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New Stanford research reveals the secrets of stishovites, a rare form of crystallized sand

Lasers are nothing like meteor strikes, but in the nanosecond when each strike silicon dioxide, the main ingredient in coastal sand, stishovites form. Understanding how this rare crystal form will help improve laser technology and allow Earth scientists to better understand meteor impacts.

An asteroid smashing into the earth and a laser pulsing through optical glass are very different phenomena, but both produce the extreme heat and pressures that instantaneously fuse silicon dioxide – the compound found in sand and glass  into the hard, dense and rare crystal known as stishovite.

Named after Sergey M. Stishov, the Russian physicist who first synthesized it in 1961, stishovite means different things to different scientists.

To geologists, stishovite provides residual proof of a meteor impact.

But for laser scientists, these ultra-hard crystalline lumps mar the optics needed to focus light beams, potentially resulting in catastrophic failure of the optical element.

Although scientists knew that it took intense heat and pressure to fuse sand into stishovite in meteor craters and generate it in smooth glass laser components, they didn’t know why this same exotic crystal could be formed in these two very different environments.

Metoer Crater in Arizona

Meteor Crater in Arizona, formed by a meteorite impact 50,000 years ago, contains bits of a hard, compressed form of silica called stishovite. (National Map Seamless Server/USGS)

Now a Stanford team led by materials science professor Evan J. Reed has provided the most detailed picture yet of how stishovite forms in the very different environments of meteor impact craters and laser pathways.

Based on computer simulations published in the journal Nature Materials, the team revealed a secret of stishovite formation: when this rare silicon variant is created, everything happens really, really fast.

Their findings could lead to more efficient lasers, a better understanding of impact craters in Earth sciences and even new ways to store computer data.  

Heat, pressure, time

Sand, the most common form of silicon dioxide, is composed largely of minute quartz crystals. Quartz has a tetrahedral crystalline structure: each silicon atom is bound to four oxygen atoms.

Glass is another form of silicon dioxide, but its atomic structure is amorphous or irregular rather than crystalline.

Researchers, including Stanford materials scientist Alberto Salleo, had previously shown that crystalline sand and amorphous glass both form stishovite when subjected to massive shock and heat of a meteor strike or laser pulse respectively.  In either case, the silicon dioxide becomes stishovite, an octahedral crystal in which every silicon atom is connected to six oxygen atoms.

But until now, scientists didn’t understand how such different events as a meteor impact and a laser pulse could produce the same crystalline structure. This is surprising because the amorphous form of silica is known to crystallize very slowly, much more slowly than the timescales of laser pulses.

To get the answers the Stanford team ran computer simulations of meteor impacts and laser pulses that produced stishovite crystals in sand and glass, respectively.

They discovered that silicon dioxide molecules crystallize into stishovite almost instantaneously when hit by shock waves moving faster than seven kilometers per second  forces typical of both meteor impacts and laser bursts.

These sudden impacts heat the silicon dioxide to around 3,000° C and subject the molecules to pressures exceeding half a million atmospheres. About one-billionth of a second later, the affected silicon dioxide molecules form octahedral stishovite.

“Ultimately, we demonstrated that silicon dioxide can crystallize into stishovite in nanoseconds, even picoseconds, during both meteor impact events and laser pulses,” said Yuan Shen, a graduate student at Stanford’s Department of Physics and first author of the Nature Materials paper.

The Stanford team, which included Shai B. Jester and Tingting Qi, also showed that meteor impacts and laser pulses produced stishovite crystals of size comparable to those found in the simulations.

“Though these events are on the macro and nano scales respectively, though they differ in magnitude by a million times or more, we end up with the same material in the predicted sizes,” Shen said.

The team’s work provides baseline information on crystallization speed that could prove valuable to various scientific endeavors.

For one thing it could lead to improved laser systems. The production of stishovite in laser optical elements is highly undesirable; even a few grains of the mineral may damage optics and affect function.

“Ultimately, our findings may suggest possible solutions to stishovite damage in optical elements,” said Shen. “For example, laser pulse duration may be a good thing to explore. It’s possible you may be able to avoid damage with shorter pulses. The shock wave from a pulse of a specific ‘short’ duration may be insufficient to cause phase transition of SiO2 to stishovite.”

In the geosciences, this knowledge may allow better assessment of the scale of past meteor impacts.

Shen said the team’s findings may also have applications in computer phase-change memory, a type of random access memory that uses a special material that changes from amorphous to crystalline when heated, allowing it to store information at various intermediate states.

“What’s really compelling about this work is that it provides specifics on just what happens when SiO2 goes from an amorphous to a crystalline state due to pressure and temperature,” Shen said. “We knew that these transitions happen. Now we have the details on how they happen, and that may aid research in a number of different fields.”