Crystalline Silicates Research Articles

Peculiarities of xonotlite synthesis from the raw materials with different SiO2 activities

In this work, the peculiarities of xonotlite hydrothermal synthesis at 200 °C from lime and SiO2 materials with different pozzolanic activities (AP) were investigated: calcinated opoka (Ap = 170.1 mgCaO kg−1), granite sawing waste (Ap = 52.2 mgCaO g−1) and reagent SiO2·nH2O (Ap = 336.8 mgCaO kg−1). By XRD, DSC, TG, SEM, FT-IR methods have been shown that the formation of crystalline calcium silicate hydrates and the sequence of the intermediate phases existence are influenced not only by SiO2 component activity, but by other factors too. The use of the most active raw meal with SiO2·nH2O results in a very rapid formation of z-phase, C–S–H(I) and gyrolite, which hardly recrystallize into thermodynamically stable mineral—xonotlite. The impurities in the starting materials may promote the formation of some other compounds and retard the synthesis of stoichiometric ones: high content of Al-containing minerals in granite sawing waste (15.41% of Al2O3) predetermines that 1.13 nm tobermorite even after 72 h of hydrothermal curing did not recrystallize into xonotlite. Regardless of its average activity, calcinated opoka is an excellent material for the synthesis of crystalline calcium silicate hydrates. Amorphous SiO2 from opoka begins to react first, followed by tridymite and cristobalite. 1.13 nm tobermorite and xonotlite are formed at the beginning of the hydrothermal synthesis (4 h), and this greatly reduces the probability of the existence of amorphous phases.

Characterization of airborne dust samples collected from core areas of Kathmandu Valley

Kathmandu Valley is reported to be one of the highly polluted and populated cities in the world. Particulate matter is one of the major contributors of unhealthy air in Kathmandu. Although there are several reports on spatial and temporal variation of air quality of Kathmandu Valley, the morphological and mineralogical characteristics of particulate matter are very limited or none. In this study, we report on the mineralogical and morphological analysis of airborne particulate matter collected from densely populated core areas of Kathmandu Valley using spectroscopic and microscopic techniques. The Fourier Transform Infrared (FTIR) and X-ray Diffraction (XRD) data showed the presence of clay minerals, crystalline silicate mineral, carbonate minerals, and asbestiform mineral in the dust samples. The field emission scanning electron microscopic analysis confirmed the existence of particles having diverse morphology with some of the particles having aspect ratio as high as twenty; indicating the existence of asbestiform type minerals. Based on SEM-EDX data, we found that the relative distribution of elements to be different in different samples and C, O, Mg, Ca, and Si were the major elements in the dust samples. Interestingly, the XRD data analysis showed that in all the samples quartz mineral having high degree of crystallinity was present. The XRD measurement was also carried out in three different brands of cement samples. Few minerals present in dust samples were also identified in the cement samples. This observation could indicate that cement is one of the sources of minerals in the airborne particulate matter in the Kathmandu Valley.

Basic mechanisms of afwillite seeding for acceleration of tricalcium silicate hydration

Afwillite is a crystalline calcium silicate hydrate (3CaO·2SiO2·3H2O) that can be used for the acceleration of tricalcium silicate hydration. Experimental data was recorded in this study to shed further light on the basic mechanisms for this acceleration effect. It is observed that afwillite addition favours growth of the afwillite seeds and hardly any C-S-H is formed. The pore solution chemistry during hydration closely resembles that of unseeded tricalcium silicate indicating that pore solution chemistry is not triggering the accelerating effect of afwillite. Much higher degrees of hydration are observed after the main hydration period in the presence of afwillite seeds. This is explained by the formation of relatively large afwillite crystals in some distance to the surface of tricalcium silicate. This is different to the hydration of tricalcium silicate in the absence of afwillite seeds when C-S-H is precipitated on the surface of the tricalcium silicate particles.

In vitro bioactivity of silicophosphate glasses doped with ZnO, SrO or CuO

Silicophosphate glasses with variable divalent ion dopants (Zn2+, Sr2+ or Cu2+) were prepared via melt annealing route. Parent and doped glasses were thermally heat-treated through a two-step regime to be converted into their glass–ceramic derivatives. Fourier transform infrared (FT-IR) spectral data were recorded for parent glasses and their glass–ceramics derivatives to identify the structural building units which reveal vibrational bands due to both main phosphate and some silicate groups and to verify the bioactivity behavior after immersion in diluted phosphate solution. X-ray diffraction studies indicate the formation of different phosphate and silicate crystalline phases in the derived glass–ceramics which varied with the type of dopant oxide. SEM investigations of the glass–ceramics before and after immersion in phosphate solution showed multicomponent microcrystalline textures in the studied micrographs before immersion. Nodular-shaped microcrystalline features were identified after immersion in phosphate solution referring to the formation of crystalline hydroxyapatite. The undoped glass–ceramic is identified to crystallize in almost equal silicate and phosphate phases, while with the dopant ZnO, SrO or CuO, the crystalline phases are only of phosphates, while the silicate phase is assumed to be retained in the surrounding remaining vitreous boundaries.

Crystalline silicate absorption at 11.1 μm: ubiquitous and abundant in embedded YSOs and the interstellar medium

ABSTRACT Utilizing several instruments on 4–8 m telescopes, we have observed a large sample of objects in the mid-infrared (8–13 μm). These comprise a few evolved stars, multiple envelopes of embedded young stellar objects (YSOs) or compact H-II regions, and several sightlines through the interstellar medium (ISM). The latter is where dust resides – and is potentially modified – between its formation in evolved stellar outflows and deposition in molecular clouds. In most objects, we detect not only the well-known 9.7 μm absorption feature of amorphous silicates but also a second absorption band around 11.1 μm whose carrier is attributed to crystalline forsterite. We propose that crystalline silicates are essentially ubiquitous in the ISM and earliest phases of star formation, and are evolutionary precursors to T-Tauri and Herbig stars where such silicates have been commonly found. Modelling shows that in most YSOs, H-II regions and ISM cases, the forsterite mass fraction is between 1 and 2 per cent, suggesting that the younger phases inherit their abundance from the ISM. However, several sources show much stronger features (abundances ≥3 per cent). This suggests that significant processing, perhaps crystallization by thermal annealing, occurs early on in star formation. Most intriguing is the first detection of crystalline silicate in the diffuse ISM. We propose that our observed abundance is consistent with a mass fraction of crystalline silicates of 10–20 per cent injected into the ISM, along with commonly accepted lifetimes against their destruction, but only if cosmic ray-induced amorphization is insignificant over a few Giga years.

Flash Catalytic Pyrolysis of Polyethylene over (Alumino)silicate Materials

AbstractA series of amorphous and crystalline (alumino)silicate catalysts, with variable acidity, are evaluated for the flash pyrolysis of polyethylene (PE). Catalysts morphology, acidity as well as textural properties are influencing the process of decomposition of PE. Incomplete PE conversion and significant wax formation are obtained under the reaction condition with pure amorphous and MFI‐type silica. Complete conversion, and limited formation of wax, are observed with amorphous and MFI‐type Si−Al containing catalyst. Acidity has a significant influence on the molecule size and nature. Then, the increase of the catalyst acidity leads to the decrease of the molecular weight of pyrolysis products and the increase of the gas and the light liquid fractions. In addition, the acidity increase promotes cyclization reaction, resulting in the increase of the cycled hydrocarbon/aromatic proportion in the liquid fraction. Then, from PE catalytic pyrolysis, up to 25 wt.% of aromatics can be obtained in the liquid fraction with Si/Al ZSM‐5 zeolite, while olefins remain the major fraction with the less acidic amorphous aluminosilicate catalyst.

АНАЛІЗ ПАРАМЕТРІВ АКУСТИЧНОЇ АНІЗОТРОПІЇ ПІРОКСЕН-МАГНЕТИТОВИХ ПОРІД ПІЩАНСЬКОЇ СТРУКТУРИ

The analysis of the results of acoustic properties of rocks study of Pischans`ka iron-ore structure is presented. The aim of the work is to establish the features of the distribution of acoustic properties and parameters of acoustic anisotropy in samples of core rocks selected from the well No. 3 of the Pischans`ka structure to determine the nature of its occurrence. A sample of 35 samples from the depth range 144-273 m is divided into 3 groups of rocks, namely: magnetite-pyroxene, quartz-magnetitepyroxene and biotite-amphibole crystalline shales. Based on an invariant polarization method, a number of acoustic laboratory measurements have been carried out. The values of the measured phase velocities "quasi-longitudinal" and two "quasi-transverse" waves at the stage of measurements showed significant acoustic anisotropy of the rocks. The ranges of the measured speeds of the collection samples are 7661 ÷ 5046 m / s for longitudinal waves and 4232 ÷ 2648 m/s for transverse ones. The difference in values measured for each of the sides of the cubic rhombic dodecahedron is from 100 to 800 m / s and from 0 to 500 m/s for Vp and Vs, respectively. The parameters of an acoustic ellipsoid were calculated, on the basis of which the division of samples into 3 main groups has been performed, according to the acoustic texture: acoustically linear, shale and rhombic. Separately, a group of samples with a more complex texture was discovered. The analysis of coefficients of anisotropy by different methods is carried out: longitudinal, transverse and relative acoustic anisotropy. Most of the samples are characterized by low or average acoustic anisotropy (from 2 to 7 %). A group of highly anisotropic rocks (11–14 %), represented by samples of biotite-amphibole crystalline silicates, is singled out. According to the parameters of the acoustic tensor of most samples, the transverse isotropic type of symmetry inherent to samples from the depth intervals 174–220 m and 222–232 m, while the smaller part is rhombic, is inherent. Differences in the parameters of anisotropy of samples can be explained by the significant heterogeneity of their textures, namely: micro cracks, minerals of various sizes, shapes and orientations. The results of the research show that the acoustic properties of the samples are quite heterogeneously distributed along the investigated depth range. This indicates the difficult conditions for the formation of rocks at different depths and the presence of different types of deformations, which accompanied the formation of the Pischans`ka structure.

Spectral Evolution and Radial Dust Transport in the Prototype Young Eruptive System EX Lup*

Abstract EX Lup is the prototype of a class of pre-main-sequence eruptive stars defined by their repetitive outbursts lasting several months. In 2008 January–September EX Lup underwent its historically largest outburst, brightening by about 4 mag in visual light. In previous studies we discovered ongoing silicate crystal formation in the inner disk during the outburst, but also noticed that the measured crystallinity fraction started decreasing after the source returned to the quiescent phase. Here we present new observations of the 10 μm silicate feature, obtained with the MIDI and VISIR instruments at Paranal Observatory. The observations demonstrate that within five years practically all crystalline forsterite disappeared from the surface of the inner disk. We reconstruct this process by presenting a series of parametric axisymmetric radiative transfer models of an expanding dust cloud that transports the crystals from the terrestrial zone to outer disk regions where comets are supposed to form. It is possible the early Sun also experienced similar flare-ups, and the forming planetesimals might have incorporated crystalline silicate material produced by such outbursts. Finally, we discuss how far the location of the dust cloud could be constrained by future James Webb Space Telescope observations.

On micro-level vulnerability of concrete surface layer in process of crystallization of hydrating Portland cement and upon mechanical load

The target of this research is to make a system of data on discontinuity of the cement stone microstructure in concrete. To achieve this, the following issues have to be addressed: a) a justification for the presence of dislocations in cement stone and definition of their type needs to be provided; b) the source of the considered material weakness has to be identified; c) an explanation of formation and propagation of micro cracks starting at the surface and going deeper into the concrete needs to be given; d) an educated guess has to be made for the opening/depth ratio of micro cracks; e) information about the distances between micro cracks in the capillary walls of the cement stone should be obtained; e) binding energy in the calcium silicate crystalline hydrate has to be determined. The research results show that the mechanical properties of crystals and polycrystals in the cement stone (concrete) depend on the ratio of small-angle (θ≤10…150) and large-angle boundaries. Due to a relatively small number of large-angle boundaries, intergranular and plastic flow deformations are dampened under straining, resulting in cleavage in the areas where stress-raisers are found (pores, capillaries, submicrocracks, etc.).

Facile Preparative Access to Bioactive Silicon Oxycarbides with Tunable Porosity.

In the present work, Ca-containing silicon oxycarbides (SiCaOC) with varying Ca content have been synthesized via sol-gel processing and thermal treatment in inert gas atmosphere (pyrolysis). It has been shown that the as-prepared SiCaOC materials with low Ca loadings (Ca/Si molar ratios = 0.05 or 0.12) were X-ray amorphous; their glassy network contains Q3 sites, indicating the presence of Ca2+ at non-bridging-oxygen sites. SiCaOC with high Ca content (i.e., Ca/Si molar ratio = 0.50) exhibits the presence of crystalline calcium silicate (mainly pseudowollastonite). Furthermore, it has been shown that the incorporation of Ca into the SiOC glassy network has a significant effect on its porosity and specific surface area. Thus, the as-prepared Ca-free SiOC material is shown to be non-porous and having a specific surface area (SSA) of 22.5 m2/g; whereas SiCaOC with Ca/Si molar ratio of 0.05 exhibits mesoporosity and a SSA value of 123.4 m2/g. The further increase of Ca content leads to a decrease of the SSA and the generation of macroporosity in SiCaOC; thus, SiCaOC with Ca/Si molar ratio of 0.12 is macroporous and exhibits a SSA value of 39.5 m2/g. Bioactivity assessment in simulated body fluid (SBF) confirms the hydroxyapatite formation on all SiCaOC samples after seven days soaking, unlike the relatively inert ternary silicon oxycarbide reference. In particular, SiCaOC with a Ca/Si molar ratio of 0.05 shows an increased apatite forming ability compared to that of SiCaOC with Ca/Si molar ratio of 0.12; this difference is considered to be a direct consequence of the significantly higher SSA of the sample with the Ca/Si ratio of 0.05. The present work indicates two effects of Ca incorporation into the silicon oxycarbide glassy network on its bioactivity: Firstly, Ca2+ is shown to contribute to the slight depolymerization of the network, which clearly triggers the hydroxyapatite formation (compare the bioactive behavior of SiOC to that of SiCaOC with Ca/Si molar ratio 0.12 upon SBF exposure); secondly, the Ca2+ incorporation seems to strongly affect the porosity and SSA in the prepared SiCaOC materials. There is an optimum of Ca loading into the silicon oxycarbide glassy network (at a Ca/Si molar ration of 0.05), which provides mesoporosity and reaches maximum SSA, both highly beneficial for the bioactive behavior of the materials. An increase of the Ca loading leads, in addition to the crystallization of calcium silicates, to a coarsening of the pores (i.e., macroporosity) and a significant decrease of the SSA, both negatively affecting the bioactivity.

Unidentified infrared emission features in mid-infrared spectrum of comet 21P/Giacobini-Zinner

Comet 21P/Giacobini-Zinner (hereafter, comet 21P/G-Z) is a Jupiter-family comet and a parent comet of the October Draconids meteor shower. If meteoroids originating from a Jupiter-family comet contain complex organic molecules, such as amino acids, they are essential pieces of the puzzle regarding the origin of life on Earth. We observed comet 21P/G-Z in the mid-infrared wavelength region using the Cooled Mid-infrared Camera and Spectrometer (COMICS) on the 8.2 m Subaru Telescope on UT 2005 July 5. Here, we report the unidentified infrared (UIR) emission features of comet 21P/G-Z, which are likely due to complex organic molecules (both aliphatic and aromatic hydrocarbons), and the thermal emission from amorphous/crystalline silicates and amorphous carbon grains in its mid-infrared low-resolution spectrum. The UIR features at ~8.2 μm, ~8.5 μm, and ~11.2 μm found in the spectrum of comet 21P/G-Z could be attributed to polycyclic aromatic hydrocarbons (or hydrogenated amorphous carbons) contaminated by N- or O-atoms, although part of the feature at ~11.2 μm comes from crystalline olivine. The other feature at ~9.2 μm might originate from aliphatic hydrocarbons. Comet 21P/G-Z is enriched in complex organic molecules. Considering that the derived mass fraction of crystalline silicates in comet 21P/G-Z is typical of comets, we propose that the comet originated from a circumplanetary disk of giant planets (similar to Jupiter and Saturn) where was warmer than the typical comet-forming region (5–30 au from the Sun) and was suitable for the formation of complex organic molecules. Comets from circumplanetary disks might be enriched in complex organic molecules, such as comet 21P/G-Z, and may have provided pre-biotic molecules to ancient Earth by direct impact or meteor showers.

Preforming abundant surface cobalt hydroxyl groups on low crystalline flowerlike Co3(Si2O5)2(OH)2 for enhancing catalytic degradation performances with a critical nonradical reaction

Low crystalline flowerlike cobalt silicate hydroxide (Co3(Si2O5)2(OH)2, CoSiOx) is synthesized by a one-pot hydrothermal approach. Abundant highly active CoOH+ is preformed on the surface of CoSiOx, while the low crystallinity of CoSiOx leads to the formation of active 1O2 caused by the generation of oxygen vacancy in the valence change cycle between Co2+ and Co3+. The optimal CoSiOx with a high specific surface area of 665.2 m2 g−1 could expose surface CoOH+, thus rapidly activate peroxymonosulfate (PMS) and accelerate catalytic efficiencies. As a result, at least 11 kinds of organic pollutants could be degraded with 33–242% higher efficiencies than reported. In addition to the common SO4- and OH, large amounts of active 1O2 endow the CoSiOx/PMS system with superb catalytic performances in diverse conditions, suggesting the excellent adaptability for practical usage. This work provides a new protocol to advance the PMS activation process for high-performance cobalt based heterogeneous catalysts.

Transformations in the photoluminescent, electrical and structural properties of Tb3+ and Eu3+ co-doped ZnO films under high-temperature annealing

abstract The effect of thermal annealing on optical, electrical and structural properties of Tb and Eu co-doped ZnO films grown by magnetron sputtering on Si and Al2O3 substrates was investigated by X-ray diffraction, photoluminescence, micro-Raman and IR reflection methods. It is shown that incorporation of rare earth ions in ZnO is accompanied by the formation of intrinsic defects. The as-deposited and annealed at 600 °C films demonstrate Tb3+ emission and no Eu3+ one. Higher intensity of Tb3+ photoluminescence in the films on Al2O3 substrate as compared with that on Si is ascribed to higher content of Tb3+ emitting centers. The model of these centers including the substitutional Tb and interstitial oxygen is proposed. In the excitation spectra of Tb3+ emission, no features connected with light absorption in ZnO are observed. An annealing at 900 °C is found to result in the formation of crystalline terbium oxide and silicate phases. In the photoluminescence spectra, the decrease of Tb3+ emission and the appearance of two sets of Eu3+ related bands caused by energy transfer from Tb3+ to Eu3+ ions are found. This is ascribed to segregation of rare earth ions in the additional phases and the decrease of the distance between the ions.

Graphene oxide nanoparticles in the interstellar medium

ABSTRACT Dust particles play a major role in the formation, evolution and chemistry of interstellar clouds, stars, and planetary systems. Commonly identified forms include amorphous and crystalline carbon-rich particles and silicates. Also present in many astrophysical environments are polycyclic aromatic hydrocarbons (PAHs), detected through their infrared emission, and which are essentially small flakes of graphene. Astronomical observations over the past four decades have revealed a widespread unassigned ‘extended red emission’ (ERE) feature which is attributed to luminescence of dust grains. Numerous potential carriers for ERE have been proposed but none has gained general acceptance. In this Letter it is shown that there is a strong similarity between laboratory optical emission spectra of graphene oxide (GO) and ERE, leading to this proposal that emission from GO nanoparticles is the origin of ERE and that these are a significant component of interstellar dust. The proposal is supported by infrared emission features detected by the Infrared Space Observatory (ISO) and the Spitzer Space Telescope.

Influence of Polyfunctional Additive on Hardening Process and Properties of Cement Concrete

The paper presents results of research aimed at developing a new semi-functional concrete additive that provides an increase in rate and level of its strength growth while reducing energy costs to accelerate hardening process, as a basis for reducing energy intensity in manufacturing of concrete and reinforced concrete products and structures. Experimentally a rational ratio of components for a polyfunctional additive has been found of mass cement: a superplasticizer based on polycarboxylate resins (for example, “Stachement 2000” or “Relamiks PC”) – 0.5 %, ultradispersed microsilica (SiO2) – 1.0 %, sodium sulfate (Na2SO4), hardening accelerator – 0.5 %, aluminum sulfate (Al2(SO4)3), sealing additive structure ‒ 0.25 %. The mentioned components ensure the largest increase in strength of cement stone and structural heavy concrete. Results of derivatographic and X-ray phase analyses have shown that strength growth is based on formation of a fine-crystalline form of low-base crystalline silicates of CSH-silicate group, which complements traditionally formed C2SH by the reaction of threeand two-calcium silicate cement with water, as well as it is based on an increase in the number of neoplasms due to the reaction of Ca(OH)2 with amorphous SiO2 and ettringite 3CaO × Al2O3 × 3CaSO4 × 32H2O, being formed due to reactions with cement aluminates these are accelerating-compacting additive components, that in total provides an increase in density and strength of cement stone. While having the case with concrete, the effect is complemented by hardening the zone of contact between aggregate surface and cement stone due to the reaction between Ca(OH)2 and SiO2. These effects have been confirmed by growth (up to 38 %) of water which is chemically bound with cement in presence of a multifunctional additive in samples of cement stone, which is characterized by the largest strength. While using standardized testing methods, effectiveness of a multifunctional additive has been experimentally confirmed and it has been expressed in growth of quality characteristics and properties of structural heavy concrete: compressive strength – up to 40–60 %, flexural strength – up to 15 %, reduction of shrinkage – up to 50 % and water absorption – by 1.5–2 times, increase in frost resistance from brand F250 to F500, water resistance – from W6–W8 to W20.

Tb and Li doped Zinc Silicate green nanophosphors synthesized by solution combustion method

Zn1.99SiO4:Tb0.01 and Zn1.98SiO4:Tb0.01, Li0.01 green nanophosphors were prepared by solution combustion method. The phosphors are calcined at 950 °C for 3 h. The structural details of Zn2SiO4 powders are investigated based on the measurement of XRD. The doped Zn2SiO4 has a rhombohedral space group R3H and the crystalline size of the particles are around 80 nm. FTIR spectra analysis proves the presence of pure crystalline zinc silicate. The TEM graphs indicate that the materials are nano structured Photoluminescence measurement of the samples shows that the phosphors emit strong green light centered at 524 & 547 nm under UV light excitation. The excitation wavelength is attributed to 4f-5d transition of Tb3+ ion and the green emissions are attributed to 5D4→7Fj (j=5,6) transitions. The presence of co-dopant Li+ ion increases the luminescence property. The addition of Li+ ions along with Tb3+ act as self-promoter for a better crystallization. Luminescence decay curve measurement shows double exponential decay behavior and fast decay time has observed.

Self-synthesize and flexural strength test porcelain from Indonesian natural sand

Dental porcelain is usually used for aesthetic indirect restoration. Porcelain powder is still imported, although Indonesia is rich with natural sand as porcelain raw material. Flexural strength is a mechanical property which is related to porcelain’s ability to resist occlusal force. The objective of this study is to synthesize porcelain from the mixture of Sumateran and Javanese natural sand and evaluate its flexural strength. This experiment used explorative descriptive method. Feldspar Pangaribuan (65% wt), silica Belitung (25%wt), kaolinite Sukabumi (5%wt), and potassium salt (5%wt) was mixed with wet-ball mill and dissolved by fritting to produce porcelain powder. Five samples were made by liquid-phase method in size 3mm x 2cm x 7cm. XRD test was used to analyse crystalline formed and flexural strength of these samples was measured by Netzch Universal Testing Machine with 7.5kg initial load and calculated by flexural strength formula. The result was porcelain powder which has been sintered on 1150 °C contain leucite (potassium aluminium silicate) crystalline on XRF test with average flexural strength 26.678 MPa. Leucite crystalline is the main indicator for synthesizing dental porcelain because leucite is not usually found on other ceramic. Leucite crystalline is known for increasing strength, preventing crack propagation, and resisting pyroplastic flow. Ten percent of leucite on X-Ray Fluoroscence shows the percentage is less than other dental porcelain. The conclusion of this research is that porcelain succeeded to be self-synthesized from the mixture of Sumateran and Javanese natural sand and having flexural strength 26.678 MPa.

Evidence for the absorption of crystalline silicates at 11.1 $\mu$m in the spectra of the nucleus of the Circinus galaxy

Abstract Several mid-infrared spectra of the Circinus galaxy nucleus taken with T-ReCS on Gemini South and MIDI on VLTI have consistently shown an inflection centred around 11 $\mu$m. We ascribe this feature to the absorption of crystalline silicate, based on their similarity in profile shape and improvement in fitting quality using a partially crystalline silicate model, compared to entirely amorphous models. Spectral fits reveal a fraction of 0.6–2.0 per cent of crystalline forsterite in the nucleus of the Circinus galaxy, which is similar to the values obtained for the interstellar medium (ISM) of the Milky Way. This is probably the first detection of crystalline silicate absorption in the nucleus of this Seyfert 2 galaxy. In addition, the presence of large grain-size amorphous silicates, together with the similarity in profile shape of the optical depth of Circinus with those of young stellar objects in the Milky Way, implies that most of the contribution to the spectra of Circinus comes from dust in the star formation regions near the centre of the nucleus or along the line of sight to the Earth, rather than in the ISM of Circinus. We also compare our optical depths of Circinus with those in previous studies.

Reaction of pseudowollastonite with carbonate-bearing fluids: Implications for CO2 mineral sequestration

The kinetics of silicate carbonation in aqueous solutions are typically sluggish, especially at neutral to alkaline conditions. This hampers the complete understanding of the mechanisms and parameters that control mineral carbonation during carbon capture and storage (CCS). Here we study the hydrothermal dissolution and carbonation of pseudowollastonite (psw; α-CaSiO3), one of the most reactive silicates known, under a range of geochemical conditions ranging from acidic to strongly alkaline pH, presence/absence of different background alkali metal ions and carbonate sources (K2CO3 and Na2CO3, pH ~13, or NaHCO3 and KHCO3, pH ~9). We show that in addition to amorphous silica precipitation, the formation of secondary Na + Ca- or K + Ca-silicates in the presence of Na+ and K+ background ions, respectively, fosters the progress of psw carbonation. However, the formation of Ca-containing secondary crystalline silicates and Ca-containing amorphous silica is shown to be a strong handicap for a fully effective carbonation. In all cases a higher conversion into CaCO3 (up to ~70 mol%) is achieved when using bicarbonate salts (i.e., lower initial pH). By using a reactor with a pressurized CO2-solution, with and without Na+ or K+ background ions, rapid and nearly complete conversion of psw with a CaCO3 yield ~92 mol% is achieved because, in addition to the initial low pH (~3.7) that favored α-CaSiO3 dissolution, abundant Ca-free non-passivating amorphous silica formed along with calcite. These results imply that the presence (e.g., use of sea water during CO2 injection or mixing with saline formation solutions) or the release of different alkali metal ions (e.g., after feldspar and/or basaltic glass dissolution) in combination with a reaction-induced pH increase during in situ CCS scenarios may strongly limit carbonation due to the capture of alkaline-earth metals in secondary silicates and a reduction in reaction rates. In turn, our results show that the high conversion achieved in pure CO2-aqueous systems, while relevant for ex situ CCS, may not reflect the actual conversion in multicomponent natural systems following reactive transport during in situ CCS. Moreover, the precipitation of secondary silicate and calcium carbonate phases have a direct cementing effect, which could be detrimental for in situ CCS, as it would likely reduce host rock permeability, but would be relevant and beneficial for the setting of novel CaSiO3-based non-hydraulic cements with reduced CO2 footprint.

Rietveld Quantitative Phase Analysis of Oil Well Cement: In Situ Hydration Study at 150 Bars and 150 °C.

Oil and gas well cements are multimineral materials that hydrate under high pressure and temperature. Their overall reactivity at early ages is studied by a number of techniques including through the use of the consistometer. However, for a proper understanding of the performance of these cements in the field, the reactivity of every component, in real-world conditions, must be analysed. To date, in situ high energy synchrotron powder diffraction studies of hydrating oil well cement pastes have been carried out, but the quality of the data was not appropriated for Rietveld quantitative phase analyses. Therefore, the phase reactivities were followed by the inspection of the evolution of non-overlapped diffraction peaks. Very recently, we have developed a new cell specially designed to rotate under high pressure and temperature. Here, this spinning capillary cell is used for in situ studies of the hydration of a commercial oil well cement paste at 150 bars and 150 °C. The powder diffraction data were analysed by the Rietveld method to quantitatively determine the reactivities of each component phase. The reaction degree of alite was 90% after 7 h, and that of belite was 42% at 14 h. These analyses are accurate, as the in situ measured crystalline portlandite content at the end of the experiment, 12.9 wt%, compares relatively well with the value determined ex situ by thermal analysis, i.e., 14.0 wt%. The crystalline calcium silicates forming at 150 bars and 150 °C are also discussed.