Showing papers on "Tridymite published in 2004"
TL;DR: It was concluded that higher potassium levels were responsible for this difference in the crystalline silica content in these RHA samples, as the combustion of rice husk was considered to cover the demands for energy and silica resource in Asian countries.
Abstract: The quantities of two forms of crystalline silica, tridymite and cristobalite, in heated rice husk ash (RHA) samples were determined by X-ray diffraction (XRD) and chemical methods. Two RHA samples, containing 93% SiO2 and 2-3% K2O, were prepared from charcoaled rice husk products and heated to above 900 degrees C. The crystalline silica made up over 60-80% of the total silica in the heated RHA samples based on the XRD analysis. The crystalline phases in the two samples were somewhat different: The sample heated in the temperature range of 900 to 1,200 degrees C contained 52-62% cristobalite and 10-17% tridymite, but the other sample heated at a comparable temperature, above 1,100 degrees C, contained 46-66% tridymite and 37-16% cristobalite. Based on a correlation of lower tridymite crystallization temperature with higher potassium content, it was concluded that higher potassium levels were responsible for this difference. The pyrophosphoric acid analysis did not give exact results in the evaluation of total crystalline silica content in these RHA samples. As the combustion of rice husk was considered to cover the demands for energy and silica resource in Asian countries, cristobalite and tridymite crystallized in RHA by burning of rice husk should be assessed precisely by XRD analysis and the airborne dust in relevant workplace be controlled.
138 citations
TL;DR: This paper examined 39 samples of siliceous sinter from the low-temperature (35°C) palisade microfacies at Orakei Korako and Te Kopia geothermal fields, New Zealand, in three different postdepositional environments (least-disturbed, heat-affected, weathered).
Abstract: The opal-A to opal-CT silica phase transformation in New Zealand's siliceous hot-spring deposits (sinter) is accompanied by recurring textural changes at the micron scale. Recognition of these changes is essential to extract paleohydrological, paleoenvironmental, and paleobiological signatures from ancient hydrothermal systems, and to recognize diagenetic overprints upon primary depositional signals. We examined 39 samples of siliceous sinter from the low-temperature (< 35° C) palisade microfacies at Orakei Korako and Te Kopia geothermal fields, Taupo Volcanic Zone, New Zealand, in three different postdepositional environments (least-disturbed, heat-affected, weathered). Friable to indurated samples were collected from modern, presently silicifying microbial mats in the outflow of nearly neutral alkali chloride springs, to sinter 3,500 years old. The progressive mineralogical change from noncrystalline opal-A to paracrystalline opal-CT was traced by X-ray powder diffraction and scanning electron microscopy. One diagenetic sequence with two morphological pathways, spherical and smooth (polymeric and monomeric deposition, respectively), was identified at the micron scale. Both pathways ultimately lead to formation of typical, opal-CT bladed lepispheres. Along the spherical pathway, initial morphological restructuring includes formation of circular holes (< 0.1 to 1.0 µm in diameter) in opal-A spheres (< 3.0 µm in diameter). These changes correlate with a shift in the maximum intensity of the opal-A X-ray scattering broadband, from 4.0 A to 4.09 A, the characteristic position for opal-CT. An early transitional diagenetic stage is indicated in both sphere and smooth silica pathways by development of hexagonal platelets and a composite opal-A/-CT scattering broadband that is sharp-peaked in comparison, and centered at 4.09 A. A still sharper-peaked, composite opal-CT/-A X-ray diffraction band follows, which has developed an incipient tridymite shoulder; it is found in samples that display both hexagonal platelets and incipient "fuzzy" opal-CT lepispheres. At this late transitional stage, patchy replacement textures are visible at the macroscale. Only with the appearance of numerous, well-developed, opal-CT bladed lepispheres is a typical, sharp-peaked (4.09 A), opal-CT X-ray trace evident, with a well-defined tridymite shoulder. This diagenetic process can produce either porous or vitreous fabrics, depending on the degree to which infill of silica has reduced porosity and increased density of the deposit. This study provides a textural-mineralogical context for understanding silicification of microbial communities in geothermal settings and their subsequent diagenetic modifications. These modifications continue long after geothermal activity has ceased, with little or no burial, and occur in horizons or localized patches of a given deposit. Moreover, postdepositional conditions, such as heat overprinting or weathering, influence the diagenesis of siliceous sinter by increasing its maturation rate.
103 citations
TL;DR: In this paper, the electrochemical properties of nanoparticle crystalline iron phosphates were characterized with a voltage window of 2.4-0 V. The iron phosphate anodes exhibited the highest reported capacity, while the cathode LiFePO4 has an ideal capacity of 170mAh∕g.
Abstract: Nanoparticle crystalline iron phosphates (FePO4∙2H2O and FePO4) were synthesized using a (CTAB) surfactant as an anode material for Li rechargeable batteries. The electrochemical properties of the nanoparticle iron phosphates were characterized with a voltage window of 2.4–0 V. A variscite orthorhombic FePO4∙2H2O showed a large initial charge capacity of 609mAh∕g. On the other hand, a tridymite triclinic FePO4 exhibited excellent cyclability: the capacity retention up to 30 cycles was ∼80%, from 485 to 375mAh∕g. The iron phosphate anodes exhibited the highest reported capacity, while the cathode LiFePO4 has an ideal capacity of 170mAh∕g.
94 citations
TL;DR: The Shergotty meteorite as discussed by the authors contains three dense silica polymorphs in distinct petrographic settings: (1) two post-stishovite SiO 2 polymorphs, in individual multiphase grains coexisting with glass with nearly labradorite composition, and (2) large individual stishovites in shock-melt pockets which also contain the new CAS phase (Calcium-aluminosilicate; CaAl 4 Si 2 O 11 ).
37 citations
TL;DR: In this paper, a series of SAPO-11 catalyst samples were synthesized by varying source of silica and synthesis time, and products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption and FTIR spectroscopy of pyridine adaption as well as evaluated towards 1-butene skeletal isomerization.
Abstract: A series of SAPO-11 catalyst samples were synthesized by varying source of silica and synthesis time. Products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption and FTIR spectroscopy of pyridine adsorption as well as evaluated towards 1-butene skeletal isomerization. By using waterglass as a source of silica, tridymite phase instead of SAPO-11 was obtained. The combination of colloidal silica and aluminum oxide as sources of silica and alumina resulted in SAPO-11 phase but small amounts of other phases, such as tridymite, were obtained. The number of the acid sites was observed to increase if synthesis time was prolonged indicating more silicon incorporation into the framework. Acidity and phase distribution had an influence on the conversion of 1-butene and selectivity to isobutene. The selectivity to isobutene was higher with catalysts, which contained more SAPO-11 phase.
32 citations
TL;DR: In this article, a mass-dependent fractionation of 18O/16O between tridymite and bronzite with apparent oxygen isotopic heterogeneity was measured with an in situ UV laser microprobe technique.
21 citations
TL;DR: In this paper, a series of nominal compositions Al1−xFexPO4 with 0≤x≤0.8 calcined at various temperatures have been synthesized by the co-precipitation method.
19 citations
TL;DR: In this paper, the structure and morphology of pure AlPO-5s and nickel containing AlPO5s were affected by the preparation conditions such as pH value, nickel incorporated method (hydrothermal method or traditional incipient wetness impregnation).
Abstract: The structure and morphology of both pure AlPO-5s and nickel containing AlPO-5s were affected by the preparation conditions such as pH value, nickel incorporated method (hydrothermal method or traditional incipient wetness impregnation). The dense tridymite phase of AlPO-5 can be formed by two controlling factors. One is controlling the pH at lower value to the AlO4 and PO4 moleculars to form AlPO4 with dense phase, and the other is used the excess nickel adding to increase the crystallinity of AlPO-5 to form dense tridymite phase. The reactivity of PEG-400 amination was also affected preparation methods and conditions. At higher nickel loading, the nickel supported on AlPO-5 prepared by traditional incipient wetness impregnation forms the surface nickel species, and both the surface nickel species and nickel incorporated species form by the hydrothermal method. The amination catalysis indicates that the reactivity of PEG-400 prepared by incipient wetness impregnation method was higher than that prepared by hydrothermal method. The formation of the dense tridymite phase of AlPO-5 can be further determined by the X-ray powder diffraction (XRD). The nickel containing AlPO-5s prepared with lower pH value possesses only the tridymite phase and exhibit higher catalytic reactivity. It appears that the trydimite structure can improve the catalytic activity for PEG-400 amination due to the grain size of the formed crystal of NiAlPO-5 is smaller than the catalyst without the formation of the trydimite phase.
8 citations
TL;DR: In this paper, it was shown that the stuffed tridymite-like oxides show a dependence of the expansion of their tetrahedral framework on their directedness: the larger the directedness, the lower the expansion.
Abstract: The new oxide K0.75Ba0.25Fe1.25Ge0.75O4 has been prepared at 1000 °C in air. This germanoferrite exhibits the real tridymite type structure—hexagonal cell, a
= 5.38 A, c
= 8.92 A—which is fully stuffed by K+ and Ba2+ cations. A combined X-ray and neutron powder diffraction Rietveld analysis gives evidence for a large off-centering of the bridging oxygen atoms along the c axis. This is likely to be connected with the absence of long range ordering of the tetrahedral and stuffed cations, as well. An analysis of the directional sequences of the hexagonal rings in the parent oxides KFeGeO4 and BaFe2O4 allows an understanding of why the real tridymite type is retained for a composition much closer to KFeGeO4 than BaFe2O4. More generally, it is proved that the stuffed tridymite-like oxides show a dependence of the expansion of their tetrahedral framework on their directedness: the larger the directedness, the lower the expansion of the structure.
5 citations
Journal Article•
TL;DR: In this article, a suitable curve of temperature rise is determined on the basis of crystal structure inves- tigation at different temperature ranges, based on the crystal structure of fine silicon powder.
3 citations
01 Jan 2004
TL;DR: In this paper, Fenner et al. determined the triple point α-quartz + high tridymite + β-cristobalite at about 1460 K and 0.15 GPa.
Abstract: Several polymorphs of silica are known and their phase relations are reviewed by a number of authors (see [79WEA/CHI], [84TAY], [86LIUBAS], [86KUS/FAB], [94SWA/SAX]). The principal polymorphs stable at pressures below 3 GPa are quartz, tridymite and cristobalite; each of them have a low-temperature (α) and a high-temperature (β) form. The α = β-quartz transformation occurs at temperature of 846.5 ± 1.5 K at atmospheric pressure (see [49TUT1), β-quartz transforms to tridymite at temperature of 1140 K (see [13FEN]). Cristobalite is stable from 1743 K (see [13FEN]) up the to melting point 1999± 5 K (see [82RICBOT]). Several modifications of low-temperature tridymite are known (see [13FEN], [84TAY1). According to the available experimental phase equilibrium data of Fenner [13FEN], Kennedy et al. [62KEN/WAS], Ostrovsky [6605T1, Grattan-Bellew [78GRA] the triple point of (β-quartz + high-tridymite + β-cristobalite is placed at about 1460 K and 0.15 GPa. Tridymite is not stable above 0.15 GPa, cristobalite is stable up to 0.6 GPa according to data of Jackson [76JAC]. Quartz polymorphs are stable at pressures up to 3.5 ± 1 GPa (see [80MIR/MAS], [90KAN]). The P-T boundary of α = β-quartz transformation was studied by Cohen and Klement ([67COH/KLE]), they determined the triple point α-quartz + β-quartz + coesite at 3.75 GPa and 1673 K.