Tridymite

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

Thermal emission spectroscopy of the silica polymorphs and considerations for remote sensing of Mars

Abstract: [1] The possible existence of silica-rich rocks on Mars requires consideration of all of the candidate forms of silica that might exist there We synthesized coesite and cristobalite, and obtained natural samples of tridymite, opal-A, and opal-CT The thermal emission spectra of these silica phases were analyzed and related to their crystal structure and crystallinity Structural differences between the polymorphs result in unique spectral character and the degree and type of crystal ordering in opal-CT results in cristobalite- or tridymite-like spectral features These spectra will be useful for analysis of Thermal Emission Spectrometer (TES) data from Mars While silica polymorphs are not modeled in deconvolution of the two main TES surface-type spectra, this does not preclude the possible detection of local abundances of these minerals on Mars Spectral detection of silica phases on Mars would have implications for the volcanic and alteration history of the planet

Seifertite, a dense orthorhombic polymorph of silica from the Martian meteorites Shergotty and Zagami

Abstract: Seifertite is a dense orthorhombic polymorph of silica with the scrutinyite (α-PbO2) type structure that was found as lamellae occurring together with dense silica glass lamellae in composite silica grains in the heavily shocked Martian meteorite Shergotty. The mineral is also intergrown in some grains with minor stishovite and a new unnamed monoclinic dense silica polymorph with a ZrO2-type structure. Seifertite has also been found in the Martian shergottite Zagami and is a minor constituent in other Martian shergottites. Chemical analyses of seifertite in Shergotty indicate major SiO2 with minor concentrations of Al2O3 and Na2O. Selected-area electron diffraction (SAED) and X-ray diffraction can be interpreted in terms of an orthorhombic pattern from a scrutinyite (α-PbO2) structure. The cell parameters are a = 4.097(1) A, b = 5.0462(9) A, c = 4.4946(8) A, V = 92.92 A3, Z = 4, and the space group is Pbcn or Pb 2 n . Density is (calc.) = 4.294 g/cm3 (with pure SiO2), 4.309 g/cm3 (with empirical formula). It is inferred that seifertite was formed by shock-induced solid-state transformation of either tridymite or cristobalite on Mars at an estimated minimum equilibrium shock pressure in excess of 35 GPa. The new mineral is named after Friedrich A. Seifert (b. 1941), founding Director of the Bayerisches Geoinstitut, Universitat Bayreuth, Germany, for his seminal contributions to high-pressure geoscience.

The action of different forms of pure silica on the lungs of rats.

Abstract: Pure silica (Si02 ) exists at room temperature in the four different modifications : quartz, tridymite, cristobalite, and fused silica. The first three are crystalline, but fused silica is a non-crystalline solid, a glass. Tridymite, cristobalite, and fused silica are metastable, but at room temperature they do not invert within measurable time to the only thermo dynamically stable modification, quartz. It can be regarded as proven that quartz can produce tissue changes in the lungs of animals similar to those found in human silicosis. It was also thought up to quite recently that silica was unique in producing such changes. Experiments with rand /quartz (King, Rogers, Gilchrist, and Nagelschmidt, 1945) had shown that there was no difference in the action of either of the optically active forms from the effect of the racemic type. It was thought desirable next to test the effect of the four different modifications of silica under comparable conditions. The only previous informa tion in this field came from Gardner (1938) who had injected rabbits by the ear vein with 1 g. lots and had found progressive nodular fibrosis of the liver with quartz and fused silica, but a more severe diffuse fibrosis with cristobalite and a still more rapid reaction with tridymite which resulted in the death of all his animals within three months or less. Gardner stated that his samples were analysed and were all below 3 micron diameter, but he did not give any results of analyses or counts. We decided to make comparable sized fractions of the four silica modifications and to use them for

Preparation of phosphoric acid-based geopolymer foams using limestone as pore forming agent – Thermal properties by in situ XRPD and Rietveld refinements

Abstract: In this work, geopolymer foams were obtained by reacting metakaolin with phosphoric acid and using natural calcite/dolomite as foaming agent. Total porosity and thermal conductivity were ca. 70% and 0.083 ± 0.008 W/mK, respectively. Rietveld refinements, using both ex - and in situ XRPD data, were performed in order to elucidate the phase stability of the formed binder up to 1200 °C. The results showed that the amorphous matrix partially crystallized in tridymite and cristobalite type structures of AlPO 4 –SiO 2 solid solutions at about 700 °C. At 1000 °C, 3:2 mullite started to crystallize, possibly from unreacted metakaolinite, resulting in co-crystallization of SiO 2 cristobalite. At the same time, the amount of tridymite-type structure decreased, possibly due to selective phase transformation of AlPO 4 tridymite to cristobalite, leaving behind the SiO 2 isostructure. The geopolymer paste composition allows to tailor the mullite content in the refractory foam.