Showing papers on "Tridymite published in 1994"

A thermodynamic assessment of silica phase diagram

Abstract: An internally consistent data set on the thermodynamic properties of the silica polymorphs stable up to 15 GPa (α-quartz, β-quartz, tridymite, cristobalite, coesite, and stishovite) and the liquid phase is presented The data set was produced through a computer-based assessment of the properties in which the available thermochemical (calorimetric), physical (bulk modulus and thermal expansion), and solid-state and melting transition data (including some newly determined data on the high-pressure polymorphs cosite and stishovite) were considered The data set can be used to calculate phase relations at pressures of 01 MPa to 15 GPa and temperatures of 300 to 3200 K The calculated phase diagram using these data agrees quite well with the phase equilibrium determinations except for the high-temperature part of the coesitestishovite boundary The properties of the liquid phase obtained are also in good agreement with the available data

NMR, XRD and IR study on microcrystalline opals

Abstract: Microcrystalline opal-CT and opal-C were investigated by 29Si MAS NMR and 29Si {1H} cross polarisation MAS NMR spectroscopy, X-ray small angle scattering, X-ray powder diffraction and infrared absorption spectroscopy. The results are compared with those for non-crystalline precious opal (opal-AG), non-crystalline hyalite (opal-AN), moderately disordered cristobalite and with well ordered low-cristobalite and low-tridymite. Opal-C is confirmed to be strongly stacking disordered low-cristobalite with about 20 to 30% probability for tridymitic stacking. More extensively stacking disordered opal-CT does not contain detectable domains of low-cristobalite or low-tridymite. The stacking sequence is close to 50% cristobalite and 50% tridymitic. The local order decreases with increasing stacking disorder, so that the structural state of microcrystalline opals lies between cristobalite, tridymite and non-crystalline opals.

Wet chemical syntheses of ultrafine multicomponent ceramic powders through gel to crystallite conversion

Abstract: Coarse Bn+On/2·xH2O (10 < x < 120) gels, free of anionic contaminants react with A(OH)2 solutions under refluxing conditions at 70–100 °C giving rise to nanoparticles of multicomponent oxides (A = Ba, Sr, Ca, Mg or Pb; B = Zr, Ti, Sn, Fe, Al or Cr). These include ABO3 perovskites and their solid solutions, polytitanates, hexaferrites and related phases, aluminates with spinel or tridymite structure and chromates. The nanosized crystallites are often in metastable phases, such as cubic BaTiO3 at room temperature or superparamagnetic hexaferrites. Through the same route, luminescent phosphors of aluminates doped with rare-earth metals could be prepared. The present results indicate the general features of the gel–crystallite (G–C) conversion involving the instability of the metal hydroxide gel brought about by the disruption of the ionic pressure in the gel as a result of the faster diffusion of A2+ ions through the solvent cavities within the gel frame work. This is accompanied by the splitting of the bridging groups like B—(OH)—B or B—O—B, leading to the breakdown of the gel into crystallites. G–C conversion has advantages as a method of synthesis of ceramics in terms of operational cost and procedural simplicity.

An electron diffraction study of the polymorphs of siO2-tridymite

Abstract: The results of a detailed TEM study of the low frequency modes of distortion of high temperature SiO2-tridymite and their relationship to the extensive structural polymorphism of tridymite are presented. It is found that low energy modes of distortion of the ideal tridymite tetrahedral framework structure give rise to a strong and extremely characteristic diffuse intensity distribution (which can be broken into two component types) for the two highest temperature polymorphs of tridymite. Experimental results strongly suggest that this observed diffuse distribution is not a result of irreversible beam damage but is rather an intrinsic property of the ideal tridymite tetrahedral framework structure. The diffuse intensity distribution is closely related to the lower temperature polymorphs of tridymite — in particular, the primary modulation wave-vectors of these low temperature polymorphs always fall on the higher temperature diffuse distribution. The first type of diffuse distribution appears to result from coupled tetrahedral edge rotation of 〈110〉 columns of corner-connected SiO4 tetrahedra (uncorrelated from column to column as a result of the tetrahedral connectivity of the ideal tridymite framework structure). The real space structural origin of the second curved type of diffuse distribution, however, remains unclear.

Glass-ceramics containing lithium disilicate and tridymite

Abstract: This invention is directed at the makings of glass-ceramic which is uniquely suited for use as a disc substrate for utilization in a magnetic memory storage device. The glass ceramic material exhibits a crystal phase assemblage comprised predominately of a mixture of lithium disilicate and tridymite which are uniformly interspersed with a residual glass phase and form an interlocked microstructure with the glass. The composition consists essentially, expressed in terms of weight percent on the oxide basis, of 75-95% SiO 2 , 3-15% Li 2 O, 0-6% Al 2 O 3 , and 0-6% K 2 O. The nucleating agent for this glass-ceramic is selected from the group consisting of 0-0.1% Pd and 0-5% P 2 O 5 ; however if Pd is absent the P 2 O 5 amount is at least 0.5% and if P 2 O 5 is absent then the Pd amount is at least 0.005%. Additionally, up to 15% of optional ingredients may be added, including, B 2 O 3 , Na 2 O, ZnO, MgO, CaO, SrO, ZrO 2 , TiO 2 , F, Sb 2 O 3 , As 2 O 3 , PbO and BaO.

Modifications of structure and Si environment upon heating of SAPO-5, SAPO-34 and SAPO-37

Abstract: XRD, IR and 29Si MAS NMR are used to characterize three SAPO's(5, 34 and 37). The changes in structure or arrangement of T atoms are shown to strongly depend on heating conditions(muffle oven, vacuum or flow of gas). Even when the initial crystalline structure is maintained at 1173 K, a solid state transformation is shown to occur in the three compounds, gathering the Si atoms in Si islands. The order of stability of the three structures with regard to rehydration or phase transformation to tridymite + cristobalite structures is shown to be SAPO-5 >SAPO-34 ≥ SAPO-37. This is parallel to the rank of topological densities. The most stable structures are the denser.

Crystal Growth and Structure Determination of the Monoarsenate CsBeAsO4.

Abstract: Solid-state synthesis of a new caesium-beryllium monoarsenate and its crystal growth by an original flux process are detailed and followed by a complete crystallographic characterization pointing out it belongs to the well-known stuffed tridymite structural family Caesium atoms are located in cavities delimited by the chains of tetrahedra and the eight nearest oxygen atoms are distributed over a half-space It is proposed that the phase transition at 832K proceeds according to a displacive mechanism from Pna2 1 at low temperature to Pnam space group at high temperature

TIBePO4 : un nouveau phosphate ferroélectrique

Abstract: The thallium-beryllium monophosphate is related to the β-SiO 2 tridymite structural kind. The phase II, stable at room temperature, is found to have orthorhombic symmetry mm2. It sustains a displacive transformation at 919 o C leading to centric phase I with symmetry mmm. In the low-temperature form, the 6s 2 electron pair of Tl + cation is oriented along polar axis, inducing a significant distortion of the oxygen framework; the ferroelectric nature of phase II is pointed out by the strong increase of the dielectric stiffness around Curie temperature. Phase transition results of bound rotations of PO 4 and BeO 4 tetrahedra, the rise of symmetry order leading to the loss of hybridization of the lone pair. According to the displacive transition mechanism, both thermal and dielectric properties show typical second order features

Petrology and geochemistry of an unusual tridymite-hercynite xenolith in tholeiite from southeastern China

Abstract: A rare variety of Fe-Al-rich xenolith, tridymite-hercynite rock, was entrained in the Cenozoic basalt from Niutoushan, southeastern China. The tridymite-hercynite rock consists of hercynite (40–50%), tridymite (35–45%), ilmenite (≈2%), and glass (10–20%). Mineral grains are smaller than 0.05 mm with a granular texture. Compared to “normal” igneous rocks, this tridymite-hercynite rock has higher FeO, Al2O3, and lower SiO2 CaO, MgO, and alkalis (Na2O + K2O).

[Studies of structure-determined biological aggressiveness of chrysotile asbestos. I. Model of chrysotile crystalline structure]

Abstract: Chrysotile is built out of two kinds of layers placed alternately, namely tridymite (silico-oxygenic) and brucite. A tridymite layer is composed of silico-oxygenic tetrahedrons laid planary in the pseudohexagonal order, while magnesium ions coordinated octahedrally in hydroxyl groups produce a brucite layer. Two layers are bound in a chrysotile molecule through substitution of a part of hydroxine groups of the basis of the brucite layer by oxygen ions from the vertex of the tridymite layer tetrahendrons. The size of an elementary chrysotile cell is as follows: a = 5.3A, b = 9.2A, c = 7.3A, beta = 93 Positions (x, y, z) of all atoms in the elementary chrysotile cell were calculated. In calculations hydrogen atoms were neglected because of small atomic dissipation factor (contribution to the structure) and difficulties in their precise location in the elementary cell. The elementary chrysotile cell is composed of two molecules Mg3 [Si2O5] (OH)4. Crystallographic axes of chrysotile structure were directed in relation to the fibre axis. The values of the elementary chrysotile fibre radii are as follows: R = 208A (starting value) and r = 18.2A (minimum value) A theoretic chrysotile pulverulent diffraction pattern was calculated and drawn. The theoretical pulverulent diffraction pattern of chrysotile was compared with chrysotile diffraction pattern in the card JC PDS21-543. A great similarity between the two diffraction patterns was found.

Diagenetic Microcrystalline Opal Varieties from the Monterey Formation, CA: HRTEM Study of Structures and Phase Transformation Mechanisms

Abstract: Microcrystalline opal varieties form as intermediary precipitates during the diagenetic transformation of biogenically precipitated non-crystalline opal (opal-A) to microquartz With regard to the Monterey Formation of California, X-ray powder diffraction studies have shown that a decrease in the primary d-spacing of opal-CT toward that of cristobalite occurs with increasing diagenesis The initial timing of opal-CT/quartz formation and the value of the primary opal-CT d-spacing, are influenced by the sediment lithology Transmission electron microscopy methods (CTEM/HRTEM) were used to investigate the structure of the diagenetic phases and establish transformation mechanisms between the varieties of microcrystalline opals in charts and porcelanites from the Monterey Formation HRTEM images revealed that the most common fibrous varieties of microcrystalline opals contain varying amounts of structural disorder Finite lamellar units of cristobalite-and tridymite-type layer sequences were found to be randomly stacked in a direction perpendicular to the fiber axis Disordered and ordered fibers were found to have coprecipitated within the same radial fiber bundles that formed within the matrix of the Most siliceous samples HRTEM images, which reveal that the fibers within radial and lepispheric fiber bundles branch non-crystallographically, support an earlier proposal that microspheres in chert grow via a spherulitic growth mechanism A less common variety of opal-CT was found to be characterized by non-parallel (low-angle) stacking sequences that often contain twinned lamellae Tabular-shaped crystals of orthorhombic tridymite (PO-2) were also identified in the porcelanite samples A shift in the primary d-spacing of opal-CT has been interpreted as an indication of solid-state ordering g toward a predominantly cristobalite structure, (opal-C) Domains of opal-C were identified as topotactically-oriented overgrowths on discrete Sections of opal-CT fibers and as lamellar domains within relict opal-CT fibers These findings indicate that the type of transformation mechanism depends upon the primary structural characteristics of the authigenic opaline varieties that are in turn influenced by the sediment lithology

Oxygen isotopic constraints on the origin of nodular silica-apatite from the Har Peres pyroclastics, Golan Heights, Israel

Abstract: Oxygen isotope composition of three types of unique nodules which consist of amorphous silica-apatite, cristobalite-apatite and tridymite-apatite associations interspersed amidst basaltic pyro- clastics from the Hat Peres volcano, Golan Heights, Israel is reported. Unusual isotopic temperature (75~ estimated from oxygen isotope fractionation between cristobalite (6'80 = +25.5%0-apatite (6'80 = + 12.90/~) pair suggests that the nodule was not formed by present-day pedogenesis as has been previously proposed, but was a xenolith incorporated probably from the underlying siliceous phosphorites at a higher temperature. An observed negative oxygen isotopic fractionation (6180 = -5.1% 0 between tridymite (6'80 = +9.9%~) and associated apatite (6180 = -~ 15.0%0) is indicative of the nodular formation under dis- equilibrium conditions. A plausible mechanism of formation of the apatite (and calcite) associated with tridymite is an epitaxial overgrowth on template tridymite ofmagmatic origin under the current weathering regime. Oxygen isotopic evidence indicates a complicated origin for the nodules.