Showing papers on "Tridymite published in 1991"

X-ray powder diffraction patterns and phase relationship of tridymite modifications

Abstract: Calculated and observed X-ray powder patterns of three ambient temperature modifications of tridymite: L1-T O (=ordered low-tridymite MC), L2-T D (=disordered low-tridymite PO 5/10 ) and L3-T o (=ordered low-tridymite MX-1) are reported. Deviations in the diffractograms due to texture effect, stacking disorder and intergrowth of different modifications which complicate identification and indexing are discussed. Changes of unit cell dimensions due to thermal expansion and phase transitions were measured from room temperature to about 700 o C. Near 160 o C all ordered and slightly disordered tridymite polymorphs transform to monoclinic high tridymite H3-T (OS) with non-integral modulation, at about 210 o C to orthorhombic high tridymite H2-T (OC) and finally at about 400 o C to hexagonal high-tridymite H1-T (HP)

Tridymite and maghemite formation in an Fe-SiO smoke

Abstract: Grains produced in porous Fe-SiO smoke are analyzed by means of electron microscopy to determine their crystallographical, morphological, and chemical properties The nucleation of metal oxide condensates takes place as the vapor evolves in a condensation-flow apparatus, initially producing simple crystalline phases Fe nucleation follows, and the subsequent heterogeneous nucleation of quenched mixed high-Fe silica grains exhibits associated Fe-depletion, eventually producing pure silica grains The coalescence exhibited by the pure silica grains is compatible with the higher surface energy found in the pure silica phase During coagulation the grains melt and anneal to a chemically pure mixture of alpha- and beta-tridymite, and the precipitation of amorphous silica material is observed The settling of grains in evolving solar nebula is theorized to be related to the stickiness of this material, and maghemite is suggested to contribute to the Fe/Si fractionation in solar nebula

Periodic Hartree-Fock study of minerals: Tetracoordinated silica polymorphs

Abstract: Periodic Hartree-Fock STO-3G calculations have been performed on several tetracoordinated silica polymorphs: low and high quartz, low and idealized high cristobalite and prototype tridymite. The optimized structural parameters are in overall good agreement with experimental data. In the particular case of α-quartz, the SiO4 tetrahedra are found to be irregular. The optimized values of the two different SiO bond lengths are respectively 1.608 A and 1.613 A. The potential energy versus tilt angle curves suggest a picture of the high temperature phases in terms of delocalized oxygen atoms which is consistent with a disordered structure. Finally, the bonding in silica polymorphs is discussed from electron density maps and Mulliken population analysis.

Alkali-silica reaction—Canadian experience

Abstract: The first case of alkali-silica reaction (ASR) in Canada was discovered in the mid 1950s, by which time most of the investigative techniques currently used had already been developed. Three types of reactive aggregates are recognised in the current Canadian Standards:(1) Alkali-silica reaction, occurring with opal, chert, chalcedony, volcanic glass, cristobalite and tridymite.

Crystallization of Porous Aluminophosphates and Metal Substitutions

Abstract: Crystallization phenomena of a series of the molecular sieves MeAPO4-5 and MeASPO-5 have been studied under the influence of different Mecomponents. As Me components Be, Mg, Zn, Ni and Fe have been used. These components have been offered in some excess to study the extent of the incorporation which was determined by SEM-EDAX at the product crystals. For Be and Mg almost no influence on the kinetics of crystallization can be observed compared with a pure ALPO4-5 batch. Mg enters the structure with about 0.12 MgO for one Al2O3 P2O5 unit. A batch with Zn shows a reduced crystallinity and a cocrystallization of an unknown phosphate. The ZnO content of the crystals shows about the same molar ratio as the MgO. Extended experiments with NiO show that the Ni is not incorporated into ALPO4-5 and SAPO-5 crystals. In the presence of NiO pure ALPO4-5 and SAPO-5 crystallize which undergo recrystallization to a cristobalite like phase. Fe is incorporated into the crystals for both valency states taking up about 0,1 Fe2O3 for one Al2O3 P2O5 unit in the structure. Generally the presence of an excess of the Me-ions favours some recrystallization to denser phosphate phases or to cristobalite or tridymite.

Crystal Growth of Mg2SiO4 and MgSiO3 Single Crystals by the Flux Method

Abstract: Single crystals of Mg2SiO4 and MgSiO3 have been synthesized by the evaporation-aided slow-cooling flux method. Acidic oxides such as V2O5, MoO3, and WO3 were used as the flux, and alkali metallic oxides or calcium oxides were added to the MoO3 and WO3 as flux modifiers. The starting mixture was first held at 1400°C and then slow cooled to 1000°C at a rate of 10 C/h. The temperature difference between the upper and lower limits of the melt was ∼25°C. Using an Li2O─MoO3 system flux with an Li2O:MoO3 ratio of 1:2 produces a polycrystalline clinoenstatite with a 9-mm crystal size; a 1:1 ratio, however, results in forsterite crystals of 90-μm size. The metallic oxides (Na2O and K2O) as flux modifiers produce forsterite with a crystal size of 65 and 40 μm, respectively. Similar results are observed when a Li2O─WO3 flux is used. Enstatite (10-mm size) and forsterite (1.6-mm size) are produced when the Li2O:WO3 ratios are 1:2 and 1:1, respectively. A V2O5 or CaO·WO3 flux produces no forsterite. The V2O5 flux results in tridymite as a primary phase, whereas calcium and magnesium tungstate were grown from a CaO·WO3 flux. The forsterite single crystals are tabular in form, generally bounded by the planes (001), (100), (010), (110), and (021), and are transparent. The enstatite consists of an aggregation of individual single crystals. The refractive indexes of the forsterite are α= 1.636 ± 0.001, β= 1.653 ± 0.001, and γ= 1.671 ± 0.001 (20°C).

On the anomalous mechanism of ionization of impurity atoms and stabilization of ions in the silicon dioxide structure

Abstract: The anomalous mechanism of ionizing impurity atoms and stabilizing ions which are formed under the action of crystal lattice elastic forces is systematically studied. The possibility of the existence of the above mechanism is proved theoretically and by experiment. The experimental investigations show the correlation between the values of a fixed positive charge, rate of crystallization, and presence of modifiers in the boundary layer of silicon dioxide films. The formation of a fixed charge is shown to occur in the cases the silicon dioxide has impurities which stimulate the crystallization of the silica into one of the impurity phases: cristobalite or tridymite. The impurities serving as nuclei of crystallization are ionized under the effect of elastic forces of the crystal lattice of the originated nuclei. The possibility of stabilizing the ionized state of impurity cations implanted into interstitial lattice sites by the field of elastic matrix forces is also found. [Russian Text Ignored].

Crystal Growth of Mg2SiO4 and MgSiO3 Single Crystals by the Flux Method.

Abstract: Single crystals of Mg2SiO4 and MgSiO3 have been synthesized by the evaporation-aided slow-cooling flux method. Acidic oxides such as V2O5, MoO3, and WO3 were used as the flux, and alkali metallic oxides or calcium oxides were added to the MoO3 and WO3 as flux modifiers. The starting mixture was first held at 1400°C and then slow cooled to 1000°C at a rate of 10 C/h. The temperature difference between the upper and lower limits of the melt was ∼25°C. Using an Li2O─MoO3 system flux with an Li2O:MoO3 ratio of 1:2 produces a polycrystalline clinoenstatite with a 9-mm crystal size; a 1:1 ratio, however, results in forsterite crystals of 90-μm size. The metallic oxides (Na2O and K2O) as flux modifiers produce forsterite with a crystal size of 65 and 40 μm, respectively. Similar results are observed when a Li2O─WO3 flux is used. Enstatite (10-mm size) and forsterite (1.6-mm size) are produced when the Li2O:WO3 ratios are 1:2 and 1:1, respectively. A V2O5 or CaO·WO3 flux produces no forsterite. The V2O5 flux results in tridymite as a primary phase, whereas calcium and magnesium tungstate were grown from a CaO·WO3 flux. The forsterite single crystals are tabular in form, generally bounded by the planes (001), (100), (010), (110), and (021), and are transparent. The enstatite consists of an aggregation of individual single crystals. The refractive indexes of the forsterite are α= 1.636 ± 0.001, β= 1.653 ± 0.001, and γ= 1.671 ± 0.001 (20°C).