Low cristobalite

The transformation of opal-A to opal-CT generally begins at 35-50 °C, corresponding to burial depths of hundred of meters. In some environments this temperature may be as low as 17-21 °C (Matheney and Knauth, 1993 Monterey Formation) or even 0-4 °C (Botz and Bohrmann, 1991 Antarctic deep sea). The acoustic properties of the sediment are altered during the transformation to opal-CT, typically providing an acoustic reflector of the diagenetic front (Calvert, 1983 Tribble et al., 1992). Opal-CT, also known as porcelanite, exhibits X-ray characteristics of low cristobalite and tridymite (Figure 4). This mineral exists as... 

The enthalpy of formation is calculated from that of low cristobalite by addition of difference between H (543 K)-H (298.15 K) for low and high forms. 

Low cristobalite is metastable with respect to quartz but persists up to is taken as the temperature at the peak... 

Figures 1 and 2 show the change in solubility of the acid-cleaned radiolarian and sponge spicule assemblages at 26° 1°C and 3° 1°C as a function of geologic age. Also shown are the estimated values of the high and low cristobalite and low quartz solubilities at these two temperatures. The open and crossed circles represent the initial leveling off of the concentration of Si (OH) 4 vs. time curves, and the dots are the...

In this chapter, the main lines of this new theory of chemical bond are presented in details and then it is applied to representative tetracoordinated and hexacoordinated silica polymorphs, namely low quartz, low cristobalite stishovite, the recently discovered CaCl2-like high pressure phase and a prototype fluorite structure in which silicons are octacoordinated. 

We performed MD simulations of low-quartz, low-cristobalite, coesite and stishovite, i.e., virtually all the natural polymorphs of silica so far known. These polymorphs correspond to different pressure-temperature regimes but can also exist at normal pressure and temperature. Our interests are (1) whether the interatomic potential derived from the cluster calculation is applicable to bulk crystals, (2) whether various physical properties of polymorphs are reproduced by the same interatomic potentials, and (3) whether pair-potential approximation is valid for silica. 

Figure 4. Atomic configurations averaged over time steps obtained in the MD simulation of low-quartz (a), low-cristobalite (b), eoesite (c) and stishovite (d).

Another novel crystalline structure is derived from low-cristobalite at —17 GPa (Fig. 9(b)). The new structure has a space group of Cmcm and... 

Figure 9. Novel structures of silica obtained by compression of known polymorphs, (a) The Cl phase from low-quartz, and (b) the Cmcm phase from low-cristobalite.

Although the Cmcm phase of silica has not been found by real experiments, it has been shown that low-cristobalite in fact undergoes pressure-induced structural transformations at room temperature [19, 50], Very recently it was also confirmed that the displacive transformation from low-cristobalite structure to the Cmcm structure really occurs in case of c-GaP04, which probably has larger ionicity than silica. It should also be mentioned that Ga atoms easily turn into sixfold coordination, while P preserves fourfold coordination at high pressure. This tendency is in harmony with the alternate arrangement of the fourfold and sixfold coordinated cations in the Cmcm phase. 

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