Cristobalite molar volume

Silica (Si02) exists in several forms, including quartz (molar volume 22.69 cm moU ) and cristobalite (molar volume 25.74 cm moU ). 

As shown in Table VI, the tetrahedra in PON are less regular than in Si02 because of the presence of two different anions. The molar volumes of the ct-quartz and /3-cristobalite phases of PON are compared in Fig. 7, as a function of pressure (200, 203). 

Fig. 7. Molar volumes of quartz and cristobalite polymorphs of PON as a function...

The occurrence of the moganite-type phase in PON confirms that moganite is really a new structure type in AX2 compounds. The molar volumes at ambient of the three phases, cristobalite, moganite, and quartz (22.58, 21.05, and 20.64 cm3, respectively) follow the same trend as in silica. The smaller size of the P02N2 tetrahedron relative to that of Si04 (cf. Table VI) widens the pressure fields of stability, so that PON appears to be very useful to shed light on the detailed behavior of silica, which is still ill defined. 

Fig. 26.6. Variation in mineral volumes over a kinetic reaction path designed to illustrate Ostwald s step sequence. The calculation traces the reaction at 25 °C among the minerals amorphous silica (tine line), cristobalite (medium line), and quartz (bold line). The top diagram shows results plotted against time on a linear scale the time scale on the bottom diagram is logarithmic. The decrease in total volume with time reflects the differing molar volumes of the three minerals.

Indeed, the quantum-chemical structure optimizations confirm the preference of V + for lower coordination numbers. Under the assumption of zero pressure, the lowest energy is found for the case of a-cristobalite (four-fold coordination, molar volume = 41.1 cm /mol), followed by quartz (four-fold, 33.2 cm /mol), followed by anatase (six-fold, 19.8 cm /mol), followed by bad-deleyite (seven-fold, 16.6 cm /mol) and several others. In order to arrive at stable structures, however, a minimum thermochemical requirement is that the total energy of VON is lower than those of its educts. Here we rely on the plausible reaction... 

Generally speaking, an increase in density can occur upon a polymorph transformation even without change in coordination of the nearest atoms in the structure. An example of such densiflcation is presented by the SiOz modifications, where a more compact packing of silicon-oxygen tetrahedra reduces the molar volumes in the succession cristobalite (25.9cm ) keatite (24,0cm ) quartz (22.7 cm ) -> coesite (20.0cm ). However, the transition under pressure from coesite (Nc=4) to stishovite (Nc = 6) reduces this volume much more drastically, to 13.8 cm. ... 

The study of the products of the oxidation reaction of powders or monociystals (SiC) up to temperatures of 1,600°C shows that the silica layer is amorphous and then crystallizes gradually into cristobalite finm 1,100°C onwards. For sintered materials that contain impurities or sintering additives, the nature of the formed products is much more complex (silicates, aluminosilicates, etc.). However, for all these materials, the oxidation reaction results in a volume expansioa The coefficient of expansion, which represents the ratio of the molar volume of silica or silicates on the molar volume of SiC (or Si3N4>, is higher than one. The reaction product is covering and protective. 

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