SiO₂ Polymorphs and Meteorite Impacts

Meteorite impacts create extreme conditions of high pressure and temperature. Certain minerals, particularly polymorphs of silicon dioxide ($SiO_2$), can form or transform under these shock conditions. Understanding the stability fields of $SiO_2$ polymorphs helps identify which ones are likely to form during such events.

Analyzing SiO₂ Polymorphs

Let's examine the options in the context of meteorite impacts:

• Quartz: The most common form of $SiO_2$, stable at Earth's surface pressures and moderate temperatures. It can transform under impact conditions but is not the characteristic high-pressure product.
• Tridymite: A high-temperature polymorph of $SiO_2$. While high temperatures occur during impacts, the extreme pressures favor denser phases.
• Opal: An amorphous (non-crystalline) hydrated form of $SiO_2$. It is not a distinct polymorph formed under the high-pressure, anhydrous conditions typical of impact events.
• Stishovite: A very dense polymorph of $SiO_2$ with a crystal structure similar to rutile. It is stable only at extremely high pressures (above 8 GPa), conditions achievable during meteorite impacts and nuclear explosions. Stishovite is a key indicator mineral for shock events.

Conclusion: Impact Formation

The extreme pressures generated by a meteorite impact favor the formation of denser, high-pressure polymorphs. Among the given options, Stishovite is the $SiO_2$ polymorph specifically known to form under such shock-metamorphic conditions.