Tridymite

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

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.

Transformation of tridymite to cristobalite below 1470° C in silica refractories

Abstract: Because silica refractory has good volume stability and creep properties at high temperature, it has been used in several furnaces. However, silica has three polymorphs (quartz, tridymite and cristobalite) and each polymorph has anα-β type transformation. It is known that cristobalite is the stable phase of silica between 1470° C and the melting point of silica refractories. However, sometimes cristobalite was found in silica refractories used in the stable temperature region of tridymite. Therefore, the cause and mechanism of this tridymite-to-cristobalite transformation below 1470° C was studied. Although the transformation temperature of tridymite to cristobalite was also 1470° C in the sample used in this research, it decreased on addition of Al2O3. The apparent activation energy of the tridymite-to-cristobalite transformation was found to be 787 kJ mol−1 above 1470° C and 176 kJ mol−1 below 1470° C with Al2O3 by measuring the transformation rate. It was also observed using EPIVIA that the tridymite included CaO; however, CaO and Al2O3 were located on the outside of the cristobalite which was produced below 1470°C. Therefore, it is supposed that the liquid phase was produced by the penetration of Al2O3, and impurities in the tridymite crystal diffused outside and then silica was precipitated as cristobalite.

Two forms of aluminium phosphate tridymite from X-ray powder data

Abstract: Monoclinic and triclinic (pseudo-orthorhombic) AlPO4 tridymites have been refined from X-ray powder diffraction data using the silica analogues as starting models. The framework structures of both forms of tridymite are made up of six-membered rings of tetrahedra which differ in the distortion patterns of the ring shapes. Ordered occupation of alternate tetrahedra by Al and P leads to a doubling of the a lattice parameter for monoclinic AlPO4 tridymite (space group Pc) and loss of the C-centring with respect to the isotypic silica tridymite (space group Cc). Triclinic AlPO4 tridymite was refined in the same space group (F1) as the SiO2 analogue.

Comparison of radial distribution function for silica glass with those for various bonding topologies: Use of correlation function

Abstract: Radial distribution functions (RDF's) calculated from the bonding topologies of quartz, cristobalite, tridymite and a 1412 atom model of silica glass (35 A in diameter) have been compared with the experimental RDF for silica glass. The functions, 4πr3(ϱ(r)−ϱ0), computed over the range 0 < r < 20 A from the known structures of quartz, cristobalite, tridymite and the 1412 atom model have been compared with the corresponding function for silica glass by means of correlation functions, giving 0.26, 0.69, 0.82 and 0.91, respectively (0.00 would indicate no correlation, and 1.00, perfect positive correlation). Cristobalite and tridymite are composed entirely of six-membered rings of silicate tetrahedra, whereas the model contains both six- and five-membered rings in a ratio of 2.6:1. The correlation coefficients suggest that the six-membered rings of the type present in tridymite play a dominant role in silica glass, but that other ring sizes are important and result in the higher correlation coefficient for the 1412 atom model. While the correspondence in RDF's is encouraging, the model RDF does not fit the experimental curve to within the accuracy of the experiment. Perhaps the model is not large enough to represent adequately, in a statistical fashion, all the configurations present in a macroscopic sample.