A team of scientists at Carnegie University has discovered a new form of silicon dioxide, or silica, one of the most abundant natural materials on Earth, as well as a very common material in Earth's crust and mantle.
Silica is also a much-studied material due to its properties that imitate what happens in deep Earth as it transitions through phases based on temperature and pressure.
Most fascinating, though, is that the Carnegie scientists discovered a new transitory form of the material by exerting extreme pressure on other phases of silica at room temperature.
"Scientists have long debated whether a phase exists between the four- and six-oxygen phases," says Dave Mao, one of the Carnegie University researchers. "These newly discovered four transition phases and the new phase of post-stishovite we discovered show the missing link for which we've been searching."
Before this new discovery, scientists knew of two distinct phases of silica, including the high-pressure, high-temperature dense coesite. We're most familiar with this phase of silica, which is also known as quartz, a major component of sand. Coesite contains atoms of silicon surrounded by four oxygen atoms and is often used for the manufacture of products such as microchips, glass and cement.
When exposed to even more pressure and higher temperature, coesite converts to stishovite, an even denser form of the material that has atoms of silicon surrounded by six oxygen atoms.
However, it's the transition between these phases that most intrigued the Carnegie team because that transition could tell us much about the pressures of deep Earth. Other studies have shown phases with even higher pressures than stishovite, most often referred to as post-stishovite.
In their study, Carnegie's researchers took one crystal of coesite at room temperature and exerted pressure on it from 257,000 to 523,000 times normal atmospheric pressure. The coesite converted to four other crystalline phases before becoming an extremely dense post-stishovite. This uncovered a new unknown phase of silica and debunked previous theories that the transition phase wasn't crystalline.
In the below image, we see the transition from coesite with its four oxygen atoms (blue) to its post-stishovite form with its six oxygen atoms (green). Between the two orange peaks are the new phases scientists discovered.
"This work sheds light on the long-debated pressure-induced amorphization phenomenon of SiO2, but also provides new insights into the densification mechanism of tetrahedrally bonded structures common in nature," write the study authors.
