Meteorite Impacts May Create an Unexpected Form of Silica

X-ray diffraction images showing the new form of silica created by sending an intense shock wave through a sample of quartz using a specialized gas gun. Credit: Sally June Tracy, Stefan Turneaure, and Thomas Duffy.

Quartz, one of the most abundant minerals in Earth’s crust, has been one of the most extensively studied materials in the field of meteorite impacts. Despite 50 years of active research, questions remain concerning the structure and transformation of quartz under shock compression. New work led by Sally June Tracy of the Carnegie Institution for Science examined the crystal structure of the silica mineral quartz under shock compression and obtained results that challenge longstanding assumptions about how this ubiquitous material behaves under such intense conditions. To mimic meteorite impacts, the team used a specialized cannon-like gas gun to accelerate projectiles into quartz samples at extremely high speeds, resulting in shock pressures up to 65 GPa. In-situ synchrotron x-ray diffraction instruments were used to determine the crystal structure of the material that forms less than one-millionth of a second after impact.

The results suggest a quartz structure transformation that was not previously anticipated. Generally, it has been assumed that silica minerals such as quartz, under these conditions, would transform into either a dense crystalline form known as stishovite or a dense glassy structure. However, the results reveal that quartz undergoes a phase transformation to a disordered metastable phase, whose structure is intermediate between fully crystalline stishovite and a fully disordered glass. Once the initial peak pressure subsided, the new structure could not be maintained, but the material may be left in a densified glass state, rather than returned to its original low-pressure form. These new results should help scientists build a better understanding of how silica-rich materials behave in high-temperature, high-pressure settings such as terrestrial impact craters or the deep mantle, as well as during planetary formation and evolution. The chemical phases produced by the sudden shock, pressure, and heat of a meteorite impact can be incredibly short-lived, but these processes helped form planets. READ MORE