Tridymite

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

Fluorine mineralisation from burning coal spoil-heaps in the Russian Urals

Abstract: Anhydrous iron, aluminum and fluorine-rich paralavas were found in the burned spoil-heaps of the Chelyabinsk coal basin, Russia. The rocks contain tridymite, anorthite, ferroan fluorine-bearing cordierite, fluorine-bearing mullite, periclase, fluorapatite, micas of the F-biotite–F-phlogopite series, fluortopaz, fluorite, and sellaite. The fluorine-rich minerals formed as a result of local thermal reactions of sedimentary carbonates and silicates with gaseous fluorine. During coal combustion fluorine concentrates in the annealed ankeritic marls where the increase of F is hundreds of times over its concentration in the initial sedimentary rocks. The formation of MgF2 and CaF2 promotes local melting at relatively low temperatures (T < 1000 °C) with the residuum consisting of two immiscible liquids. One crystallises as the fluorides, the other as fluorine-substituted analogues of the hydrosilicates, which under the extremely dry conditions, produce minerals containing extremely high F-contents.

Crystallization of Amorphous SiO2 Microtubes Catalyzed by Lithium

Abstract: Amorphous silica formed by thermally oxidizing silicon is commonly eligible for lithographic patterning or can be grown on patterned substrates. However, it is still a challenge to combine controlled microstructuring with controlled crystallization of SiO 2 . Here, it is shown that traces of volatile lithium species transported through the gas phase catalyze the crystallization of silica integrated into common silicon microstructures. The selected crystallization temperature determines which polymorph forms. As an example, the formation of quartz, tridymite, and cristobalite microtubes by thermally oxidizing macroporous silicon is investigated. Lithium-induced crystallization may extend state-of-the-art silicon technology and yield nano- and microstructures consisting of different silica polymorphs, which are, in contrast to many functional oxides, nontoxic.

Oxidation of silicon carbide and the formation of silica polymorphs

Abstract: The oxidation of both single crystal and relatively pure polycrystalline silicon carbide, between 973 and 2053 K, resulted in the formation of cristobalite, quartz, or tridymite, which are the stable crystalline polymorphs of silica (SiO2) at ambient pressure. The oxide scales were found to be pure SiO2 with no contamination resulting from the oxidizing environment. The only variable affecting the occurrence of a specific polymorph was the oxidation temperature. Cristobalite was formed at temperatures ≥1673 K, tridymite between 1073 and 1573 K, and quartz formed at 973 K. The polymorphs were determined using electron diffraction in a transmission electron microscope. These results were further confirmed using infrared and Raman spectroscopies. Cristobalite was observed to grow in a spherulitic fashion from amorphous silica. This was not the case for tridymite and quartz, which appeared to grow as oriented crystalline films. The presence of a thin silicon oxycarbide interlayer was detected at the interface between the SiC substrate and the crystalline silica using x-ray photoelectron spectroscopy.

First discovery of stishovite in an iron meteorite

Abstract: The first occurrence of stishovite in an iron meteorite, Muonionalusta (group IVA), is reported. The mineral occurs intimately mixed with amorphous silica, forming tabular grains up to ~3 mm wide, with a hexagonal outline. It was identified using X-ray diffraction and Raman microspectroscopy. The unit-cell parameters of stishovite are a = 4.165(3) A and c = 2.661(6) A, and its chemical composition is nearly pure SiO2. Raman spectra show relatively sharp bands at 231 and 754 cm −1 and a broad band with an asymmetric shape and a maximum around 500 cm −1 . The rare grains are found within troilite nodules together with chromite, daubreelite, and schreibersite. From their composition and morphology, and by comparisons with silica inclusions in, e.g., the Gibeon IVA iron, we conclude that these rare grains represent pseudomorphs after tridymite. The presence of stishovite in Muonionalusta is suggested to reflect shock metamorphic conditions in the IVA parent asteroid during a cosmic impact event.