Showing papers on "Tridymite published in 1998"

Opal, cristobalite, and tridymite: Noncrystallinity versus crystallinity, nomenclature of the silica minerals and bibliography

Abstract: Cristobalite and tridymite are distinct forms of crystalline silica which, along with quartz, are encountered in industrial operations and industrial products. Because the International Agency for Research on Cancer has designated “crystalline silica” as an IARC Group 2A (probable carcinogen) and quartz and cristobalite as a Group 1 (carcinogen), it is important to properly identify and quantify the silica phase in all materials used in production and encountered in products. Opal is a form of hydrated silica which is also encountered in industry. Although some forms of opal mimic cristobalite and tridymite, they are not truly crystalline. The term “silica” in the industrial sense is used to mean any material whose composition is SiO2 whether it is crystalline or noncrystalline. Some people also consider silica to include hydrated SiO2. There are many forms of SiO2 which have both long-range and short-range order and are recognized as crystalline phases among which are quartz, cristobalite, and tridymite. The hydrated silicas, on the other hand, pose an enigma. Only a few forms show sufficient long-range and short-range order to be considered crystalline. The mineral silhydrite is an example. Opal in all its forms lacks sufficient order to be considered crystalline. Even opal-C, which produces a X-ray pattern similar to the diffraction pattern of cristobalite, lacks not only sufficient order to be considered crystalline but also contains water in the structural make-up. This paper discusses a classification and nomenclature for these forms which is critical to proper regulation. It also reviews the recent literature on tridymite, cristobalite, and opal, and provides an extensive bibliography. Modern studies have shown that opal-A is disordered, but opal-CT and opal-C contain ordered domains that mimic stacked sequences of cristobalite and tridymite sheets such that X-ray patterns show features similar to the crystalline cristobalite and tridymite. There is debate on whether the ordered regions have lost the water that characterizes the opals. In fact, heating studies have shown that all opals show changes on heating characteristic of materials that lose water in the process. The TEM evidence showing domains in the range 10–30 nm in a matrix of disordered opal suggest that the proper term for this system is paracrystalline analogous to inorganic and organic polymers.

On the Sequence of Phase Transitions in Tridymite

Abstract: We consider the phase transitions in tridymite from the perspective of the rigid unit mode model. The rigid unit modes are the low-frequency phonons of a crystal structure that consists of an infinite framework of tetrahedra linked at corners, that can propagate without the tetrahedra distorting. Because they give distortions of the structure with a low energy cost they are the natural soft modes for displacive phase transitions. We consider the normal phase transition sequence in tridymite on cooling, HP LHP ..., as a successive condensation of rigid unit modes acting as soft modes. Some of the low-temperature phases (e.g. MX-1) arise as rigid unit mode distortions of the high-temperature structure and do not follow the sequence of phases found at higher temperatures. We are able to account for all the commensurate phases and some of the modulated phase within the framework of the rigid unit mode model.

Crystal structure of hexagonal trinepheline; a new synthetic NaAlSiO 4 modification

Abstract: The crystal structure of a synthetic NaAlSiO 4 modification has been solved and refined to an R index of 0.020 for 2745 independent reflections. The compound is hexagonal with space group symmetry P6 1 , a = 9.995(2) Aa and c = 24.797(4) Aa. The crystal showed twinning by merohedry according to m 210 , which was accounted for in the calculations. The phase was named trinepheline, following prior studies, because the length of its c lattice parameter is three times the length of the c parameter in nepheline, whereas the a parameter is about 10 Aa in both phases. The crystal structure is characterized by layers of six-membered tetrahedral rings of exclusively oval conformation. The rings are built up by regularly alternating AlO 4 and SiO 4 tetrahedra. The stacking of the layers parallel to the c axis results in a three-dimensional network containing channels that are occupied by the Na cations. Although structural similarities with respect to tridymite derivatives can be found, hexagonal trinepheline represents a new type of stuffed tridymite that is not a simple superstructure of nepheline.

Surface contribution to the interfacial chemistry of potassium modified oxide catalysts

Abstract: The surface contribution to the interfacial chemistry involved in the preparation of potassium-modified metal oxide catalysts has been revealed by examining industrial 5 wt.% K+ silica–alumina versus silica and previously examined alumina. The investigation was conducted by means of potassium and nitrogen adsorption measurements, insitu IR spectroscopy, X-ray diffractometry and photoelectron spectroscopy, which allowed definition of the adsorptive and thermochemical events occurring during the impregnation, drying and calcination steps of the preparation. The oxide surface chemistry influences the adsorption amounts attained during the impregnation from aqueous K2CO3 solutions: non-specific and specific adsorption on silica (7 K+ nm−2) and only weaker specific adsorption on silica–alumina (1.1 K+ nm−2) related to the low content of very well dispersed silica. On both carriers, the presence of potassium leads to the lower amount and weaker stability of the surface hydroxy groups owing to synergetic effects with absorbed carbonate. During calcination, K+ greatly enhances the crystallization of silica into crystoballite at 973 K and tridymite at 1273 K, which is accompanied by a strong decrease in surface area, a better dispersion of K+ and an increase in surface basicity, by comparison with silica–alumina. No specific effect can be observed on silica–alumina during transformation of the boehmite form into the γ-alumina-type structure; the presence of very well dispersed silica in alumina decreases the CO2 adsorption capacity observed on pure alumina.

Characterization of the high-temperature modifications of incommensurate tridymite L3-T o (MX-1) from 25 to 250 degrees C

Abstract: Incommensurate tridymite L3-T O (MX-1) shows a cascade of five phase transitions at 65, 110, 150, 200, and 380 degrees C upon heating. The X-ray diffraction patterns were investigated with a Buerger precession camera revealing a sequence of four incommensurate phases in the range from room temperature to 200 degrees C. The phases formed between 65 and 110 degrees C and between 110 and 150 degrees C are new modifications of tridymite. At 65 degrees C the monoclinic tridymite L3-T O (MX-1) phase undergoes a first-order transformation to an orthorhombic phase. The incommensurate structural modulation of the room-temperature phase with the wavevector q1 = 0.663 a (super *) -0.498 c (super *) flips to q2 = 0.042 a (super *) -0.388 c (super *) . Simultaneously, a commensurate modulation with tripled b lattice parameter is formed. The wavelengths of both modulations do not depend appreciably on the temperature. The incommensurate modulation discontinuously disappears near 110 degrees C whereas the commensurate modulation along the b axis becomes non-integral with a temperature-dependent wavelength varying between 115 and 100 Aa. At 150 degrees C the symmetry is reduced to monoclinic again with gamma = 90.4 degrees . Between 150 and 200 degrees C the monoclinic angle gradually decreases to 90 degrees and the wavelength of the modulation from about 90 to 65 Aa. At higher temperatures, the satellite reflections fade into weak streaks and the normal orthorhombic high-temperature modification of tridymite is formed. The phase transitions are reversible upon cooling except for the first transformation that is partly irreversible for single crystals and reversible but incomplete for pulverized material at room temperature.

Analysis of the incommensurately modulated OS phase of SiO2 tridymite

Abstract: The structure, symmetry and origin of the incommensurately modulated OS phase of tridymite (SiO2) and its lock-in to the OP three-fold superlattice structure are discussed in a computational study. The structure of the OS phase (which has not been determined experimentally) is deduced as the only geometrically possible structure derived from the parent OC phase by rotation and translation of the SiO4 tetrahedra without significant distortion of these units. It can be visualised conveniently in terms of the McConnell formalism of two component difference structures C1 and C2 whose space group symmetries are derived. The results are in accordance with the known lock-in structure at wave vector Q=\(\)a*. In the latter, the ±C1 regions expand and the structure can square up in a very general way to take advantage of the lost symmetries (lost compared with the OC phase).

Crystal structure determination of the hydrated gallium phosphate GaPO4·2H2O, analog of variscite

Abstract: The crystal structure of the hydrated gallium phosphate, GaPO4·2H2O, analog of variscite, has been determined by means of single-crystal X-ray diffraction characterisation. The structure is built from the connection of PO4 tetrahedra with GaO4(H2O)2 octahedra. The 3D framework consists of the stacking of 63 nets and is related to the topologies of metavariscite, the polymorphs of silica (tridymite, cristobalite) and zeolite Li-A(BW). A formation pathway of the hydrated gallium phosphate with leucophosphite structural type is proposed from that of the variscite one. Crystal data: Mr = 200.72 g mol−1 , orthorhombic, space group Pbca (no. 61), a = 9.9260(1) A, b = 8.6189(1) A, c = 9.7622(1) A, V = 835.17(2) A3, Z = 8, R1(F) = 0.0361, wR2(F2) = 0.0922 for 1255 reflections with I > 2σ(I).

Tridymite-based processing for high purity quartz

Abstract: A process of purifying crystalline quartz to remove impurities within the lattice structure comprising the steps of heating the quartz to the tridymite polymorphic form and evaporatively reacting the heated quartz with an aqueous acid solution reactive with the impurities in the quartz to form water soluble metal salts. The residue of the evaporative reaction is washed with water to dissolve the metal salts leaving purified crystalline quartz. The disclosure also includes the product of the process.

Piezoelectric Properties in α-, Incommensurate- and β-Phases of Quartz SiO2

Abstract: Piezoelectric properties of quartz SiO2 were obtained using a self-resonance method at 150–200 kHz over a range from room temperature to 960°C, and in detail particularly in the incommensurate phase. Their large elastic anomalies were observed. Piezoelectricity was found to be preserved up to 960°C. The "tridymite phase" was not detected.

Structure and texture of AlPO4-cesium oxide (20 wt.%) catalysts obtained by impregnation with cesium chloride

Abstract: AlPO4-cesium oxide (20 wt.%) materials retain a low degree of crystallinity on calcination at 873 K for 3 h. After DTA measurements (1423 K), AlPO4 crystallizes (tridymite form). Furthermore, DRIFT spectroscopy shows that the P−OH stretching vibration (at 3670 cm−1) of AlPO4 support disappears upon cesium oxide loading. Moreover, Al and P atoms remain in tetrahedral coordination as in unmodified AlPO4. Furthermore, the incorporation of cesium oxide leads, simultaneously, to a progressive decrease in surface area and pore volume as well as to an increase in the most frequently occurring pore radius.