A-cristobalite

The behavior of cristobalite PON has been studied as a function of pressure. No in situ evidence for pressure-induced amorphization was noticed. Whereas cristobalite Si02 displays four crystalline phases up to 50 GPa (195), PON remains in a cristobalite phase (193, 196). By using Raman spectroscopy and synchrotron X-ray diffraction, Kingma et al. (193, 197) observe a displacive transformation below 20 GPa to a high-pressure cristobalite-related structure, which then remains stable to at least 70 GPa. The high value of the calculated bulk modulus (71 GPa) (196) is indicative of the remarkable stiffness of the phase. 

In addition to the external forces, the catalyst must also resist internal forces imposed on the pellet as phase transitions in the catalyst material progress. These transitions, including e.g. transformation of the amorohous silica carrier into crystalline a-cristobalite, precipitation of V4+ and compounds, and destruction of the carrier by the melt, may eventually cause the catalyst to break up in smaller particles or even to catalyst powder. 

Figure 21. Experimentally (trydimite) and theoretically (a-cristobalite) structures proposed for the polymeric carbon dioxide.

Figure 3.37. XRD spectra of (a) tetragonal a-cristobalite and (b) chemically stabilized cristobalite (CSC, CaO 2Al2O3 40SiO2 or 1 2 40 composition) at room temperature with Ca and A1 dopants, which exhibits the structure of the high-temperature 0-phase.

In earlier literature reports, x-ray data of a-based ceramics, the /3-like phase observed in certain silica minerals was explained by a structural model based on disordered Q -tridymite. However, others have suggested that the structure of the stabilized jS-cristobalite-like ceramics is closer to that of a-cristobalite than that of Q -tridymite, based on the 29Si nuclear magnetic resonance (NMR) chemical shifts (Perrota et al 1989). Therefore, in the absence of ED data it is impossible to determine the microstructure of the stabilized jS-cristobalite-like phase. ED and HRTEM have provided details of the ceramic microstructure and NMR has provided information about the environments of silicon atoms in the structure. Infrared spectroscopy views the structure on a molecular level. 

Figure 3.38. (a) High concentration of twins and faults in a-cristobalite in (010) (b) CSC with no faults (inset ED pattern with streaks). 

Cristobalite is transparent. We do not normally consider devitrified glass as a transparent material. However, once fused silica has cooled below 250°C, B-cristobalite is transformed into a-cristo-balite. This substance is the white opaque material we usually associate with devitrified silica. When fused silica is reheated into the devitrification range, the a-cristobalite turns back into B-cristobalite. However, because a-cristobalite has many fissures and cracks, the opacity remains when it is reheated back into B-cristobalite. 

To check on the dependence of AHt on crystallinity, a cristobalite sample was prepared at temperatures high enough to cause a phase transformation (22), and it was assumed that at this temperature any amorphous surface layer would also crystallize. As predicted, the Afi/s increased (approximately 200 ergs per sq. cm.) above the original quartz values. Difficulties of interpretation were not completely obviated, since the surface areas differed by a factor of 2. Also, the Aff/s were for two different crystalline modifications and thus would not be expected to display exactly the same interaction energies with water. 

ZrSiO and low-intensity lines of monoclinic (0.316, 0.284, 0.219, 0.180 nm) and tetragonal (0.295 nm) zirconium dioxide and a-cristobalite (0.405, 0.169 nm). The X-ray pattern of mechanically activated mixture 3 contains lines of zircon and only two low-intensity lines of a-cristobalite. The X-ray pattern of a mechanically activated and thermally treated mixture 2 is practically identical to that of mixture 3, and X-ray pattern of mixture 4 contains weak reflections of monoclinic and tetragonal zirconium dioxide. 

Table 7.2. Experimental and calculated unit-cell parameters and atom positions for a-cristobalite, and their percent difference. The numbers in parentheses are the standard deviations of the last significant digit for the time-of-flight neutron powder diffraction data. The atom locations are given in units of the primitive translation lengths a = b and c...

Newton, M. D., M. O Keeffe, and G. V. Gibbs (1980). Ab initio calculation of interatomic force constants in HjSijO, and the bulk modulus of a-quartz and a-cristobalite. Phys. Chem. Mineral. 6, 305-12. 

Figure 4 X-ray diffraction patterns contrasting various crystallinities of silica (a) radiolarian silica, Porcelanite (opal-CT) and a-Cristobalite (made by heating silica gel at 1,350 °C for 4 h) from Calvert (1983) (b) diatom assemblage from Antarctic plankton tow, deep-sea siliceous ooze (Holocene in age) from beneath the Antarctic Polar Front, and two chert deposits from state of New York. The sharpness of the silica peak(s) between 20° and 26° two theta increases as silica undergoes diagenetic transformation from a fresh-diatom assemblage to buried sediment for...

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