Tridymite

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

Redox equilibria and the structural states of ferric and ferrous iron in melts in the system CaO–MgO–Al2O3–SiO2–Fe–O;: relationships between redox equilibria, melt structure and liquidus phase equilibria

Abstract: Relationships between melt structure and redox equilibria of iron in CaO-AlrOr-SiOr-Fe-O and MgO-AlrOr-SiOr-Fe-O melts with CalAl and Mg/Al > 0.5 have been determined with M 0.5, and undergoes a gradual coordination transformation in the Fe3*pFe range between 0.5 add 0.3. In this Fe3+pFe-range, tetrahedrally- and octahedrally-coordinated ferric iron may coexist. The temperature-dependence of the Mcissbauer hyperfine parameters and the temperatureindependence of the intensity ofthe absorption envelope are consistent with a local structural unit that may be stoichiometrically similar to Fe.On. The Fe2+/Fe3+ is linearly correlated with polymerization (nonbridging oxygens per tetrahedrally coordinated cations, NBOru) and Al/(Al + Si) of the melt. There are linear relationships between log (Fe2*7Fe3+) and log fo, and between log (Fe2+/Fe3*) and 1/T (absolute temperature). The standard-state free energy of reduction of ferric to ferrous iron, calculated from these lines, decreases with increasing Zlrz (ionization potential) of the alkaline earth metal cation, with decreasing bulk melt NBO/T (more polymerized melts) and with decreasing AV(AI + Si) of the melt. In magnesium aluminosilicate melts with NBO/[:0.6 and AV(AI + Si):0.2 (typical values for quartz tholeiite and basaltic andesite) with 10 wt.% iron oxide added as FerO, the liquidus phase is tridymite when equilibrated with air. Calculations indicate that at fo" between 10-2 and 10-3 atm the liquidus phase changes to protoenstatite, and then to forsterite at even lower oxygen fugacities. Substitution of Ca (or Na) for Mg results in expansion of the metasilicate (pyroxene) liquidus field and contraction of that of tridymite. Fractional crystallization trends of magmatic liquids are, therefore, significantly dependent on oxygen fugacity, degree of polymerization of the magma (NBOA), Al/(Al + Si) and the relative abundance of alkali metals and alkaline earths.

DTA and X‐Ray Analysis Study of Nucleation and Crystallization of MgO‐Al2O3‐SiO2 Glasses Containing ZrO2, TiO2, and CeO2

Abstract: Crystallization sequences of glasses with compositions in the tridymite primary phase field of the MgO-Al2O3-SiO2 system were studied by DTA, X-ray diffraction, and other techniques. Crystallization was catalyzed by the addition of 7 wt% of either ZrO2 or TiO2. Up to 10 wt% CeO2 was also added to some glasses. Metastable solid solutions with the high-quartz structure exhibiting varying lattice parameters commonly occurred at low temperatures, transforming into a high cordierite at higher temperatures. Depending on the composition and heat treatment, other phases also appeared, e.g. Ce2Ti2O4 (Si2O7). The rate of crystallization was markedly dependent on the catalyst. Colloidal precipitation of the catalyst accompanied by bulk crystallization of the glass was observed with ZrO2, but no crystalline TiO2 was detected. In the presence of CeO2, TiO2 was a more effective catalyst than ZrO2. Although CeO2 lowered the melting temperatures of the glass-ceramics, it increased the stability of the glasses and inhibited volume nucleation, causing coarse structures to form on crystallization.

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.