Showing papers on "Tridymite published in 2009"

Silica in protoplanetary disks

Abstract: Mid-infrared spectra of a few T Tauri stars (TTS) taken with the Infrared Spectrograph on board the Spitzer Space Telescope show prominent narrow emission features indicating silica (crystalline silicon dioxide). Silica is not a major constituent of the interstellar medium; therefore, any silica present in the circumstellar protoplanetary disks of TTS must be largely the result of processing of primitive dust material in the disks surrouding these stars. We model the silica emission features in our spectra using the opacities of various polymorphs of silica and their amorphous versions computed from earth-based laboratory measurements. This modeling indicates that the two polymorphs of silica, tridymite and cristobalite, which form at successively higher temperatures and low pressures, are the dominant forms of silica in the TTS of our sample. These high-temperature, low-pressure polymorphs of silica present in protoplanetary disks are consistent with a grain composed mostly of tridymite named Ada found in the cometary dust samples collected from the STARDUST mission to Comet 81P/Wild 2. The silica in these protoplanetary disks may arise from incongruent melting of enstatite or from incongruent melting of amorphous pyroxene, the latter being analogous to the former. The high temperatures of ∼ 1200–1300 K and rapid cooling required to crystallize tridymite or cristobalite set constraints on the mechanisms that could have formed the silica in these protoplanetary disks, suggestive of processing of these grains during the transient heating events hypothesized to create chondrules.

H-Bond Features of Fully Hydroxylated Surfaces of Crystalline Silica Polymorphs: A Periodic B3LYP Study

Abstract: Different models of hydroxylated surfaces of quartz, cristobalite, and tridymite have been studied with the hybrid B3LYP functional using the Gaussian basis set of polarized double-ζ quality with periodic boundary conditions. Starting from the optimized bulk structures of the polymorphs, 2D slabs exhibiting low (hkl) crystallographic planes have been cut, dangling bonds healed by hydroxyl groups, and the final structures fully optimized. The H-bond pattern at a given surface depends on the (hkl) plane and on the OH group density, exhibiting isolated, weakly interacting pairs, short chains, or strings extending through the whole surface. Cases in which no H-bonds are present envisage either a very low OH density or slab structural rigidity which hinders the OH groups to establish H-bond contacts. The thermodynamics of surface hydroxylation of the considered polymorphs has been shown to correlate with the strength of the H-bonds formed at the surfaces measured by the bathochromic shift of the ν(OH) stretchi...

A rapid method for quantifying single mineral phases in heterogeneous natural dusts using X-ray diffraction

Abstract: Quantification of potentially toxic single mineral phases in natural dusts of heterogeneous composition is critical for health hazard assessment. For example, crystalline silica, a human carcinogen, can be present as respirable particles in volcanic ash such as quartz, cristobalite, or tridymite. A method to rapidly identify the proportions of crystalline silica within mixed dust samples, such as volcanic ash, is therefore required for hazard managers to assess the potential risk of crystalline silica exposure to local populations. Here we present a rapid method for quantifying the proportions of single phases in the mineral assemblage of mixed dusts using X-ray diffraction (XRD) with a fixed curved position-sensitive detector. The method is a modified version of the whole-pattern peak-stripping (PS) method (devised by Cressey and Schofield [Powder Diffr.11, 35–39 (1996)]) using an internal attenuation standard (IAS) but, unlike the PS method, it requires no knowledge of other phases present in the sample. Ten synthetic sample mixtures were prepared from known combinations of four pure phases (cristobalite, hematite, labradorite, and obsidian), chosen to represent problematic constituents of volcanic ash, and analyzed by XRD. Results of the IAS method were directly compared with those of the PS method. The proportions of cristobalite estimated using the methods were comparable and accurate to within 3 wt %. The new IAS method involved less sample preparation and processing and, therefore, was faster than the original PS method. It therefore offers a highly accurate rapid technique for determination of the proportions of individual phases in mixed dusts.

Polymorphism and variable structural dimensionality in the iron(III) phosphate oxalate system: a new polymorph of 3D [Fe2(HPO4)2(C2O4)(H2O)2]·2H2O and the layered material [Fe2(HPO4)2(C2O4)(H2O)2]

Abstract: Two new iron (III) phosphate oxalates have been isolated under hydrothermal conditions as phase-pure samples and their crystal structures determined from single crystal X-ray diffraction. [Fe2(HPO4)2(C2O4)(H2O)2]·2H2O, I, is a polymorph of a known phase and differs in the local arrangement of hydrophosphate, oxalate and coordinated water about iron, presenting a mer orientation of three coordinated phosphates, rather than fac as previously seen. The structure of I is three-dimensionally connected with similar network connectivity to the known phase but different overall topology. [Fe2(HPO4)2(C2O4)(H2O)2], II, has a layered structure constructed from octahedral Fe(III) centres coordinated by hydrophosphate, oxalate and water in a fac arrangement. The amount of water used in synthesis is one of the key experimental parameters in stabilising one phase over the other. Thermogravimetric analysis shows that both I and II ultimately collapse into dense tridymite type FePO4 above 600 °C and variable temperature powder XRD shows that this occurs via crystalline intermediate phases. Variable temperature magnetisation measurements show that both materials order antiferromagnetically at low temperatures, with similar Neel temperatures (∼29 K) despite their long-range structural differences.

Cathodoluminescence characterization of tridymite and cristobalite: Effects of electron irradiation and sample temperature

Abstract: Cathodoluminescence (CL) spectra of tridymite and cristobalite have broad peaks around 430 and 400 nm, respectively, both of which can be assigned to the [AlO 4 /M + ] 0 defect. The CL intensities of these spectral peaks in the blue region decrease with prolonged exposure to electron irradiation, similar to the short-lived luminescence observed in quartz, although quartz shows a lower decrease in CL intensity compared to these minerals. Cristobalite has a higher CL intensity reduction rate during irradiation than does tridymite. Irradiation of these minerals at low temperatures results in a more rapid decay of CL emission, whereas that of quartz shows no apparent change in the rate of CL emission at similar temperatures. Confocal micro-Raman spectroscopy of the electron irradiated surface of these minerals reveals the amorphization caused by the interaction of the electron beam with the surface layer to a depth of several micrometers. This suggests that such structural destruction diminishes the activity of CL emission centers related to the [AlO 4 /M + ] 0 defects by migration of monovalent cations associated with exchanged Al in the tetrahedral sites. Both samples present a considerable reduction of their CL intensities at higher temperature, suggesting a temperature quenching phenomenon. The activation energy in the quenching process was evaluated by a least-squares fitting of the Arrhenius plots, assuming the Mott-Seitz model. The result implies that the energy of non-radiative transition in this process might be transferred to lattice vibrations as phonons in two different manners. This might be related to different irradiation responses of the CL with a change in sample temperature.

Effect of Interfacial Interaction on Properties of Gamma Ray-Irradiated High Density Polyethylene Reinforced by Sericite-Tridymite-Cristobalite

Abstract: A Gamma-ray irradiation was employed to modify high density polyethyelene (HDPE) in order to impart poor interfacial interaction between hydrophobic matrix and hydrophilic Sericite-Tridymite-Cristobalite (STC) filler. The STC filler was further treated with a silane coupling agent. The FTIR, SEM and rheological observations confirmed enhanced interactions between oxygen containing groups on the irradiated matrix (γ-HDPE) and NH2 groups of silane-treated filler (t-STC), resulting in improved mechanical properties of the composites (10 times in case of impact strength). The parameter of the theoretical model used characterizing interfacial interaction increased from 2.69 for HDPE/STC up to4 for irradiated and surface treated material.

Caracterización por XANES, análisis mineralógico y aplicación industrial de un depósito de caolín de México

Abstract: A kaolin obtained from Villa de Reyes, a region near to San Luis Potosi (Mexico) was characterized by means of X-ray powder diffraction (XRD, optical microscopy (OM), scanning electron microscopy (SEM), X-ray fluorescence (XRF), X-Ray Absorption Near Edge Spectroscopy (XANES), thermal analysis (DTA/TGA), dilatometry (DIL), and chemical analysis. Mineralogical and morphological characteristics of the mineral are presented. The kaolin sample was formed mainly by kaolinite, but other minor phases were also detected such as quartz, cristobalite, tridymite, and dolomite. The high content of volcanic glass detected, by optical microscopy, revealed an incomplete kaolinization process of the raw material. The reddish color of the kaolin was associated with the free iron content in the form of limonite [FeO(OH)], which was determined by XANES. The influence of the particle size on the whiteness of kaolin was evaluated. Dilatometric analysis revealed a strong thermal expansion between 110 y 240 °C, which would difficult the use of this material in traditional ceramic applications. On the other hand the presence of glass and high temperature phases of SiO2, such as cristobalite and tridymite will favor its use in the cement industry.

Polymorphic Mn/W/Na(K,Rb,Cs)/SiO2 catalysts for oxidative coupling of methane

Abstract: The phase composition of Mn/W/Na(K,Rb,Cs)/SiO2 composites-catalysts was determined by X-ray diffraction. Polymorphic transformations involving tridymite (a SiO2-based nonstoichiometric crystalline phase) were shown to be a reason for the high catalytic activity of Mn/W/Na(K,Rb,Cs)/SiO2 in heterogeneous oxidative coupling of methane.

Fayalite from Fe-rich paralavas of ancient coal fires in the Kuzbass, Russia

Abstract: Fayalite is a common mineral of Fe-rich paralavas related to spontaneous combustion of coal seams. Fayalite has also been found in parabasalts from burned coal waste piles of the Chelyabinsk coal basin. Among paralavas from different combustion metamorphic (CM) complexes of the world, fayalite is the most widespread in the fused rocks of the Kuznetsk coal basin (Kuzbass) and the Ravat area in Tajikistan. The optimal conditions for fayalite formation as products of coal fires in the Kuzbass and Ravat resulted from a favorable combination of the composition of fused protolith (parental rocks) composed of pelitic and Fe-rich sediments and the redox conditions of the deep subsurface (\( f_{O_2 } \) is lower than the QFM buffer). In the Kuzbass, fayalite is commonly hosted in high-silica aluminous Fe-rich paralavas composed of Fe-cordierite (sekaninaite), tridymite, hercynite-magnetite, cristobalite, aluminous clinoferrosilite, and Al-K silicic glass. The composition of all Kuzbass fayalites is close to the Fe2SiO4 end member. Kuzbass fayalites are characterized by a negligibly low CaO content and higher MnO and P2O5 contents like fayalites from burned rocks of other CM complexes. In Kuzbass paralavas, Fe-olivine is the late phase that crystallized after sekaninaite and tridymite, immediately before melt quenching.

Modulated crystal structure of incommensurate low tridymite.

Abstract: The incommensurately modulated crystal structure of low SiO2 tridymite was refined based on single-crystal X-ray diffraction data in the superspace group Cc(α0γ)0 The data set consists of 885 main reflections, 1751 first-order, 924 second-order and 119 third-order satellite reflections with I > 3σ(I) The modulation is mainly made up from cooperative twistings of the rigid SiO4 tetrahedra Two orders of displacement waves are used to describe the modulation of the Si atoms and three orders for the O atoms The maximal amplitudes are ca 06 A O atoms bridging pairs of tetrahedra in cis and trans configurations show different positional modulation patterns The anisotropic displacement parameters (ADPs) are also modulated A correlation between ADP modulations and positional modulations was found

Effect of impurities on cathodoluminescence of tridymite and cristobalite

Abstract: Cristobalite and tridymite in andesite and obsidian exhibit bright blue emissions in color cathodoluminescence (CL) images. CL spectra of cristobalite and tridymite show emission bands at 400 and 430 nm, respectively. Their CL intensity decreases with an increase in the electron irradiation time, similar to the short-lived blue luminescence reported in hydrothermal quartz. The reduction rate of the CL intensity of hydrothermal quartz is small as compared to cristobalite and tridymite. The initial CL intensity of both minerals immediately after the electron irradiation is positively correlated with the Al2O3 content. This suggests that the blue emissions of cristobalite and tridymite can be attributed to the [AlO4/M+]0 defect center (M: H+, Li+, Na+, and K+). The electron irradiation of cristobalite and tridymite diffuses the monovalent cations of H+, Li+, and Na+ and destroys their crystal structure, resulting in short-lived blue luminescence. The CL intensity after 600 s of electron irradiation depends on the K2O impurity content. In contrast to other monovalent cations, the K+ cation is not diffused because of its large ionic radius. The CL emissions of cristobalite and tridymite after 600 s of electron irradiation can be attributed to the [AlO4/M+]0 defect (M: K+). CL images of cristobalite in obsidian from Utah, USA, show a heterogeneous distribution of CL intensity, with oscillatory CL zoning on the rim and radial lath-shaped textures with bright CL superimposed on weak luminescence background. This implies that lath-shaped textures with bright emissions might have crystallized from rhyolitic magma at temperatures higher than the recrystallization temperature minerals in the other areas.

Einfluss korrosiver Bedingungen auf die Eigenschaften feuerfester Werkstoffe in Anlagen der thermischen Abfallverwertung

Abstract: The aim of this work was the evaluation of corrosion mechanisms of different refractories in operation of waste incineration plants. A testing method in steam atmosphere according ASTM was used to determine the oxidation resistance. Four different silicon carbide materials with various amounts of α-Si 3 N 4 , β-Si 3 N 4 and Si 2 N 2 O within their bonding phase composition were exposed to a constant steam flux at 1000 °C for time range between 100 and 500 h. The experiments at 1000 °C proved an excellent stability in shape dimensions of the refractories and low volume change below 1 %. Formation of silica phases such as cristobalite and tridymite could be observed for some samples after the oxidation tests. Changes of the bonding phase microstructure are discussed in relation to the oxidation resistance, their original chemical composition and resulting properties of the refractory materials. Furthermore vapour phase corrosion experiments with SiC mortars and evaluation of slag corrosion tests of high alumina bricks after use in a waste incineration plant have been performed. For the vapour phase corrosion the reaction mechanisms of evaporation and condensation of salt compounds are discussed. The melt corrosion with the slag leads to phase formation of anorthite and hedenbergite. Precipitated alkaline oxides lead to a drastic lowering of the refractoriness. The influence on property changes such as thermal expansion and microstructural changes are explained.

Silica refractory material with high tridymite content and preparation method thereof

Abstract: The invention relates to a silica refractory material with high tridymite content and a preparation method thereof, and belongs to the field of refractory materials The silica refractory material comprises the following raw materials in percentage by weight: 55 to 74 percent of crystal silica particles with the grain diameter less than or equal to 226 mm, 20 to 30 percent of mixed fine powder which has the grain diameter less than or equal to 0074 mm and is prepared from crystal silica and waste brick, 2 to 3 percent of mineralizing agent calcium oxide, 1 to 2 percent of mineralizing agent ferrous oxide, 2 to 8 percent of additive silicon nitride and 1 to 2 percent of additive silicon carbide The preparation method comprises the following steps: performing dry mix on the raw materials for one minute, and adding concentrated pulp waste liquor into the mixture to be mixed for 8 minutes, wherein the molding pressure of a blank is more than or equal to 100 MPa; and drying the blank for 10 hours at the temperature of 60 to 100 DEG C, performing hard fire on the blank at the temperature of 1,400 to 1,430 DEG C, and keeping the temperature for 5 to 7 hours The tridymite content and the compactness of the silica refractory material are higher than those of a control sample, and the invention can provide a novel silica refractory material for metallurgical coke and the coal chemical industry

Effect of TiO2 on the Thermal Conductivity of Eco-Friendly Silica Bricks Fabricated by Yellow River Silt

Abstract: Silt from the Yellow River was mixed with a certain amount of SiO2 powder to fabricate eco-friendly silica bricks and different amounts of TiO2 were added to ameliorate the thermal conductivity of the bricks. Results showed that the phases of as-fabricated silica bricks are cristobalite and tridymite with cristobalite predominating. With increasing amounts of TiO2, the bulk density and thermal conductivity of the samples increase. Although the bulk density (1.65~1.76 g/cm3 ) than the ideal density 1.85 g/cm3, the thermal conductivity is significantly ameliorated. The thermal conductivity of the sample with 1.5% TiO2 addition is 2.11 W/m.°C, which is nearly three times higher than that of commercial silica bricks (0.8 W/m.°C).

Subsolidus Phase Relations in the System MgO–SiO2–Cr–O and Thermodynamic Properties of Related Cr2+–Containing End–members

Abstract: Subsolidus phase relations have been determined in the systems SiO2–Cr–O and MgO–SiO2–Cr–O in equilibrium with metallic Cr, at 1100 to 1500°C and 0 to 2.88 GPa. The results show that there are no ternary phases in the SiO2–Cr–O system at these conditions, i.e., only the assemblage eskolaite–Cr–metal–quartz (or tridymite) is found. In the MgO–containing system, however, extensive substitution of Cr2+ for Mg is observed in (Mg, Cr2+)2SiO4 olivine, (Mg, Cr2+)2Si2O6 pyroxene, and (Mg, Cr2+)Cr2O4 spinel. Cr3+ levels in olivine and pyroxene are below detection limits. The pyroxene is orthohombic at XPxCr2+ 0.2. The experimental data have been fitted to a thermodynamic model, and the authors obtained the mixing parameter (W) of Mg-Cr2+ in olivine, pyroxene and spinel, and the relation between temperatures and free energies of formation for the end–members: Cr2+–olivine (Cr2SiO4), Cr2+–pyroxene (Cr2Si2O6) and Cr2+–spinel (Cr3O4).