Search results for "Cristobalite"
showing 6 items of 16 documents
Luminescence of ODC(II) in quartz and cristobalite glasses
2022
Abstract The results of the optical spectroscopy of twofold coordinated silicon centers – ODC(II) in quartz and cristobalite glasses are presented. The luminescence and excitation spectra attributed to different local symmetry of ODC(II) were investigated under synchrotron excitation in the VUV region. The observed differences in the luminescence and excitation spectra of ODC(II) are caused by the environment and, therefore, short-range order in the samples.
Origin of SiO2-rich components in ordinary chondrites
2006
Abstract Silica-rich objects are common minor components in ordinary chondrites (OC), occurring as fragments and as chondrules. Their typical paragenesis is orthopyroxene + SiO 2 (with bulk SiO 2 >65 wt%) and occasionally with additional olivine and/or spinel. Individual silica-rich components (SRC) have previously been studied in various types of OCs, although there is only one comprehensive study of these objects by Brigham et al. [Brigham, C.A., Murrell, M.T., Yabuki, H., Ouyang, Z., El Goresy, A., 1986. Silica-bearing chondrules and clasts in ordinary chondrites. Geochim. Cosmochim. Acta 50, 1655–1666]. Several different explanations of how SRCs formed have been published. The main ques…
The stoichiometry of blue vanadium doped zircon obtained by sol-gel methods
1992
Abstract In this study, Sol-Gel processes in aqueous solution were used to investigate the stoichiometry of the V/1bZrSiO 4 solid solutions achieved by this method. Compositions with a formal stoichiometric defect in: (a) ZrO 2 ([ZrO 2 ] 1−2x [SiO 2 ] [V 2 O 5 ] x ), and (b) in SiO 2 ([ZrO 2 ] [SiO 2 ] 1−2x [V 2 O 5 ] x ), x=0.02, 0.04, 0.05 and 0.10 have been prepared. X Ray Diffraction analyses in fired samples (1200°/3 days) showed that cristobalite and monoclinic zirconia appear as residual crystalline phase in (a) and (b) compositions respectively, except in sample (a) with x=0.02 in which no residual phase was detected and therefore is considered stoichiometric. This sample and sample…
Influence of Sodium on the Structure and HDS Activity of Co–Mo Catalysts Supported ON Silica and Aluminosilicate
2000
Abstract Structural changes and catalytic performances of hydrodesulfurization (HDS) Co–Mo catalysts, supported on amorphous aluminosilicate (ASA) and amorphous SiO 2 were investigated as a function of the amount of sodium ions added to the supports. The catalysts were prepared according to the incipient wet impregnation method using (NH 4 ) 6 Mo 7 O 24 as molybdenum precursor and CoCl 2 or Co(NO 3 ) 2 as cobalt precursor. Structural and morphological characterisations of the materials were performed with X-ray diffraction (XRD) and surface area measurements (BET). In the case of the ASA-supported catalysts, increasing the amount of sodium resulted in a gradual decrease of the catalyst surf…
Influence of the preparation method on the thiophene HDS activity of silica supported CoMo catalysts
2002
Abstract The effect of the preparation method of CoMo/SiO2 catalysts with and without sodium ions, on the hydrodesulfurization (HDS) of thiophene was investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and N2 physisorption (BET). The samples were prepared by total sol–gel route, by classic wetness impregnation and by co-impregnation, in the presence of nitrilotriacetic acid (NTA), of commercial and sol–gel prepared silica. The influence of sodium ions added to the sol mixture and to the silica before the impregnation with Co and Mo salt solution, was also considered. The presence of sodium favoured the phase transition of amorphous silica to cristobalite with con…
Volcanic SiO2-cristobalite: A natural product of chemical vapor deposition
2020
Abstract Cristobalite is a low-pressure, high-temperature SiO2 polymorph that occurs as a metastable phase in many geologic settings, including as crystals deposited from vapor within the pores of volcanic rocks. Such vapor-phase cristobalite (VPC) has been inferred to result from silica redistribution by acidic volcanic gases but a precise mechanism for its formation has not been established. We address this by investigating the composition and structure of VPC deposited on plagioclase substrates within a rhyolite lava flow, at the micrometer to nanometer scale. The VPC contains impurities of the form [AlO4/Na+]0—coupled substitution of Al3+ charge-balanced by interstitial Na+—which are ty…