Showing papers on "Tridymite published in 1990"

X-ray diffraction and scanning electron microscope studies of processed rice hull silica

Abstract: The structure and composition of processed rice hull ash were studied to better understand its adsorption of the soy oil components, lutein and free fatty acids. The silica-rich ash was composed largely of low order cristobalite and tridymite in the form of opal CT, with variable particle size. On heating of the ash there was conversion to a more ordered form of opal C, fusing of particles, and loss of potassium. Acid washing, following heating, also removed potassium and slightly reduced particle size further. Crystal form, particle size, particle fusion and potassium content may be important factors influencing ash adsorption of soy oil components.

Crystal structures and ferroelectric properties of AIBIIPO4 (AI = monovalent cation and BII = divalent cation)

Abstract: Some of the title compounds have been found to exhibit ferroelectricity at room temperature Their structure types can be classified in three series according to the cations coordination number (CN) and size: 1- single cation with CN 4: olivine and glaserite; 2- two cations with CN 4: Beryllonite and stuffed tridymite; 3- all cations with CN 4: α-LiZnPO4 is the unique case known

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.

Sister chromatid exchanges in human lymphocytes treated with silica.

Abstract: The abilities of Min U Sil quartz or tridymite particles to induce sister chromatid exchanges (SCEs) in cultures of human lymphocytes plus monocytes or of human purified lymphocytes were investigated. With cultures of lymphocytes plus monocytes the level of SCEs was significantly enhanced after treatment with tridymite at the highest dose tested (50 micrograms/cm2). No effect was observed with purified lymphocytes. Quartz did not give clear cut results. Complementary experiments with tridymite filtrates suggested that phagocytosis of tridymite particles by monocytes was a necessary step for the induction of SCEs in human lymphocytes.

A study of the thermal behavior of terrestrial tridymite by continuous X-ray diffraction

Abstract: Phase transition behavior of two types of terrestrial tridymite; crystals from Topaz Mountain in the Thomas Range of northwestern Utah and Smith Peak in Plumas County, California; were investigated. The Topaz Mtn. samples were characterized at room temperature using optical, X-ray diffraction (Debye-Scherrer, rotation, and Laue), and transmission electron microscopy (TEM) methods. Least-squares refinement of the powder data yielded an orthorhombic cell with a = 17.072 A, b = 9.923 A, and c = 16.291 A. The tridymite is designated PO-2 (nomenclature of Nukui and Nakazawa 1980). This material is complexly twinned and has severe stacking disorder parallel to [001]. Additional X-ray experiments were performed on one Plumas County crystal and four Topaz Mtn. crystals during heating from room temperature to a minimum of 320° C with one run attaining a maximum temperature of 532° C. Crystal to crystal behavior was somewhat variable suggesting kinetic barriers to transition controlled by the detailed structural state of each crystal. The Laue results indicated that the transition behavior of these multiple twinned tridymites could be conveniently divided into two classes: discontinuous and continuous. The discontinuous diffraction effects were generally associated with major structural transitions. Continuous diffraction effects involved gradual structural distortions of the tridymite framework that occurred over discrete ranges of temperature. Upon cooling, many of the minor effects were not reversible while the major transitions were reversible with some temperature hysteresis. Comparison of initial and final diffraction patterns indicated that the starting and ending structures were very similar but not identical. From the five experiments, three distinct patterns emerged based on the major transitions observed: Type I behavior (Topaz Mtn.) characterized by three major changes at 175–187° C, 283–302° C, and 348–352° C, Type II behavior (Plumas Co.) characterized by one major transition between 283–295° C, and Type III behavior (Topaz Mtn.) characterized by one major change between 159–165° C.

The systematic enumeration of (4;2)-connected 3D nets related to the tridymite group

Abstract: Forty-two hypothetical 3D nets are derived by applying four types of transformations (or combinations thereof) to the previously described tridymite group of structures. Two novel transformations are proposed, one preserving the (hexagonal or orthorhombic) symmetry and another converting the hexagonal symmetry into orthorhombic. Correspondingly, groups of hypothetical 3D framework structures are derived. The properties of frameworks belonging to these groups are compared and discussed.

Framework structures derived from a novel type of trigonal secondary building unit with on-axis T atoms: the tridymite group

Abstract: An infinite series of secondary building units generated around a trigonal axis, with the top and/or bottom T atom coinciding with the symmetry axis allows the systematic enumeration of a novel class of (4; 2)-connected 3D framework structures. Known members of this group are the framework topologies of tridymite, cristobalite, MAPSO-46 (AFS), CoAPO-50 (AFY) and beryllophosphate-H (BPH). Adjacent T atoms on a trigonal axis with a T-O-T bond angle of 180° (T2 unit) present in all structures confer some specific properties on this class of compounds.

Phase Relations in the System SiO2-AlPO4 up to 400°C

Abstract: The phase relationships along binary join of SiO2-AlPO4 were investigated up to 400°C using several starting materials made by a partial solution route. Precursors used were boehmite (AlOOH), H3PO4, non crystalline silica (Ludox, Cab-O-Sil), and quartz. Studies up to 400°C showed that SiO2, AlPO4, and its hydrate were the only crystalline and non-crystalline phases present along the binary join, and no substantial crystalline solution of any ternary phase was observed. Three polymorphic forms of AlPO4, namely, berlinite, tridymite form, and cristobalite form, coexisted at as low as 200°C. The nature of the silica precursors greatly influenced the development of the polymorphic phases of AlPO4. The low quartz precursor suppressed the formation of the cristobalite form of AlPO4 and favored berlinite production. On the other hand, non crystalline silica with a cristobalite-like broad XRD peak suppressed the formation of berlinite and enhanced that of the cristobalite form of AlPO4. The silica precursors acted as structural seeds for the growth of AlPO4. These precursor effects indicate that heteroepitaxy is very significant during the nucleation and growth of AlPO4 phases on the surface of SiO2 particles even in these low temperature reactions. The influence of other precursors and route during the syntheses of AlPO4 and other phases in the SiO2-AlPO4 are discussed.