Tridymite

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

Über Erdalkalimetalloxogallate, VI Zum Problem der Mischkristallbildung zwischen CaGa2O4 und CaAl2O4 / On Alkaline Earth Metal Oxogallates, VI About Mixed Crystal Formation of CaGa2O4-CaAl2O4

Abstract: Mixed crystals of the formulae CaGaAlO₄ (1) and CaGa₀.₅Al₁.₅O₄ (2) were investigated by X-ray single crystal work. 1 shows a tetrahedral orientation like that of CaGa₂O₄ (monokl.), 2 another one of CaAl₂O₄. A discussion of the different polyhedral arrangement of the stuffed tridymite structures is given.

High-temperature transformation of changbaishan diatomite and the formation mechanism of cristobalite

Abstract: The samples of Changbaishan diatomite and amorphous silica (99.9% purity) were thermally treated at 1250 ℃. The products were analyzed by X-ray diffraction and FTIR. The results showed that Changbaishan diatomite was transformed to cristobalite at about 1100 ℃ and amorphous silica at about 1200 ℃, which are in the thermodynamic stable condition of tridymite. The authors proposed that the metastable formation mechanism of cristobalite is controlled by dynamical process and is related to the similarity in the intermediate range order structure between diatomite/amorphous silica and cristobalite, which lowed the nucleation activation energy of cristobalite so that the nucleus of cristobalite was easy to form. In the serial reactions of diatomite/amorphous silica→cristobalite→tridymite, the reaction of diatomite/ amorphous silica that is transformed to cristobalite is dominant to the reaction of cristobalite to tridymite, and the latter transformation reaction is almost prohibited under experimental temperature conditions. The temperature difference of about 100 ℃ between Changbaishan diatomite and pure amorphous silica is possibly related to the defects and impure elements such as Na, Al in diatomite. An essentially large amount of water in diatomite is propitious to lower the transformation temperature of amorphous silica to cristobalite. The dehydration of Si-OH group under high temperature led to the transformation of abridging oxygen of OH to bridging oxygen, which caused the disappearance of structure defects and formed the long-range order structure of cristobalite. Impure elements such as Na, K, Ca, Fe formed the multi-component system with SiO_(2), which lowered the crystalline temperature of SiO_(2). On the other hand, the strong diffusion property of impure elements is favorable to promoting the dehydration velocity of Si-OH group and defects transfer ratio, and also increasing the structural stability of cristobalite.

Synthesis of zeolite beta directly from rice husk ash: effect of reaction composition on crystallinity of zeolite beta

Abstract: White rice husk ash obtained from complete uncontrolled burning of rice husk contains more than 94% silica. The ash, which consists of crystalline silica of the type tridymite and -crystobalite, was used directly as a source of silica in the synthesis of zeolite beta. The mole oxide ratio of the initial gel of 1.25-8Na2O: 10-120SiO2: Al2O3: 1-20TEA2O: 150-1000H2O was prepared and heated at 150C in a static condition for 6 d. The solid phases formed were monitored by XRD technique. Influence of reaction mixture ratio in the initial gel to the crystalline products formed was studied. Results showed that the pure zeolite beta was formed in a certain range of reaction mixture, i.e.: 1.25-4Na2O : 15-45SiO2 : Al2O3 : 4-10TEA2O : 240-480H2O. The other ratio of reaction mixtures produced crystalline phases such as analcime, Na-P, mordenite, and gismondine, and non-reacted of -crystobalite and tridymite.

Phase relations in the system ZnONb2O5SiO2

Abstract: Phase relations in the ternary system ZnONb2O5SiO2 have been determined at liquidus and sub-solidus temperatures. The following compounds have been encountered as primary phases: ZnO, 3ZnO·Nb2O5, α-ZnO·Nb2O5, β-ZnO·Nb2O5, 2ZnO·17Nb2O5, HNb2O5, silica (either as tridymite or as cristobalite) and α-2ZnO·SiO2. No ternary phase has been encountered in this system. Five ternary invariant points have been located, four of which are eutectics, the remaining one being a peritectic. Sub-solidus compatibility relationships have also been determined. This ternary system was studied using hot-stage microscopy, quenching, and X-ray diffraction techniques.