Showing papers on "Tridymite published in 1987"

Vibrational interactions of tetrahedra in silicate glasses and crystals

Abstract: Normal coordinate calculations, producing synthetic infrared and Raman spectra, were carried out on melilites, pyroxenes, silica polymorphs and feldspars. Atomic motions are complex in the high-frequency Raman modes of melilites and aluminous pyroxenes. The symmetric T-Onb stretching vibrations of Si and Al tetrahedra with different numbers of bridging oxygens are separate from each other, but may combine individually with oscillation of bridging oxygens between Si and Al tetrahedra. The latter type of vibration tends to dominate as Al/Si increases. The frequencies of these vibrational components and the degree of such intermixing depend on T-O force constants, which vary greatly depending on local bonding configurations; individual bands in the high-frequency Raman cannot in general be assigned to single structural entities or fixed combinations thereof. Calculations confirm that in some Al-Si glasses such as jadeite and spodumene, i.e. those in which all Al can be tetrahedral without non-bridging oxygens, Al-O-Al linkages or linkage of more than two tetrahedra by a single oxygen, aluminum is predominantly in tetrahedral coordination. Other Al-Si glasses which are richer in aluminum or which have non-bridging oxygens may contain Al tetrahedral triclusters, non-tetrahedral Al, or both. On the basis of distinctive 450–750 cm−1 infrared bands, both silica and feldspar glasses resemble tridymite and related stuffed derivatives, not other crystalline silica polymorphs or feldspars. Either these glasses have a structure like that of tridymite on a local scale, or the disorder of the glasses causes drastic modification to the vibrations in question.

Structural defects in microcrystalline silica

Abstract: The structure of the microcrystalline silica varieties chalcedony, flint, moganite, opal-C and -CT is characterized by X-ray powder diffractometry and transmission electron microscopy (TEM). The role of impurities is investigated by infrared spectroscopy and chemical analysis. Microcrystalline opal, chalcedony and flint have a disordered intergrowth structure composed of cristobalite and tridymite domains in opal, and quartz and moganite domains in chalcedony and flint. Each constituent phase has different cell dimensions and symmetry. The main impurity is water which is enriched at the intergrowth interfaces. Density and refractive indices of microcrystalline silica depend on the water content.

Evaluation of Occupational Exposure to Mixed Dusts and Polycyclic Aromatic Hydrocarbons in Silicon Carbide Plants

Abstract: Workers in the silicon carbide industry have experienced occupational health diseases, particularly lung disorders such as silicosis The silicon carbide production process mainly employs petroleum coke, sawdust, pure crystalline silica and graphite Since crystalline silica is present in the occupational environment, the airborne dust content of various polymorphs of silica, especially quartz, cristobalite and tridymite, was determined by X-ray diffraction analysis The analytical method was modified to eliminate graphite, since it overlaps with the main diffraction plane of quartz Exploratory field surveys were conducted to identify the minerals present in that occupational environment and to evaluate the validity of the analytical method The surveys provided information on the mineralogical nature of the dust, its respirable content and the concentration of silica polymorphs Polycyclic aromatic hydrocarbons also were measured, and the effect of their adsorption on graphite was evaluated

Transformation of tridymite to cristobalite below 1470° C in silica refractories

Abstract: Because silica refractory has good volume stability and creep properties at high temperature, it has been used in several furnaces. However, silica has three polymorphs (quartz, tridymite and cristobalite) and each polymorph has anα-β type transformation. It is known that cristobalite is the stable phase of silica between 1470° C and the melting point of silica refractories. However, sometimes cristobalite was found in silica refractories used in the stable temperature region of tridymite. Therefore, the cause and mechanism of this tridymite-to-cristobalite transformation below 1470° C was studied. Although the transformation temperature of tridymite to cristobalite was also 1470° C in the sample used in this research, it decreased on addition of Al2O3. The apparent activation energy of the tridymite-to-cristobalite transformation was found to be 787 kJ mol−1 above 1470° C and 176 kJ mol−1 below 1470° C with Al2O3 by measuring the transformation rate. It was also observed using EPIVIA that the tridymite included CaO; however, CaO and Al2O3 were located on the outside of the cristobalite which was produced below 1470°C. Therefore, it is supposed that the liquid phase was produced by the penetration of Al2O3, and impurities in the tridymite crystal diffused outside and then silica was precipitated as cristobalite.

The mineralogy of the Tokomaru silt loam and the occurrence of cristobalite and tridymite in selected North Island soils

Abstract: The origin of cristoba lite has been invest igated in a se lect ion of soils f rom North Island . These include Tokoma ru silt loam in the Manawatu , and Hamilton clay loam, Naike clay and Te Kowhai s i lt loam in the Hamilton basin . The occurrence of cristobal ite , in con j unct ion with a s s ociated sand and silt mineralogy, has been used to interpret both the provenance of soil parent materials and aspects of soil gene s i s . Cristoba lite isolated from the soils is a-cristobalite ( opal-C) and is inva riably accompanied by t ridymite ( t ridymite-M st ructure ) . SEM data suggest that rather than discrete phases , cri stoba l ite and t ridymite usually occur intergrown with each other . Togethe r they const itute up to 15% of the soils examined and usually occur in higher concent rat ions in upper soil hori zons . Oxygen isotope abundances for cristoba lite and tridymite range from 7 . 5 -8 . 4 °/00 and are independent of grain s i ze and relat ive proportions of c ristobalite and t r idymite. These data , together with the highly ordered c ryst al st ructure and subhedral morphologies of grains , indicate that c r istobalite and tridymite formed at high temperature . Both are shown to occur in a number of the rhyolitic tephras f rom the Taupo Volcanic Zone ( e . g . Aokautere and Okareka ashes) and leucocratic lavas from the Egmont cent re . Cristobalite and t ridymite in the soils the refore probably o riginated from volcanic sources . Various forms of amorphous silica were also identified in the soils of the Hamilton basin . In upper soil horizons, microfossils and phyt o l iths dominate but at depth the Naike c lay and Hamilton clay loam cont a in two inorganic forms of amorphous silica . These comprise

The Structure of the (001)Si/SiO2 Interface

Abstract: We show that a careful examination of previous microscopic structural data from the Si/SiO2 interface reveals that the presence of an epitaxial interfacial oxide cannot be ruled out, and describe the conditions necessary for a definitive search for an intervening layer between c-Si and a-SiO2. We present electron diffraction and lattice imaging data, which establish the c-Si→a-SiO2 transition to take place via a crystalline layer ~7 A thick. Modelling of lattice images in two projections indicates the crystalline oxide to be tridymite, a stable, bulk phase of SiO2.

Preparation of Crystalline Aluminum Phosphate by the Reaction Using Boron Phosphate

Abstract: The use of BPO4 in the synthesis of crystalline AlPO4 has been investigated. The best results are obtained if BPO4 and hydrated alumina are heated in molten KCl: at 1050°C the conversion degree of BPO4 into AlPO4 is >90% (in 4 h). The product consists of the tridymite AlPO4 form.

Thermodynamic Properties of Silicalite SiO2.

Abstract: Silicalite, a polymorph of SiO 2 , is a new and interesting molecular-sieve material. Low-temperature (5 to 350 K) heat-capacity, flourine-combustion, high-temperature (350 to 1500 K) drop-calorimetric, and reaction-calorimetric with HF(aq) measurements have been performed on this material. At T ′ = 298.15 K, Δ f H m o ( T ′) = −(905.20±0.84) kJ·mol −1 ; C p , m o ( T ′) = (46.34±0.23) J·K −1 ·mol −1 ; { Δ 0 T ′ S m o ( T ′)} = (46.29±0.23) J·K −1 ·mol −1 ; and Δ f G m o ( T ′) = −(852.23±0.84) kJ·mol −1 . The enthalpies of reaction with 24.4 mass per cent hydrofluoric acid of silicalite at mass concentrations of 0.5, 1, 2, and 4 g·dm −3 were determined to be −(144.91±0.23), −(144.80±0.10), −(145.07±0.12), and −(145.06±0.14) kJ·mol −1 , respectively. The enthalpies of reaction of silicalite with 20.1 and 15.0 mass per cent hydrofluoric acid at a silicalite mass concentration of 1 g·dm −3 were determined to be −(143.85±0.19) and −(142.68±0.08) kJ·mol −1 , respectively. Because silicalite reacts rapidly with HF(aq) at 298.15 K, it has potential as a reference material for thermochemical measurements on inorganic silicates and silicate minerals. Silicalite is less stable than quartz, cristobalite, or tridymite, but more stable than SiO 2 (glass) or the high-pressure forms coesite or stishovite.

Padfor sliding nozzle

Abstract: PURPOSE:To permit easy sliding of a sliding nozzle of a vessel for a molten metal, to stabilize a rate of aperture and to hold the molten metal for a long period of time by using a specific content or above of a siliceous raw material subjected to a heat treatment at a specific temp. or above as pad. CONSTITUTION:Silicic acid has a tricrystal system of quartz, cristobalite and tridymite. The quarts transfers from the alpha structure to the beta structure at 573 deg.C, the cristobalite at 275 deg.C and the tridymite at 163 deg.C, respectively. Particularly the quartz makes large volumetric expansion. The pad makes a less volumetric change at >=300 deg.C even if the temp. of the heat-insulated vessel for the molten metal near the sliding nozzle rises to >=300 deg.C if the siliceous raw material contg. >=70wt% silicic acid is used for the nozzle pad. The sliding of the nozzle is, therefore, made easy and the bridging of the pad is eliminated, by which the rate of aperture is stabilized and the leakage of the molten metal is eliminated. The molten metal is thus held for a long period of time.

Preparation of Crystalline Aluminum Phosphate by the Reaction Using Boron Phosphate.

Abstract: The use of BPO4 in the synthesis of crystalline AlPO4 has been investigated. The best results are obtained if BPO4 and hydrated alumina are heated in molten KCl: at 1050°C the conversion degree of BPO4 into AlPO4 is >90% (in 4 h). The product consists of the tridymite AlPO4 form.