Recrystallized cristobalite

The phosphates of the incorporated metals are usually Insoluble in water. The crystallization of these phosphates or of an aluminophosphate with higher density may take place in different ways. It can be amorphous, crystallize in different structures on or beside the ALPOij-5 or SAPO-5 structure and may cause a recrystallization cristobalite- or tridymite-. like aluminophosphate. 

Siliceous oozes are accumulations of opaline silica (opal-A, an amorphous phase of high water content and porosity) in the tests of diatoms, radiolarians, and/or silicoflagellates. Opal-A solubility at 25 °C is 60-130 ppm Si02(aq) (e.g., Williams etal., 1985), and solubility increases with increasing temperature and pressure (Walther and Helgeson, 1977). Adsorption of aluminum and iron on the surfaces of siliceous tests decreases their solubility (Her, 1955 Lewin, 1961). Opal-A is a metastable phase that with burial eventually recrystallizes to quartz, often with another metastable intermediary phase, opal-CT (e.g., Hein et ai, 1978 Williams et ah, 1985 Williams and Crerar, 1985). Opal-CT structurally resembles an inter-layering of the two silica phases, cristobalite... 

Q-Quartz, which has a trigonal crystal structure, undergoes a rapid, reversible transition to hexagonal /J-quartz at 573 °C and then slowly changes to hexagonal /3-tridymite at about 870 °C tridymite in turn goes over slowly to cubic /3-cristobalite at 1470 °C, and this melts at 1713 °C. The reversion of cristobalite and tridymite to quartz is slow, so that these forms can exist at room temperature (as a-modifications). In addition, dense modifications with six-coordinate Si are found in shocked rocks associated with meteorite impact craters coesite forms only above 450 °C and 3.8 GPa, and stishovite requires over 1200 °C and 13 GPa. Survival of those metastable polymorphs on the geological timescale is evidence of an extremely slow recrystallization rate. 

Extended experiments with NiO show that the Ni is not incorporated into ALP0i)-5 and SAPO-5 crystals. In the presence of NiO pure ALPO/j-5 and SAPO-5 crystallize which undergo recrystallization to a cristobalite like phase. 

Generally the presence of an excess of the Me-ions favours some recrystallization to denser phosphate phases or to cristobalite or tridy-mite. 

In Fig. 4 a series of experiments with different Ni- and Si contents are summarized. It can be seen that after 6 hours a batch with 0.2 Si02 and 0.2 NiO has a crystallinity of more than 90 % compared with a SAPO-5 batch with 0.4 Si02. It can be seen that the sample containing only Si02 is stable for the 48 hours observation time. The Ni-containing phases show a recrystallization to cristobalite. The Si02 increases the life time of the metastable SAPO-5 phase. 

Fig. 5. Kinetics of the crystallization of a batch AI2O3 P2O5 0.4 FeCl2 2 PC3N 50 H2O and the recrystallization to cristobalite (open circles)...

With an increasing Si02/Al203 ratio of the reaction mixture at constant PDDA-Cl content the crystallization is accelerated, being kenyaite as the preferred phase (Table 11-4). Cristobalite is also formed upon recrystallization. 

The most common phases obtained through recrystallization from the various zeolite types are feldspars, such as celsian and high- sanidine, feldspatoids, such as the above mentioned carnegieite and nepheline, and the high-temperature silica polymorphs (tridymite and cristobalite), which are usually the recrystallization products of high-silica zeolites, such as ZSM-5 [48]. 

Posters on the following topics were prepared for display at the Steering Committee Meeting NZP Ceramic Composites, B-Cristobalite, Recrystallization and Corrosion of 6-eucryptite, Lightweight NZP Ceramics, and Thermal Properties of (Ca.Mg)NZP. 

If this phase transformation is a sequential process where even crystalline phases are transformed to other crystalline phases, characteristic changes observable in the stability range (D) of phase I must correspond to the incubation period (BO for the second crystalline phase (II), where the nucleation of the subsequent product takes place. As a consequence, the recrystallization process (period E) of phase I and the growing period (CO of phase II are of the same time scale. This sequential recrystallization process finally leads to thermodynamically more stable products such as cristobalite and quartz. 

The sequence of consecutive products and also the morphology of the synthesis products can be altered by addition of inorganic salts to the synthesis mixture [36,73]. Addition of LiCl, KCl, RbCl, and MgCl2 retards nuclei formation. However, it accelerates recrystallization to cristobalite, kenyaite, and quartz [76]. According to (DE-OS 31 23 000), the formation of magadiite from silica gel and sodium hydroxide solution is accelerated by copper sulfate and ammonia. Bergk et al. [73] observed that the sulfate anion shortens the incubation period of the magadiite crystallization process much more than nitrate or chloride anions. 

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