Showing papers on "Tridymite published in 1992"

Influence of transition metal oxide additions on the crystallization kinetics, microstructures and thermal expansion characteristics of a lithium zinc silicate glass

Abstract: Lithium zinc silicate glasses can be used to prepare moderately high thermal expansion glass-ceramics, and these materials are ideally suited for the manufacture of hermetic seals to both nickel-based superalloys and stainless steel. On the basis of earlier work by the present authors, one particular composition from the lithium zinc silicate system was chosen for detailed investigation. This composition contains Na2O and B2O3 fluxing agents, together with P2O5 as the primary nucleating agent. The crystallization kinetics and resultant microstructures of this composition have been studied as a function of the heat-treatment parameters using differential thermal analysis, dynamic mechanical thermal analysis, scanning and transmission electron microscopy, ambient and high-temperature X-ray diffraction, and small-angle neutron scattering. Indirect evidence from the dynamic mechanical thermal analysis and small-angle neutron scattering suggests that the nucleated glass is phase separated on a very fine scale, of the order of 16 nm. A number of crystalline phases have been positively identified in the heat-treated glasses, including cristobalite, quartz, tridymite, β1-Li2ZnSiO4 and γo-Li2ZnSiO4, the precise phases that are formed depending strongly on the heat-treatment parameters. The influence of a number of transition metal oxide additions on the resultant properties of the lithium zinc silicate composition has also been investigated, and it has been shown that the crystallization kinetics, microstructures and thermal expansion characteristics are all strongly affected by these additions. In particular, the activation energy for crystallization (which is related to the nucleating efficiency) is dependent on the ionic field strength of the transition metal ion species employed, with crystallization being favoured by solutes of high field strength.

Pyrometamorphism and partial melting of shales during combustion metamorphism: mineralogical, textural, and chemical effects

Abstract: Eocene shales metamorphosed by a naturally ignited coal seam in the Powder River Basin, Wyoming record a continuum of mineralogic and textural changes from relatively unaltered shale to melt developed during pyrometamorphism. Samples collected along a section 2 m in length, corresponding to a temperature range of approximately 1300°C, were examined optically and by XRD, SEM, and STEM. The low temperature samples are comprised primarily of silt-sized quartz, K-feldspar, and minor amounts of other detrital minerals in a continuous matrix of illite/smectite (I/S). Delamination of phyllosilicates due to dehydroxylation occurs early in the sequence with curling of individual layers from rim to core. Within one-half meter of melted areas, phyllosilicates have undergone an essentially isochemical reconstitution with nucleation and growth of mullite crystals with maximum diameters of 50 nm, randomly distributed within a non-crystalline phase that replaces I/S. Large detrital grains remain for the most part unaffected except for the inversion of quartz to tridymite/cristobalite. Within 1 mm of the solid/melt interface, the mullitebearing clay mineral matrix is essentially homogeneous in composition with obscure grain boundaries, caused by apparent homogenization of poorly crystalline material. This material is similar in composition to parent clays and acts as a matrix to angular, remnant tridymite/cristobalite grains. Rounded, smaller silica grains have reaction rims with the non-crystalline matrix; K-feldspar is no longer present (apparently reacted with the matrix) and the matrix contains abundant pore space due to shrinkage upon dehydroxylation. As isolated pods of paralava (glass) or fractures are approached, Fe−Ti−Al oxides become abundant. Vesicular glass is separated from clinker by a well-defined interface and contains numerous phenocrysts. XRF analyses and reduced area rastering using EDS imply enrichment of the melt phase in Fe, Ca, Mg and Mn, apparently due to vapor transport from other layers lower in the sedimentary sequence.

Infrared spectroscopy of natural, synthetic, and oxygen-18-substituted .alpha.-tridymite: structural implications

Abstract: Infrared absorption spectra from thin films of three samples of α-tridymite (natural, synthetic, and isotopically substituted 18 O) shown only 11 IR bands in common, two of which appear to be overtones. Symmetry analysis of the high-temperature (β-)polymorph and of the potential pathways of displacive tranformation imply that α'-tridymite is hexagonal (space group D 3h 2 =P6c2) and that the lowest temperature α-phase could be P321 or a similar hexagonal structure. Possibly, only one displacive transformation occurs. The orthorhombic space groups are unlikely, and a monoclinic structure is entirely incompatible with the spectroscopic evidence

Phase transitions and microstructures in natural kaliophilite

Abstract: An investigation using TEM, high temperature X-ray powder diffraction and high temperature single-crystal X-ray diffraction revealed previously unreported microstructures and phase transitions in natural kaliophilite. Measurements of unit cell dimensions from room temperature to 895°C indicate the existence of a displacive phase transition at ∼725°C between two hexagonal phases with the same unit cell. A symmetry change of P6 3 22⇄P6 3 is tentatively suggested with the concomitant development of merohedral twins. Spontaneous strain determinations suggest that the transition is close to being tricritical in character. Above ∼750°C kaliophilite crystals undergo a first order epitaxial phase transition to a different hexagonal phase with a=8.9 A and c=8.4 A. Dark and bright field images in the TEM show the presence of twins and heterogeneously distributed linear defects. These are interpreted respectively as transformation twins and stacking defects incorporated during crystal growth. The thermal evolution of kaliophilite is sufficiently different from that of kalsilite to indicate that it has a different framework topology and is not a true stuffed tridymite structure

The mineralogical and petrological factors in Alkali Silica Reactions in concrete

Abstract: Reactive silicas which can cause Alkali Silica Reactions (ASR) are opal, Si02 rich volcanic glass, chalcedony (or fibrous quartz) and tridymite. Opal is a hydrous cryptocrystalline or colloidal form of silica (Si02 nHP) with a water content of around 6% to 10%. Silica-rich volcanic glass is a component of many young volcanic rocks. Chalcedony is a compact variety of silica composed of minute (crypto- or micro-crystalline) crystals of quartz (Si02 ) enclosing submicroscopic pores. Tridymite is a rare hexagonal high temperature polymorph of quartz with a S.G. of 2.28. Opaline silica and silica-rich volcanic glass are regarded as the most reactive. Chalcedony, tridymite and some forms of crypto- and microcrystalline quartz occurring in acidic volcanics, chert and flint are regarded as of intermediate reactivity. Strained macrocrystalline quartz can cause ASR but only under certain conditions. A factor which may influence the reactivity of particular aggregate is the proportion of reaction materials that is present in the aggregates. For a very reactive opaline silica the worst expansion may occur when it is found in the order of 2% to 5% (in the coarse aggregate), that is the pessimum. The pessimum for silica-rich volcanic glass is unknown. In the case of the strained macro-crystalline quartz, the pessimum being 100% that is quartzite containing wholly of strained quartz. The pessimum for the intermediate reactive silica is not known, but could well be higher than that of the opaline silica and definitely lower than that of strained quartz.

Phase Relations in the System Iron Oxide–NiO–SiO2 under Strongly Reducing Conditions

Abstract: Liquidus and solidus phase relations have been determined for the system iron oxide–NiO–SiO2 under strongly reducing conditions obtained by using CO2–CO gas mixtures in controlled proportions. The phase relations were determined with the well-known quenching method: oxide mixtures were equilibrated in vertical tube resistance furnaces, followed by quenching to room temperature and identification of phases with transmitted- and reflected-light microscopy and X-ray diffraction. Three crystalline phases are present on the liquidus surface: olivine (Fe2SiO4–Ni2SiO4 solid solutions), oxide (“FeO”–NiO solid solutions), and silica (tridymite or cristobalite, depending on temperature). The “ternary peritectic” point where these three phases coexist with liquid is at 1571°C, with a liquid composition of approximately 19 wt%“FeO”, 47 wt% NiO, 34 wt% SiO2.

Influence of water vapour on high-temperature oxidation of Al2O3-MgO-doped hot-pressed silicon nitride

Abstract: Silicon nitride is particularly sensitive to high-temperature oxidation. The intensity of oxidation is influenced by the chemical composition of the amorphous phases present at the grain boundaries and consequently by the sintering additives responsible for their formation. The presence of water vapour increases Si 3 N 4 oxidation also in intermediate temperature conditions. In this study the influence of water vapour pressure at high temperature (1200°C) on the corrosion of hot-pressed silicon nitride (HPSN) doped with Al 2 O 3 -MgO was evaluated. The water vapour has a great influence on the devitrification of the amorphous oxide upper layer, due to the formation of crystalline oxides (primarily cristobalite and tridymite). This process increases the oxidation rate, consequently increasing the porosity of the exposed surface. The microstructural evolution of HPSN in the presence of water vapour at 1200°C was analysed by SEM and XRD.

Liquid–Solid Equilibria in the System FeO–NiO–Fe2O3–SiO2

Abstract: Liquidus and solidus phase relations have been determined for the system NiO-iron oxide–SiO2 in air and in CO2. In both cases the system is simple in that it contains only three primary crystalline phases: spinel (Fe3O4–NiFe2O3 solid solution), oxide (“FeO”–NiO solid solution) and SiO2, either tridymite or cristobalite. There is only one liquidus invariant point at which the above crystalline phases coexist with liquid (and the gas phase). In air, the invariant point (a eutectic) is at 1524°C, and a temperature maximum is present along the spinel–silica liquidus boundary curve at 1540°C. In CO2, the silica + spinel + oxide + liquid invariant temperature is considerably lower: 1433°C. Combination of these data with those obtained previously for the same system under strongly reducing conditions (CO2–CO gas mixtures of ratios only slightly above those of equilibrium coexistence of the oxide phases with Fe–Ni alloy) makes it possible to draw the main features of phase relations in the quaternary system FeO–NiO–Fe2O3–SiO2. This system also is relatively simple, in that only five crystalline phases are present in equilibrium with liquids, viz, silica (cristobalite or tridymite), olivine (Fe2SiO4-Ni2SiO4 solid solution), spinel (Fe3O4-NiFe2O4 solid solution), hematite (Fe2O3), and oxide (“FeO”–NiO solid solution). There is only one quaternary liquidus invariant point in the system, characterized by the equilibrium coexistence of silica, olivine, spinel, oxide, and liquid, at a temperature of ∼ 1410°C and an oxygen pressure of the gas phase of ∼10−5 atm.

Effect of Cations on Crystallization of Amorphous Silica(Part3)

Abstract: Amorphous silica of guaranteed chemical reagent was heated at temperatures 500 °C, 600°C, 800°C and 1000°C under the existence of various chlorides such as NaCI, KCI and LiCI, and effects of which on crystallization of the amorphous SiO 2 were investigated. At 500°C, no crystalline form was produced when small amounts of NaCI and KCI were added, but with LiCI, quartz was evident after 25 days. At 600°C, cristobalite was formed under the existence of NaCI, and tridymite and cristobalite under the existence of KCI, both crystallizations resulting from 70 days heating. Quartz and lithium silicate were formed under the existence of LiCI in 25 days' heating. When NaCl was added at 800°C, both cristobalite and tridymite were formed in one-day's time, but the amount of tridymite gradually increased with heating time where by forming quartz after prolonged heating. With large amount of NaCI, however, only tridymite remained formed for up to 37days. With KCl was added at 800°C, tridymite and cristobalite likewise were formed under the existence of small amount, and only tridymite formed under the existence of large amount. Upon addition of LiCI, products varied depending upon the amounts added. At 1000°C, tridymite and cristobalite were formed in 7 days heating under the existence of NaCI and KCl.

Method for producing silica brick

Abstract: A method which enables easy and efficient production of silica brick having small content of residual silica by firing from a siliceous stone material containing silica as the main component, even when the siliceous stone material used is of the type in which transformation of silica is not easy to occur. The method is characterized by adding 0.2 to 5 wt % of Na 2 O--CaO--SiO 2 fused and solidified material to the siliceous stone material. The Na 2 O--CaO--SiO 2 fused and solidified material, together with a binder, is added to pulverized siliceous stone material, and the mixture is then formed followed by a firing at 1350° to 1500° C., so that the Na 2 O-13 CaO--SiO 2 fused and solidified material reacts with silica in the siliceous stone material to promote transformation of the silica into cristobalite and tridymite.

Process for producing silica brick

Abstract: A process for producing silica brick by firing silica stone mainly comprising silica and containing 0.2 to 5 wt % of solidified molten soda lime silicate added thereto, whereby silica brick with a lowered content of residual quartz can be produced readily and efficiently even by using a silica stone which difficultly causes the transformation of quartz. The process comprises crushing silica stone, adding thereto solidified molten soda lime silicate and a binder, molding the mixture, and firing the molding at 1350 to 1500 °C to cause a reaction between the silicate and the silica in the silica stone to thereby cause rapid transformation of the silica into cristobalite and tridymite.

Introduction to mineral sciences: Transformation processes in minerals II: structural phase transitions

Abstract: In this chapter we discuss various aspects of the mechanism of transformations in minerals in which there is no change in chemical composition. Such polymorphic transformations are often described in terms of the degree of similarity between the structures, which in turn can be used to define the structural changes required to transform one to the other. Often a kinetic classification is also implied in this approach. For example a transformation between two very similar structures may only involve a distortion of the bonds such as that between the high and low silica polymorphs (Section 6.8.1). Such displacive transitions are generally fast and cannot be prevented from occurring even with very rapid cooling rates (i.e. they are unquenchable). Displacive transitions may be thermodynamically first- or second-order (Section 8.4.2). On the other hand, a reconstructive transition involves a major reorganisation of the structure, with bonds being broken and new bonds formed. Transformations between the silica polymorphs (quartz ⇔ tridymite ⇔ cristobalite) are typical reconstructive transitions which have high activation energies and are kinetically very sluggish. Rapid cooling can easily quench the high temperature form which may persist indefinitely at low temperatures. The other classic example is the persistence of the diamond structure at atmospheric temperatures and pressures where graphite is the stable form of carbon. Reconstructive transformations are always thermodynamically first-order. Order-disorder transitions may be slow, as in the case of substitutional disorder (e.g. Si,Al disorder in aluminosilicates) or fast, as in orientational disorder (e.g. the orientation of the CO 3 group in calcite – see Section 8.13.2 and Figure 8.38).

An HRTEM investigation of the metastable low-temperature silica phase opal-CT in cherts and porcelanites from the Monterey Formation, CA

Abstract: High resolution transmission electron microscopy (HRTEM) is used to investigate the metastable low-temperature silica phase opal-CT in cherts and porcelanites from the Miocene Monterey Formation of California. Low-dose imaging techniques developed to image highly beam sensitive proteins were used in this study and have resulted in good phase contrast images of this hydrous silica phase. Detailed X-ray powder diffraction studies of stratigraphically equivalent rocks along the Santa Barbara coast indicate that the primary d-spacing of newly formed opal-CT differs in rocks with different ratios of silica and detrital minerals. Opal-CT forms progressively later and with a smaller primary d-spacing in rocks with increasing amounts of detrital minerals. In siliceous cherts opal-CT occurs as long needles that most often form dense spherulitic fiber bundles which are randomly dispersed within the rock matrix. The random orientation of fiber bundle nucleation centers does not appear to be associated with any obvious nucleation site, unlike the length-slow opal-CT fibers known as lussatite. Opal-CT needles produce optical diffractogram patterns that are compatible with tridymite and crystobalite. Streaking in the diffraction pattern of individual needles is attributed to a high density of planar defects parallel to their length. Planar defects are not as abundant in opal-CTmore » needles formed in detrital-rich rocks suggesting the rapid growth of opal-CT in highly siliceous environments results in a greater proportion of stacking disorder in the needles. HRTEM provides a method for investigating the development of the microstructure of opal-CT during diagenesis.« less

New Tight-Binding Pair Potentials for Mineral Oxides: Application to β-Cassiterite (110), β-Tridymite (10TO) and Cristobalite (110)

Abstract: Tight-binding, total-energy (TBTE) methods have successfully predicted surface atomic geometries for a variety of semiconducting and insulating materials that are well described by a nearest-neighbor model of interatomic interactions. However, little work has been done on distant-neighbor models, which are required to study many important mineral oxides. In this paper we demonstrate one way in which the TBTE methodology can be extended to these materials. To illustrate this approach, we calculate surface atomic structures for cassiterite SnO{sub 2} (110), {beta}-cristobalite SiO{sub 2}(110) and {beta}-tridymite SiO{sub 2} (10T0).