Showing papers on "Tridymite published in 1996"

Rigid-unit phonon modes and structural phase transitions in framework silicates

Abstract: The rigid-unit mode model provides many new insights into the stability and physical properties of framework silicates. In this model the SiOo and AlO4 tetrahedra are treated as very stifl to a first approximation as completely rigid, in comparison with intertetrahedral forces. In this paper we apply the model to several important examples. The model is reviewed by a detailed study of quarlz, and it is shown that the a-B phase transition involves a rigid-unit mode that preserves the Si-O-Si bond angle. The model is used to explain the phase transitions in cristobalite and the diferent feldspar, sodalite, and leucite structures. We also use the model to explain the nature of the high-temperature disordered phases of cristobalite and tridymite, to interpret the observations of streaks of diffuse scattering in electron diffraction patterns, to interpret the structures in the kalsilite-nepheline solid solution, to explain volume anomalies in the cubic leucite structures, and to explain qualitatively the negative linear thermal expansion in cordierite. The results for the highest symmetry sodalite structure show that there is a rigid-unit mode at every wave vector, a finding with significant implications for the understanding of the sorption and catalytic behavior of zeolites.

TEM and X-ray diffraction evidence for cristobalite and tridymite stacking sequences in opal

Abstract: In an attempt to resolve the structure of opal-CT and opal-C more precisely, 24 opal samples from bentonites, Fuller's Earths, zeolite tufts, biogenic silicas and silicified kaolins have been analyzed by high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). Results of this examination demonstrate that opal-C and opal-CT are part of a continuous series of intergrowths between end-member cristobalite and tridymite stacking sequences. These findings are consistent with Fl6rke's (1955) interpretation of the most intense opal peak at -4 ,~ as a combination of the (101) cristobalite and (404) tridymite peaks. The position and width of this peak are controlled by the relative volume of the two stacking types and the mean crystallite size. Direct evidence obtained by HRTEM provides data showing various stacking sequences in opals. Broadening due to crystallite size alone was determined by directly measuring crystallite size by TEM and comparing the measured size to the apparent size calculated using the Scherrer equation. XRD peak broadening is also described in terms of various contributions from structural disorder. The mean opal crystallite size ranges from 120 to 320 A.. For samples at either end of the size range, the crystallite size plays a larger role, relative to stacking disorder, in controlling peak broadening.

Structurally relaxed models of the Si(001)-SiO2 interface

Abstract: We present a first‐principles investigation of the structural properties of two models for the Si(001)–SiO2 interface The models derive from attaching tridymite, a crystalline form of SiO2, to Si(001), and then allowing for full relaxation These models do not show electronic states in the silicon gap, as required by electrical experiments They contain the three intermediate oxidation states of silicon, consistent with photoemission experiments We study bond length and bond angle distributions and measures of local strain The strain is localized to a transition region at the interface Strain does not persist in the full oxide

Structure of carbon produced by hydrothermal treatment of β-SiC powder

Abstract: Interaction of a β-SiC powder with supercritical water has been studied in the temperature range 600–800 °C under 100–500 MPa pressure. Examination of the composition of corrosion products by Raman and IR spectroscopy, XRD, SEM and TEM demonstrated that carbon and silica are formed after the hydrothermal treatment of SiC. Their structure varies depending on the experimental conditions. Amorphous and crystalline SiO2(α-quartz, α-cristobalite and traces of tridymite) were found. Along with graphite and amorphous sp2 carbon, disordered or poorly crystallised sp3 carbon (diamond) was present in the reaction products. The formation of sp2vs. sp3 carbon is discussed.

Thermochemistry of the new silica polymorph moganite

Abstract: Samples of microcrystalline silica varieties containing variable amounts of the new silica polymorph moganite (up to R~82 wt.%) have been studied by a combination of high temperature solution calorimetry using lead borate (2 PbO · B2O3) solvent and transposed temperature drop calorimetry near 977 K, in order to investigate the thermochemical stability of this new silica mineral. The enthalpy of solution at 977 K and the heat content (H977 — H298) of “pure” moganite phase were estimated to be -7.16 ± 0.35 kJ/mol and 43.62 ± 0.50 kJ/mol, respectively. The standard molar enthalpy of formation is-907.3 ± 1.2 kJ/mol. Thus, calorimetry strongly supports results of previous X-ray and Raman spectroscopic studies that moganite is a distinct silica polymorph. Its thermochemical instability relative to quartz at 298 K of 3.4 ± 0.7 kJ/mol is marginally higher than those of cristobalite and tridymite. Structurally, this instability may be related to the presence of distorted 4-membered rings of SiO4 tetrahedra, which are not found in the quartz structure. The metastability relative to quartz may also explain the apparent scarcity of moganite in altered rocks and in rocks that are older than 130 my.

Rigid unit modes in the high-temperature phase of SiO2 tridymite: calculations and electron diffraction

Abstract: Calculations of the rigid unit mode (RUM) spectrum of the high-temperature phase of SiO2 tridymite are used to explain the patterns of diffuse scattering seen in transmission electron microscopy experiments. These results show that RUM's can occur with wave vectors on curved surfaces in reciprocal space rather than being confined to symmetry points, lines or planes. The fact that the calculations reproduce the detail seen in the diffuse scattering provides a striking nontrivial confirmation of the validity of the rigid unit mode model.

In situ 29 Si MAS NMR studies of structural phase transitions of tridymite

Abstract: Abstract The structural phase transitions of monoclinic tridymite that occur at elevated temperatures, as well as the structure of the high-temperature modifications, have been studied by 29SiMAS NMR. The sample, originating from the surface of a refractory silica brick, displays sharp discontinuities in the first moment (M1), second moment (M2), or both, of the 29Si MAS NMR spectrum at the following phase transitions: 108 ± 1 °C for the monoclinic (MC) to orthorhombic (OP) transition, 165 ± 5°C for the transition from OP to a second orthorhombic phase (OS), and 206 ± 4 °C for the transition from OS to a third orthorhombic phase (OC). M1 for the OC phase becomes more negative with increasing temperature up to ~323°C, at which there is also a small discontinuity in M2, indicating a transition from OC to a hexagonal phase (LHP). The spectrum of the MC phase consists of 12 narrow lines of equal area, consistent with the known crystal structure. The spectrum of the OP phase is broad and asymmetric, indicating a distribution oflocal environments at the Si sites. It can be simulated assuming an incommensurate structural modulation with a maximum amplitude of ~5 × 10-2 Å, but it is also consistent with a recent domain-formation model, provided that most disorder occurs at the Si6 site. The spectrum of the OS phase is asymmetric, indicating a nonlinear incommensurate modulation with a maximum amplitude of ~2 × 10-2 Å. The spectra of higher temperature phases are consistent with the single Si site of the known crystal structures, but M1 values imply significantly smaller mean Si-O-Si angles ‹α›, e.g., ‹α› = 152.6° at 220°C from NMR, compared with 168.0° from X-ray diffraction data. This difference is consistent with disorder of O atoms around Si-Si axes.

Irradiation-induced amorphization of AlPO4

Abstract: AlPO 4 , in the mineral form berlinite , is isostructural with α-quartz. We have investigated the irradiation-induced amorphization of hydrothermally-grown berlinite and found that — like quartz and other silicas but unlike most other phosphates — it undergoes solid-state radiolyis, with an efficiency fifty times that of quartz at room temperature, and amorphizes at an absorbed ionization dose of about 1 GGy. High-resolution TEM revealed that — unlike quartz in which small amorphous inclusions nucleate — electron-irradiated AlPO 4 proceeds uniformly to an aperiodic state, much as do cristobalite and tridymite, and 20 times faster. It was found also to amorphize under 1.5 MeV Kr + ion irradiation at a collisional energy density (10 eV/atom) similar to that for quartz and in keeping with the degree of structural freedom afforded by its tetrahedral network structure. The critical ion fluence for amorphization was found to increase by a factor of 5 between 300 and 600 K. Radial distribution functions derived from energy-filtered electron diffraction patterns from regions amorphized by electrons resemble those of electron-amorphized quartz with some additional features.

Fibrogenic and Other Biological Effects of Silica

Abstract: Crystalline silica, silicon dioxide (SiO2), has a simple chemical structure but potent biological effects. The structures of different crystalline and amorphous forms of silica and related molecules are reviewed by Heaney and Banfield (1993) and their biological effects are reviewed by Guthrie (1992). The commonest form is quartz, although other tetrahedral crystalline forms of silica (tridymite, crystobalite) have comparable effects. In contrast, stishovite is an octahedral form of crystalline silica, produced under conditions of high temperature and pressure. It is biologically inert, like titanium dioxide (rutile). Silica dust exposure during the mining and smelting of metal ores has long been known to produce pulmonary fibrosis (silicosis or phthisis) and to increase the incidence of tuberculosis (Collis, 1919). In some countries, such as the People’s Republic of China, large numbers of occupationally exposed workers are showing progressive impairment of lung function. It is desirable to clarify the underlying pathogenesis and, if possible, control it.

Clathrate hydrates as layer structures : comparison between the structures of type II and hypothetical type V

Abstract: Crystallographic data for cubic Type II clathrate hydrate have been used to derive a hypothetical structure for Type V clathrate hydrate. The structures of Types II and V are related by different stacking arrangements of layers of six-membered rings of pentagonal dodecahedra, which make up the water framework. The structure of Type II has an... ABCABC... stacking sequence, whereas the structure of Type V has an... ABAB... arrangement. Hypothetical Type V is hexagonal, P6 3 mmc, with a = 12.19A and c = 19.91A. The general stoichiometry for Type V, assuming fully occupied cages, is 4X.8Y.68H 2 O(X:H 2 O = 1:17). Theoretical oxygen and guest molecule positions are given for the structure of the hypothetical Type V hydrate. The relationship between the structures of Types II and V are analogous to that of the silica polymorphs tridymite and cristobalite, and the layered clathrasils. The stability of Type V and the possibility of stacking disorder between Type II and V structures, as is found for the silica polymorphs, is discussed.

The Effect of Dealumination on the Structure and Acidity of SAPO-11 Molecular Sieve

Abstract: SAPO-11 was synthesized hydrothermally and dealuminated with EDTA. The framework structure of SAPO-11 was maintained safely by 24 hours' dealumination, but further dealumination for 48 hours caused SAPO-11 collapsed and to be changed to variscite() and tridymite(). Dealuminated SAPO-11 showed two structural hydroxyl bands at 3607 and 3453 respectively. The intensities of these two bands increased according to the extent of dealumination, and disappeared by the adsorption of methylamine. Dealuminated SAPO-11 showed higher desorption temperatures and greater activation energies in desorption of water and methylamine compared to non-dealuminated SAPO-11. All the phenomena may be due to the stronger interactions of Bronsted acid sites of structural hydroxyl groups generated by dealumination with adsorbed water and methylamine molecules respectively.

Phase Relations in the System Ca3(PO4)2-MgO-SiO2

Abstract: A phase diagram in the three-component system C3P-M-S was constructed. Abbreviations for oxide formulae are used as follows: C=CaO, M=MgO, P=P2O5 and S=SiO2. Primary crystals are C3P (α′, α, β), diopside and pigeonite on the C3P-MS join, and C3P, sc (magnesium silicocarnotite, C3M2PS) and forsterite on the C3P-M2S join. Periclase appeared on the MgO corner, and tridymite, cristobalite and two liquids region appeared on the SiO2 corner. A eutectic with the lowest temperature 1245°C is located near the center portion where three crystals of β-C3P, diopside and tridymite coprecipitate. The sc compound appeared in a spindle-shaped region and disappeared below 1360°C to form β-C3P and forsterite. It is concluded that the system C3P-M-S consists of 4 composition triangles of C3P-M-sc, sc-M-M2S, C3P-M2S-MS and C3P-MS-S containing 7 invariant points and this system is pseudo-binary.

Characterisation of Siliceous Earth Materials from Rajasthan and Tamil Nadu

Abstract: Silica has various polymorphic forms such as quartz, cristobalite, tridymite, hydrated or opaline silica etc. One of the most fundamental aspects in the characterisation of any raw material is the identification of the crystalline minerals present in the same. When X-ray diffraction is used for the identification of silica minerals, it is difficult to distinguish between α-cristobalite and certain type of opaline silica. Two siliceous earth samples, one from Rajasthan and the other from Tamil Nadu have been characterised by studying their chemical properties, XRD and IR spectral analyses, behaviour towards thermal treatment and chemical reactivity. Both samples were found to be opaline silica and not α-cistobalite.

AIPO4‐containing glass‐crystalline materials in the system Li2O‐Al2O3‐P2O5

Abstract: Publication concerns the crystallization of glasses in the system Li 2 O-Al 2 O 3 -P 2 O 5 . The aim of this study was to determine the area of the occurrence of glasses in the system Li 2 O-Al 2 O 3 -P 2 O 5 to examine their tendency for crystallization of AlPO 4 phase and others in the process of thermal treatment. Applying the method X-ray diffraction the crystallization mechanism of AlPO 4 (cristobalite and tridymite form) and other accompanying phases, and the influence of the addition of small amount of SiO 2 on the crystallization of these glasses was examined. The structure of glasses and glass-crystalline materials obtained were defined by means of infrared spectra. AlPO 4 glass-crystalline materials have statisfying resistant properties to use them as coats on steel.