Tridymite thermal expansion

Irradiation by fast neutrons causes a densification of vitreous silica that reaches a maximum value of 2.26 g/cm3, ie, an increase of approximately 3%, after a dose of 1 x 1020 neutrons per square centimeter. Doses of up to 2 x 1020 n/cm2 do not further affect this density value (190). Quartz, tridymite, and cristobalite attain the same density after heavy neutron irradiation, which means a density decrease of 14.7% for quartz and 0.26% for cristobalite (191). The resulting glass-like material is the same in each case, and shows no x-ray diffraction pattern but has identical density, thermal expansion (192), and elastic properties (193). Other properties are also affected, ie, the heat capacity is lower than that of vitreous silica (194), the thermal conductivity increases by a factor of two (195), and the refractive index, increases to 1.4690 (196). The new phase is called amorphous silica M, after metamict, a word used to designate mineral disordered by radiation in the geological past (197). 

Normal ice is analogous to tridymite water has the quartz structure with a greater density. The occurrence of the density maximum at 4°, a unique property of water, must be attributed to the gradual transition of the tridymite structure into the quartz-like structure, while at higher temperature the normal thermal expansion again gets the upper hand. 

The thermal expansion curves of the individual Si02 modifications are plotted in Fig. 2, which also illustrates the discontinuous change in the specimen size occurring at the inversion temperature. The high-temperature quartz exhibits a quite rare anomaly, namely a negative coefficient of expansion in all crystallographic directions. On its expansion curve, tridymite likewise exhibits a peak followed by contraction. 

FIG. 3. Linear thermal expansion coefficient a of tridymite versus temperature (Austin, 1954). 

The variation in the coefficient of thermal expansion of quartz and cristobalite with temperature reveals very strikingly the a-P inversions, and is best seen by plotting the percentage volume expansion for each mineral against the temperature (Figure 9). Owing to the difficulty of obtaining pure tridymite its expansion... 

When standard tridymite is cooled below 380 C, several phase inversions occur with various changes in symmetry. These tend to produce a large shrinki e and therefore a high thermal coefficient of expansion between 0 -200 C, almost 400 X lO K-h... 

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