Tridymite

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

Crystallization Equilibrium in Nepheline-Albite-Silica Mixtures with Fayalite

Abstract: This paper presents the results of a thermal study of the system, $NaAlSiO_{4}-FeO-SiO_{2}$, and discusses their application to petrology. The crystalline phases formed in the various compositions are cristobalite, tridymite, albite, fayalite, nepheline, carnegieite, hercynite, and wustite. There are two ternary eutectics, one between tridymite, albite, and fayalite (temperature $980^{\circ} \pm 10^{\circ} C.$), which may be regarded as a simplified fayalite rhyolite, and the other between nepheline, albite, and fayalite (temperature $990^{\circ} \pm 10^{\circ} C.$), which may be regarded as a simplified fayalite phonolite. These and other findings are discussed in connection with the origin of natural phonolites, trachytes, and rhyolites carrying fayalite.

Effect of synthesis pH and H2O molar ratio on the structure and morphology of aluminum phosphate (AlPO-5) molecular sieves

Abstract: AlPO-5 molecular sieves were prepared by the hydrothermal reaction of a gel mixture with the following compositions: Al2O3 : P2O5 : Et3N : H2O = 1:1:1.5:x, where x is between 100 and 750 H2O molar ratio. The structure and morphology of the AlPO-5 molecular sieves depend on the gel mixture's composition, hydrothermal temperature, hydrothermal reaction time, and pH. Without pH control, the AlPO-5 structure changed from a spherical shape at H2O = 100 to a hexagonal pillar shape at H2O = 450. With pH control in the range of about 2.5-3.5, the hexagonal pillar crystals began to form at H2O = 100 and an island of hexagonal pillars with radiation form appeared at H2O = 300-450 due to the formation of a tridymite type of dense AlPO4 phase. It appears that the formation rate of hexagonal pillar crystals to form a dense AlPO4 phase is favorable under acidic conditions, and an amorphous AlPO-5 structure forms under basic conditions. Thus, the H2O concentration and pH value have a dramatic effect on the AlPO-5 structure.

Thermally induced phase transitions in tridymite: an infrared spectroscopy study

Abstract: The temperature dependence of the infrared active modes of meteoritic and synthetic tridymite have been investigated between 23 K and 1073 K in IR absorption and IR emission experiments. At room temperature both tridymite samples consist of a mixture of low temperature forms, in different proportions, due to the grinding. The sequence of phase transitions in Steinbach tridymite deduced from the IR data agrees well with recent X-ray and calorimetry studies using identical samples (Cellai et al. 1994). The previously suspected structural phase transition P6322⇔P63/mmc is confirmed by the disappearance of the 470 cm-1 mode and a temperature anomaly of the spectral shift of the 790 cm-1 mode. Changes in the infrared spectra of synthetic tridymite give a different sequence of phase transitions from those of the meteoritic sample, consistent with the structural phase transitions observed in a 29Si MAS NMR investigation using the same sample (Xiao et al. 1993).