A-tridymite

The structure of CO2-V has been determined by x-ray diffraction [343], and the observed pattern could be reasonably fitted by using a tridymite-type structure (orthorhombic P2 2 2 lattice) shown in Fig. 21. The formation and structure of polymeric carbon dioxide has been smdied by computational methods [344—348] in order to fuUy characterize this novel material however. 

SYNS CHRISTENSENITE SIUCA, CRYSTALLINE-TRIDYMITE (ACGIH, OSHA) TRIDLMITE (FRENCH) TRIDYMITE a-TRIDYMITE TRIDYMITE 118... 

Consider the following systems at 25 C and 1 atm pressure (a) equal amounts of the minerals Si02 (a-quartz) and Si02 (a-tridymite) ... 

The following sequence of inversions takes place during gradual heating up of low-temperature quartz at 573 °C, jS-quartz is very rapidly inverted to a-quartz, which is stable up to about 1025 °C when it has high purity (content of impurities < < 10 %). Itis then converted to cristobalite. If the quartz contains more impurities (solid solutions), then at about 870 °C a-quartz is converted to a-tridymite which in turn is transformed to a-cristobalite above 1470 °C, Non-equilibrium fusion of quartz at temperatures above 1400 —1450 °C was often observed cristobalite is then formed secondarily from this melt. 

Figure 3.28 Crystallization phase diagram of system 1.0AI2O3 1.0P2Os x[Co(Cp)2]-OH yPr2NH 190 H20 in the presence of two templates at 160°C. API -fa AEL noncrystalline AFI/AST AFI/tridymite A tridymite. Reproduced with permission from [119]. Copyright (1999) Witey-VCH...

When the divalent cation becomes too large for the spinel structure to be adopted (e.g. Ca +, Ba +, and Pb +), a tridymite (SiOg) structure is obtained that has the divalent cation in the interstices (see Silicon Inorganic Chemistry). These are synthesized by reaction of the oxides at temperatures over 800 °C. The barium and calcium derivatives are used in ceramics and refractories. In particular, BaAl2 04 is used... 

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