Kinetic and microstructural studies of the crystallisation of coesite from quartz at high pressure

High pressure single crystal X-ray diffraction study on α-quartz

Single crystal X-ray diffraction experiments on α-quartz (trigonal, space group P3121) were performed to pressures up to 19.3 GPa using synthetic samples.

Volumina of the unit cell were determined to 13.1 GPa, by combining the data with data taken from the literature; the bulk modulus and pressure derivative calculate to 38.7(3) GPa and 5.2(1) respectively according to a Birch-Murnaghan equation of state.

Intensity data were collected at 10.9(1), 12.0(1), 12.1(1), 12.6(1) and 13.1(1) GPa. From the five intensity data sets, one was collected using synchrotron radiation at HASYLAB/DESY and the other four using a conventional X-ray tube. Results show that the SiO4 tetrahedra, the building blocks of the structure, become increasingly distorted as pressure is increased but no significant change in the polyhedral volume is observed. The volume reduction is mainly compensated by the decrease in the inter-tetrahedral Si–O–Si-angle, which is correlated to the increase in the tilt angle φ.

In addition to the five intensity data sets collected, X-ray diffraction experiments were carried out up to 19.3 GPa in order to address the question of when the structure undergoes amorphization. It was observed that up to 19.3 GPa, no pressure-induced amorphization takes place. This result is in agreement with studies carried out on powder quartz and theoretical studies.