Tridymite

About: Tridymite is a research topic. Over the lifetime, 840 publications have been published within this topic receiving 14831 citations.

Potassium Vapor Attack in Refractories of the Alumina–Silica System

Abstract: A graphite chamber was used for the reaction between samples of 45 or 55 wt% alumina and a mixture of metallurgical coke and potassium carbonate. Thermal treatments were conducted at 1000°C. The results suggest that the potassium attack in silica-alumina bricks is controlled by the following reactions: K2O + SiO2→ K2O → SiO2 in the glassy matrix; 3(K2O · 2SiO2) + 3Al2O3→ 2SiO2· 3(K2O · Al2O3· 2SiO2) + 2SiO2 for short times; and K2O → Al2O3· 2SiO2+ 2SiO2· K2O · Al2O3· 4SiO2 for long times. In 55 wt% alumina bricks containing corundum and tridymite, potassium also attacks those phases forming a glassy phase. The formation of kaliophilite at the matrix/mullite grain interface causes a volumetric expansion of 55.5%, resulting in cracks in the matrix. Because the kaliophilite phase is not in equilibrion with mullite, the former will react with free silica to form leucite that is more thermodynamically stable.

Quartz glass elements

Abstract: Quartz glass element, such as a diffusion tube useful in the production of semiconductor elements, capable of forming an outer layer of uniformly fine crystalline silica such as cristobalite or tridymite when heated to a temperature at which such crystalline silica forms containing crystallization promoting nuclei having a rate of diffusion in quartz glass less than that of sodium at elevated temperatures. Such nuclei are preferably present in the outer half of the element wall. When the quartz glass element is exposed to elevated temperatures, the nuclei promotes the formation of the outer layer of uniformly fine crsytalline silica which imparts thermal dimensional stability for extended periods of use at elevated temperatures.

The System CaO-ZnO-SiO2

Abstract: Equilibrium relationships in this ternary system were determined by the quenching method. The only ternary compound occurring in the system was found to be Ca2ZnSi2O7, which corresponds to the natural mineral hardystonite. It has a congruent melting point (1425°C.) and a large primary-phase field in the center of the system. Primary-phase fields for cristobalite, tridymite, CaSiO3, Ca3Si2O7, Ca2SiO4, ZnO, and Zn2SiO4 were also determined in part or in full. The results of this work have some bearing on the minerals and reactions occurring in lead blastfurnace slags and in glazes containing zinc oxide.

An x-ray scattering study of vitreous KFeSi3O8 and NaFeSi3O8 and reinvestigation of vitreous SiO2 using quasi-crystalline modelling

Abstract: The structures of vitreous KFeSi3O8, NaFeSi3O8 and SiO2 have been investigated by X-ray scattering and quasi-crystalline modelling. Heavy-atom substitution of Fe3+ for Al3+ was used to improve resolution of the structural features in the RDF's of aluminosilicate glasses. However, individual FeO and SiO scattering interactions are not clearly resolved. Fe3+ is assigned to tetrahedral coordination by Mossbauer spectroscopy. The network structures of KFeSi3O8 and NaFeSi3O8 both have an apparent TO bond length of 1.70 A but differ in TT distances and angles: 3.30 a/152dg and 3.20 A/140°, respectively. Modelling of these iron-substituted feldspar composition glasses with stuffed tridymite, quartz, feldspar and pyroxene structures suggests that the KFeSi3O8 network is predominantly composed of six-membered rings of SiO4 tetrahedra similar to those found in β-Na2Si2O5. The vitreous NaFeSi3O8 RDF, however, shows aspects of both nepheline and high albite RDF's, suggesting that this glass has a local short-range structure composed of a combination of the ring structures found in these crystalline silicates. The difference is reflected in the lower angle for NaFeSi3O8 (140°) relative to KFeSi3O8 (152°) and SiO2 (151°). Vitreous SiO2 exhibits a mean SiO bond of length of 1.60 A and a bond angle of 151°, similar to values reported in previous studies. Modelling of the structure against the crystalline polymorphs of SiO2 indicates that vitreous SiO2 can best be described as a “tridymite-like” network consisting predominantly of six-membered rings of SiO4 tetrahedra of which 2 3 are tridymite-like and 1 3 cristobalite-like.

Heat capacities and inversions in tridymite, cristobalite, and tridymite-cristobalite mixed phases

Abstract: Samples of synthetic tridymite, cristobalite and tridymite-cristobalite mixed phases, examined by differential scanning calorimetry (DSC) from 360 to 770 or 840K, showed distinct peaks in heat capacity (Cp). For \"pure\" tridymite the lambda transition at 390K and two peaks near 436K and 470K, which may also be lambda transitions, appear to correlate respectively with the structural changes from (MC) monoclinic (Cc) to (OP) orthorhombic P2t2t2t, (oP) to (oS) orthorhombic with a non-integral superstructure, and (oS) to (oc) orthorhombic c222' descibed by Nukui et at. (1978) as occurring at 3g3K, 423K, and 463K respectively. It may also be possible to correlate a weak cp effect near 653K with the (oc) to (HP) hexagonal P6/mmc inversion of Nukui et al. The DSC measurements agree with the heat-content measurements by Mosesman and Pitzer (1941) and the dynamic calorimeter data of Shahid and Glasser (1970) for tridymite, although the feature observed in both these studies at 498K was absent in our tridymite sample. This 498K discontinuity probably reflects inversion in cristobalite portions of the \"tridymite\" samples used in the previous studies, as suggested by our DSC examination of tridymite-cristobalite mixed phases. The present Cp data for tridymite above 500K and for cristobalite above 560K agree reasonably well with thi polynomial flts presented by Robie et at. (1978, p.2l7, Zl8).