage and chloride impermeability coefficient of UHPC, and the effect of HFA was better than that of LFA.

Solidification/stabilization of organic matter and ammonium in high-salinity landfill leachate concentrate using one-part fly ash-based geopolymers

Landfill leachate concentrate (LLC) is a high-salinity organic wastewater produced in the process of membrane separation and has become an urgent problem because of its complex composition and high biological toxicity. Solidification/stabilization (S/S) technology using geopolymers provides a feasible and economical zero-discharge LLC treatment method. This study developed one-part fly ash-based geopolymers using LLC as the only solvent. The results showed that the solid alkali activator significantly increased the LLC treatment capacity with a high liquid−solid ratio of 0.55. The high-salinity LLC significantly promoted the substitution of Al for Si and improved the compressive strength of the geopolymers. Three leaching experiments showed that the prepared geopolymers had excellent stability, high S/S rates for organic matter (92.9%) and NH3-N (91.4%), and a high contaminant removal rate to reduce acute biological toxicity. Microscopy tests showed that sodium aluminosilicate hydrate (N-A-S-H) gel was the main component of the fly ash-based geopolymers after the addition of LLC. Cl− attached to the surface of the geopolymer gels through exchange with OH−, while SO42− was bound within the geopolymer structure in the form of Na2SO4 crystals. There was no evidence that organic matter and NH3-N were chemically combined with the geopolymer material, rather, they were physically encapsulated within the geopolymer structure. This study proposes a technical strategy for LLC S/S using fly ash-based geopolymer. The geopolymer can be used as a new building material, which provides a new perspective for the treatment of LLC and the utilization of fly ash resources.

Experimental and theoretical studies of hydration of ultra-high performance concrete cured under various curing conditions

The hydration of UHPC from 1 to 180 days was experimentally and theoretically studied. Three different curing regimes were employed during the sample preparation process in order to observe their effects on the hydration and reaction products. The samples were experimentally characterized, and the experimentally obtained degrees of the reaction of the cement and amorphous silica in the UHPC samples were used for thermodynamic calculations to predict the phase assemblages of UHPC. The decomposition of AFt phases occurred in the sample that underwent a heat treatment, which also resulted in Al-substitution for Si in C-S-H gels. The degree of the reaction of the cement and amorphous silica varied greatly depending on the curing condition applied during the first few days, and higher reaction degree was observed in the heat-treated sample, which showed a significant amount of C-S-H present at an early age, making its microstructure denser than those of the other samples.

Coordination environment of Si in calcium silicate hydrates, silicate minerals, and blast furnace slags: A XANES database

Understanding the silicate polymerization of calcium silicate hydrate (C-S-H) gel and its crystalline polymorphs is important in cement science. NMR can determine Si environments, but the measurement can be time-consuming and provides no spatial information. X-ray absorption near-edge structure (XANES) spectroscopy is a fast tool for probing Si coordination, possibly with spatial information. However, there lacks an understanding of Si K-edge XANES spectra of cement-related silicate phases. Here, a Si K-edge XANES spectral database of nanocrystalline C-S-H, C-S-H minerals, blast-furnace slags, and metakaolin is provided. Si K-edge of C-S-H minerals shifts to higher energies with higher polymerized Si and lower CaSi connectivity in the Si second nearest neighbor shell. Si K-edge energy shows weak correlations with Ca/Si ratio, average SiO bond length, and SiO4 distortion due to the structural complexity of silicates. The substitution of Al for Si shifts the Si K-edge of tobermorite and slags to higher energies.

Mineral–melt partition coefficients and the problem of multiple substitution mechanisms: insights from the rare earths in forsterite and protoenstatite

A trace element may substitute into a mineral by more than one substitution mechanism, complicating the thermodynamic description of its partition coefficients. In order to understand this phenomenon better, the mineral/melt partition coefficients for all 14 rare earth elements (REE) plus Y and Sc were measured experimentally for coexisting forsterite and protoenstatite in the system CaO–MgO–SiO2 ± Al2O3 ± TiO2 at 1406 °C and atmospheric pressure. For both phases, the results show these large trivalent cations (REE3+) replace Mg2+ on octahedral sites, but with charge-balance achieved by two different mechanisms: (1) cation vacancies (2 REE3+ + vacancy = 3 Mg2+); and (2) substitution of Al for Si (REE3+ + Al3+ = Mg2+ + Si4+). The overall REE partition coefficient is the sum of the partition coefficients for each substitution mechanism. Because the stoichiometric control is different for each mechanism, the relative importance of the mechanism varies with melt composition, including the activities of both silica and alumina in the melt ($${{a}}_{{{\text{SiO}}_{ 2} }}^{\text{melt}}$$ and $${{a}}_{{{\text{AlO}}_{ 1. 5} }}^{\text{melt}}$$). The coexistence of forsterite and protoenstatite fixes the silica activity, allowing the effect of $${{a}}_{{{\text{AlO}}_{ 1. 5} }}^{\text{melt}}$$ to be separated from that of $${{a}}_{{{\text{SiO}}_{ 2} }}^{\text{melt}}$$. The relative importance of the two mechanisms depends strongly on the identity of the REE for forsterite, but not for protoenstatite. The results are used to test the lattice strain model: the two substitution mechanisms in forsterite imply different values for the Young’s modulus in the Brice equation, despite the fact that the REE3+ cations likely occupy the same crystallographic site in both mechanisms, casting doubt on the physical basis of the lattice strain theory. Comparison with literature data confirms earlier observations that the activity coefficients of REE2O3 in silicate melts decrease with increasing SiO2 content of the melt, but the effect decreases with increasing atomic number, from La to Lu, and is almost negligible for Sc. The influence of melt composition should apply to the mineral/melt REE partition coefficients of all other minerals. Recognizing that observed mineral/melt partition coefficients are often the sums of contributions from multiple substitution mechanisms, each with its own dependence on both crystal composition and the stoichiometric control from the melt composition, will improve parameterizations of the mineral/melt partition coefficients of other rock-forming minerals. Partition coefficients for Na, Al, Ca, Ti, and Zr are also reported.

Effect of Different Aluminum Substitution Rates on the Structure of Tobermorite.

Tobermorite and Al-substituted tobermorite with varying Al/(Si + Al) molar ratios were synthesized via the static hydrothermal method. The effect of different Al substitution ratios on the structure of tobermorite was investigated by XRD, SEM–EDS, TG–DSC, FT–IR, 29Si, and 27Al MAS–NMR. The interplanar crystal spacing of (002) increased with the increment of the Al substitution ratio. The maximum ratio of the Al substitution for Si in the structure of tobermorite was about 15 at %, which was limited by the compositions of tobermorite and the electrostatic repulsive forces caused by Al substitution. The DSC curve of the tobermorite with an Al/(Al + Si) molar ratio of 0.15 showed a larger exothermic peak area at 850 °C, indicating that the Al-O-Si bond was formed in the structure of tobermorite and the Al participated in the recrystallization. The 29Si and 27Al MAS–NMR results confirmed that tetrahedral Al was introduced into the structure of the tobermorite. New silicon sites, Q2(1Al) and Q3(1Al), formed by the Al substitution for Si in bridging and non-bridging sites, were detected. Compared to the tobermorite without Al substitution, the adsorption capacity of Al-substituted tobermorite with Al/(Al + Si) = 0.15 to Na+ was improved by 44%.

Hydrothermal synthesis of chlorite from saponite: Mechanisms of smectite-chlorite conversion and influence of Mg2+ and Al3+ supplies

Conversion of smectite to chlorite (SC conversion) is a common clay transformation reaction in a range of geological settings. Yet, current models for SC conversion are still under debate. This study systematically investigated the mechanisms of SC conversion and the influences of Mg2+ and Al3+ supplies. Saponite (a representative trioctahedral smectite) was reacted with Mg2+ and Al3+ under 340 °C and autogenous pressure. Reaction was found to reach steady state around 24 h, and either randomly mixed-layered chlorite/smectite (C/S) or chlorite formed from saponite. Solution chemistry and mineralogical evidence suggested that the SC conversion occurred through solid-state transformation, characterized with gradual formation of hydroxide layers from MgAl polymers via polymerization in interlayer and concomitant substitution of Al for Si in 2:1 layers. The formation of mixed-layered C/S or chlorite depended on Mg2+ and Al3+ supplies. When Mg2+ and Al3+ were limited, mixed-layered C/S would form from saponite, and growing Mg2+ and Al3+ supplies would continuously increase the percentage of chlorite in mixed-layered C/S until chlorite formed (saponite → mixed-layered C/S → chlorite). In contrast, chlorite could form directly from saponite under sufficient Mg2+ and Al3+ supplies (saponite → chlorite). This study contributed to an improved understanding of SC conversion, and provided a general methodology and knowledge basis for future studies involving this mineral conversion.

Mechanical and electrochemical properties of Al alloyed D8m-Ta5Si3 nanocrystalline coatings

In this work, the structural stability, mechanical properties and electronic structure of β-Ta5Si3 with different Al additions were first predicted by first-principles density functional calculations. The binary and ternary β-Ta5Si3 are thermodynamically and mechanically stable, but their thermodynamic stability slightly decreases with Al additions. The elastic constants, bulk modulus (B), shear modulus (G) and Young's modulus (E) of Al-alloyed β-Ta5Si3 were calculated. According to the criteria involving the bulk/shear modulus (B/G ratio) and Poisson's ratio, the D8m-structured ternary Ta5Si3, where two Si4a atoms are replaced by two Al atoms, shows improved ductility. Based on the theoretical analysis, the β-Ta5(Si0.83Al0.17)3 nanocrystalline coating was deposited onto polished Ti-6Al-4V alloy substrates by double cathode glow discharge plasma method. The phase constitution, microstructure and composition of the coating were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM) including energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The as-deposited coating was composed of nearly rounded D8m-β-Ta5Si3 grains with an average size of ∼4 nm and exhibited strong (002) and (400) preferred orientation. After Al alloying, the hardness and the compressive residual stress of the β-Ta5Si3 decreased, and the contact damage resistance and adhesion strength increased. The electrochemical corrosion properties of the β-Ta5(Si0.83Al0.17)3 nanocrystalline coating were compared to the binary β-Ta5Si3 coating and Ti-6Al-4V in 3.5 wt% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. It was shown that the corrosion resistance of the β-Ta5Si3 was improved through the substitution of Al for Si.

Uncovering the role of micro silica in hydration of ultra-high performance concrete (UHPC)

This study aims to clarify the role of micro silica in hydration of UHPC. The behavior of two different types of micro silica in UHPC was investigated with regard to their filler effect and pozzolanic reaction by using analytical techniques; XRD, TG, 29Si and 27Al NMR spectroscopy, and MIP. The micro silica with high pozzolanic activity led to the higher level of Al-substitution for Si in C-S-H and the denser structure, thus increasing compressive strength, whereas that facilitating filler effect limited the consumption of Ca(OH)2 even after high temperature curing and led to the higher amount of AFm phase. In the UHPC showing high filler effect, additional hydration of C3S and C2S occurred at later ages, increasing the fraction of Q1 site in C-S-H and decreasing the porosity in the pore diameter region below 10nm. Consequently, a significant increase in the compressive strength of this UHPC was achieved.

γ-Alumina and Amorphous Silica–Alumina: Structural Features, Acid Sites and the Role of Adsorbed Water

The adsorption of water, pyridine, CO and CO2 on a commercial γ-Al2O3 and a commercial amorphous silica alumina (ASA) cracking catalyst are reinvestigated by means of FTIR spectroscopy. It is concluded that the real active hydroxyl groups on γ-Al2O3 are those absorbing at 3790, 3770 and 3730 cm−1 and that they are significantly acidic. The strongest Lewis acid sites are identified as Nuclear Magnetic Resonance (NMR)-silent (or still undetected) tricoordinated Al3+, becoming tetracoordinated when interacting with basic adsorbates. More than one pyridine molecule may adsorb over such highly coordinatively unsaturated sites at high pyridine vapour pressure. It has also been confirmed that the existence of Bronsted acid sites sufficiently strong to protonate pyridine on ASA is its intrinsic property, not due to coadsorbed water. A “stuffed amorphous silica” model is proposed for low-alumina ASA, where part of Al3+ ions compensate the charge in-balance due to the substitution of Al for Si in a silica-like tetrahedral framework. Interstitial Al3+ occupy 4-, 5- or 6-coordination interstices in the amorphous silica-like framework as do alkali cations in “stuffed silicas” aluminosilicates. Such “interstitial” Al3+ ions when located accidentally in opened interstices exposed at the surface play the role of strong Lewis sites. When these ions are located near a silanol group, they increase the Bronsted acidity of the terminal silanols by neutralizing the negative charge produced by their dissociation when interacting with bases.

Analysis of erionites from volcaniclastic sedimentary rocks and possible implications for toxicological research

Erionite occurs in volcaniclastic rocks and soils; in some villages in Turkey the presence of erionite in local rocks is associated with mesothelioma, a disease also associated with inhalation of airborne asbestos. Since volcaniclastic rocks containing erionite are widely present in the western U.S.A., there is a concern over potential health issues following inhalation of dust particles in these areas and thus there is a need to identify and quantify erionite particles found in air samples during hygienic investigations. Previous attempts to analyze the few micrometer-sized erionite particles found on air sample filters under transmission electron microscope (TEM) encountered difficulties due to electron beam damage. Recommendations are presented for accurate analysis by both energy-dispersive spectroscopy (EDS) and selected-area electron diffraction (SAED). Much of the work previously published to establish the crystal chemistry of erionite has involved the relatively large crystals found in vesicles in extrusive volcanic rocks. Analysis of these crystals gives a weight percent ratio of Si to Al in a narrow range around 2.7 (molar ratio 2.6), consistent with a unit-cell formula Al 10 Si 26 . In addition, the cation contents of these crystals generally meet the charge balance error formula for zeolites. However, erionites formed in volcaniclastic sedimentary rocks (tuffs) have very different Si:Al weight percent ratios, around 4.0, which is above the upper range for the analyses of the crystals found in vesicles. Analysis of many particles in samples from different locations reveal two other major differences between the erionites from the sedimentary situations and those found in vesicles. (1) The extra-framework alkali cation (Na, K, Ca) contents are lower than required for a stoichiometric balance with framework Al substitution for Si so that the cation charge balance error formula limits for zeolites are not met. (2) There is a large variability in measured cation contents from particle to particle from the same source as well as substantial differences in average compositions from different sources. However, sedimentary erionites cannot be termed a separate mineral species because the crystallographic data are consistent with erionite and new zeolite names cannot be proposed on the basis of Si:Al ratios alone. In addition to chemical differences between erionite from different sources, there are also morphological differences. By analogy with asbestos minerals, differences in composition and morphology may have implications for relative toxicity, and future research should include consideration of these aspects.

Physicochemical changes caused by reactive MgO in alkali-activated fly ash/slag blends under accelerated carbonation

Abstract This study characterizes alkali-activated fly ash/slag (AFS) blends with reactive MgO under an accelerated carbonation condition. XRD, DTG, FT-IR, solid-state 29Si, and 27Al MAS NMR analyses were conducted to investigate the microstructural and gel phase changes caused by the addition of MgO under a high CO2 condition. The addition of reactive MgO to the AFS blends confirmed the formation of a hydrotalcite-like phase with a higher Al uptake level rather than Al substitution for Si in C-S-H gel. The incorporated MgO reacted with CO2 to precipitate as magnesium carbonate and an amorphous hydrotalcite-like phase, which mitigated the carbonation of the C-S-H gel by retarding the penetration of CO2. The carbonated samples showed the presence of aluminosilicate gel with Q3 and Q4 sites stemming from the decalcification of C-(A-)S-H gel and highly cross-linked C-(N-)A-S-H gel.

Effects of Incorporating High-Volume Fly Ash into Tricalcium Silicate on the Degree of Silicate Polymerization and Aluminum Substitution for Silicon in Calcium Silicate Hydrate.

This study assesses the quantitative effects of incorporating high-volume fly ash (HVFA) into tricalcium silicate (C3S) paste on the hydration, degree of silicate polymerization, and Al substitution for Si in calcium silicate hydrate (C–S–H). Thermogravimetric analysis and isothermal conduction calorimetry showed that, although the induction period of C3S hydration was significantly extended, the degree of hydration of C3S after the deceleration period increased due to HVFA incorporation. Synchrotron-sourced soft X-ray spectromicroscopy further showed that most of the C3S in the C3S-HVFA paste was fully hydrated after 28 days of hydration, while that in the pure C3S paste was not. The chemical shifts of the Si K edge peaks in the near-edge X-ray fine structure of C–S–H in the C3S-HVFA paste directly indicate that Al substitutes for Si in C–S–H and that the additional silicate provided by the HVFA induces an enhanced degree of silicate polymerization. This new spectromicroscopic approach, supplemented with 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectroscopy and transmission electron microscopy, turned out to be a powerful characterization tool for studying a local atomic binding structure of C–S–H in C3S-HVFA system and presented results consistent with previous literature.

Hydrophilicity/hydrophobicity modulated synthesis of nano-crystalline and hierarchically structured TS-1 zeolites

Nanosized crystals and hierarchically structured materials are considered to be two important options to enhance the mass transportation efficiency in microporous zeolites. Herein, we first present a highly efficient gel-crystallization approach for the synthesis of TS-1 nanocrystals with an exceptionally high mass yield of up to 97.4% and a volume yield of ∼0.36 g ml−1. The resultant TS-1 nanozeolites (un-TS-1) possess uniform and tunable particle sizes from 80 nm to 2.0 μm. Compared to the TS-1 counterparts (ht-TS-1) synthesized by a direct hydrothermal method, the obtained un-TS-1 materials showed comparable or improved catalytic performance for epoxidation of 1-hexene and hydroxylation of phenol with dilute H2O2 as the oxidant. More importantly, a surface-hydrophilicity/hydrophobicity-modulated mechanism has been proposed to illustrate the distinct structural evolution pathway of un-TS-1 nanocrystals and single-crystalline ZSM-5 HSZs under identical synthetic conditions: the relatively stronger hydrophobicity of TS-1 due to the equal-valence substitution of Ti for Si, is suggested to be responsible for the formation of nanocrystalline TS-1, in comparison with ZSM-5 HSZs of higher hydrophilicity resulting from the tri-valence Al substitution for Si. The successful syntheses of relatively hydrophilic Al-containing TS-1 and Fe-doped silicalite-1 HSZs offer further support to this mechanism.

Vesuvianite From Pajsberg, Sweden, and the Role of Be In the Vesuvianite Structure

Vesuvianite from Pajsberg, Sweden contains about one atom of Mn, based on 50 cations per formula unit, and small amounts of Be, B, and As. Optical absorption analysis suggests that the Mn is predominantly or entirely trivalent. Crystal-structure analysis indicates that Mn is housed at the general octahedral site Y3, which exhibits only minor distortion from ideal octahedral symmetry. Arsenic is housed at Y2 and Z2, and the formula derived from electron microprobe and LA-ICP-MS analyses suggests minor substitution of Al for Si, also at Z2. Beryllium and B are at T1, between the edge-sharing trimers Y3Y2Y3, as is the case for B in the boron-dominant vesuvianite species wiluite. The total content at T1 is interpreted as 0.82Be, 0.34B, and 0.037Fe^(3+).

High-Resolution Transmission Electron Microscopy (HRTEM) Study of Stacking Irregularity in Fe-Rich Chlorite From Selected Hydrothermal Ore Deposits

AbstractThe structures of Fe-rich chlorite and berthierine and the formation mechanisms of 7 Å–14 Å interstratified minerals were not previously fully understood owing to the difficulties in analyzing them by X-ray diffraction (XRD). The present study characterizes Fe-rich chlorites in quartz veins of epithermal to xenothermal vein-type ore deposits without later structural modifications, based on high-resolution transmission electron microscopy (HRTEM) along with XRD examination and chemical analysis. Samples have a wide range of Fe/(Fe+Mg) ratios from 0.38 to 0.98 and tetrahedral Al substitution for Si from 0.94 to 1.44 atoms per formula unit (apfu). The variation in Fe content nearly parallels the tetrahedral Al content. The formation temperatures estimated by chlorite geothermometry range from 190°C to 320°C. In HRTEM, most of the samples showed interstratification between 7 Å, 14 Å, and/or (in some samples) smectite layers. Chlorites with relatively low Fe contents (Fe/(Fe+Mg) ≈ 0.4) were characterized by mostly 14 Å periodicity with the polytype IIbb. In contrast, interstratification of 7 Å and 14 Å layers predominated with increasing Fe content and the proportion of 7 Å layers exceeds 80% in Fe-rich samples with Fe/(Fe+Mg) > 0.9. The 7 Å component layer approximated Fe-rich berthierine based on the chemical composition. Layer stacking structures in the Fe-rich samples were complex, and characterized by disorder of 7 Å and 14 Å layers, differences in the polarity of the tetrahedral sheets, variations of the slant of the octahedral sheets, and positional disorder between octahedral and tetrahedral sheets involving the hydrogen bonding, as indicated from HRTEM observations along the Yi directions of the phyllosilicates. The complex stacking structures observed in Fe-rich samples suggest that irregularity was controlled by neither the Fe/(Fe+Mg) ratio nor the formation temperature; stacking was controlled by kinetic factors in the process of mineral precipitation under disequilibrium conditions.

Cathodoluminescence characterization of enstatite

Enstatite in meteorite shows various emissions of cathodoluminescence (CL), and CL emission in terrestrial enstatite has been confirmed in this study. The CL spectra of these enstatite exhibit two broad emission bands at around 400 nm in a blue region and at around 670 nm in a red region. The emission components obtained by a spectral deconvolution can be assigned to impurity centers of Cr3+ (1.70–1.75 eV) and Mn2+ (1.85–1.90 eV) and to three defect centers (2.72–2.75, 3.18, and 3.87 eV). The emission component at 3.18 eV might be associated to the defect center derived from structural distortion by the substitution of Al for Si in a tetrahedral site. Extraterrestrial enstatite gives a characteristic emission at 3.87 eV in a blue to UV region, which is not detected in the terrestrial enstatite, suggesting radiation–induced defect by cosmic rays.

Nature of C-(A)-S-H Phases Formed in the Reaction Bentonite/Portlandite

Reactions between bentonite/montmorillonite and portlandite have been studied in the context of the engineered barriers of a purpose built repository for the deep geological disposal of spent fuel and high-level radioactive wastes. Portlandite was selected as the representative material of cement leaching in the early alkaline stage expected in a repository when conventional Ordinary Portland Cement (OPC) is used. Eight different batch experiments were performed for a reaction time near to two months, including bentonite or montmorillonite at montmorillonite/portlandite molar ratios of 2 : 1 and 3 : 1 under hydrothermal conditions. Temperatures of reactions were maintained constant at either 60 or 120°C. Calcium silicates hydrates with limited substitution of Al for Si (C-(A)-S-H phases with Al/Si <0.3) were formed with different structures and compositions as a function of the reaction conditions. Orthorhombic 11 Å-tobermorite-type phase was detected in experiments at 120°C while a more disordered monoclinic tobermorite formed at 60°C. These results are useful for the interpretation of experimental data in more complex experiments using concrete or cement pastes and bentonite, where C-(A)-S-H phases of variable compositions can precipitate, in addition to the characteristic cement hydrates and other secondary minerals carbonates.