Temperature-induced phase transition in quartz nanocrystals dispersed in pseudotachylite

The size and concentration of $\alpha$-quartz nanocrystals dispersed in samples of pseudotachylite and the internal stresses in these nanocrystals have been determined using infrared spectroscopy in the temperature range 300–800 K. Pseudotachylite is a product of intense crushing of granite that undergoes in the Earth’s crust faults. It has been found that the size of the nanocrystals is $\sim$20 nm and does not depend on temperature. As the temperature increases, their concentration decreases monotonically and tends to zero at $\sim$650 K. This process is paralleled by a growth of the concentration of $\beta$-quartz nanocrystals. The $\alpha$-quartz nanocrystal concentration regains its initial level with decreasing temperature. Thus, the $\alpha\to\beta$ phase transition in quartz nanocrystals in pseudotachylite starts at temperatures lower by $\sim$500 K than that in the bulk of the macrocrystal (846 K), and is stretched by $\sim$350 K. At room temperature, the unit cell of nanocrystals is compressed by surface tension forces. These forces retard the $\alpha\to\beta$ phase transition. The thermal expansion coefficient of nanocrystals is larger than that of macrocrystals, which entails a decrease of compression and a monotonic decrease of the concentration of $\alpha$-quartz nanocrystals with increasing temperature.